The essence of the laws of inheritance of traits in humans. Patterns of inheritance
The topic “Patterns of heredity and variability” is offered for study. In this lesson we will summarize knowledge about the basic genetic concepts: heredity and variability. Let us formulate a definition of the basic genetic concepts: gene, locus, allele, homozygote and heterozygote. Let us repeat Mendel's three laws of heredity. We’ll also talk about the main types of variability: hereditary, modification and mutation, and discuss their role in evolution.
Heredity is the ability of living organisms to transmit their characteristics unchanged over generations.
Variability- the ability of living organisms to acquire characteristics that distinguish them from their parents.
In the middle of the 19th century, scientists suggested the presence in the cell of a material carrier of hereditary information about traits (Fig. 1); this carrier was called gene.
Rice. 1. DNA chain and chromosomes
Modern wording: gene- a section of DNA encoding messenger RNA containing information about the primary sequence of one polypeptide, or encoding functional RNA - ribosomal, transport and others.
Each gene has its own position on the chromosome, the so-called locus(Fig. 2).
Rice. 2. Locus
A diploid organism (human) contains a double set of chromosomes, one of which comes from the maternal organism, and the other from the paternal organism, thus, there are two copies of each gene in the cell (Fig. 3).
Rice. 3. Double set of chromosomes
Accordingly, an organism can simultaneously have two variants of genes located in the same loci of homologous chromosomes; such gene variants are called alleles.
Descendants receive characteristics from their parents in accordance with the basic laws of inheritance.
Mendel's 1st law. Law of Uniformity of First Generation Hybrids
When crossing two homozygous organisms that differ in one pair of traits, the entire first generation will be uniform in phenotype and genotype.
Mendel's 2nd law. Law of splitting
When two heterozygous organisms are crossed, the offspring exhibit cleavage in phenotype in a ratio of 3:1 and in genotype in a ratio of 1:2:1.
F1 1AA: 2Аа: 1Аа
Mendel's 3rd law. The law of independent inheritance of characters in dihybrid crossing:
When crossing homozygous individuals that differ in two or more pairs of independent traits, the combination of traits is recorded.
F1 9AB: 3Abb: 3aaB: 1aabb
Often independent traits can be inherited together; this happens if the corresponding genes are on the same chromosome; this inheritance is called linked.
Variability (Fig. 4) is required for better adaptability to changing environmental factors. Highlight hereditary And modification variability. Modification variability is not inherited. Hereditary variability can be caused by the sexual process, then it will be called combinative.
The main purpose of the separation of sexes is to ensure combinative variability.
Rice. 4. Types of variability
The second type of hereditary variability is mutational. Mutation is a violation of the nucleotide sequence of the DNA molecule - the carrier of genetic information. Mutations occur randomly and undirectedly; most often they do not benefit the body, but turn out to be destructive. Sometimes mutations lead to urgently needed changes, such individuals gain a competitive advantage, and the replaced trait is fixed in the offspring.
Combinative and mutational variability create the basis for natural selection. Modification variability is not fixed in the offspring, it represents fluctuations in the value of a trait within certain limits (Fig. 5); most often, quantitative traits are subject to modification - height, weight, fertility.
Rice. 5. Fluctuation in the value of a characteristic
Leaf plates can reach different sizes depending on environmental conditions, but these sizes will be limited by the so-called reaction norm. The reaction rate is determined genetically and is inherited.
Similarly, the skin color of a European, depending on the tan, can vary from milky white to dark.
The amount of modification variability is important only for the convenience of a particular individual; it is not transmitted to offspring, therefore the role of modification variability in the evolutionary process is small.
Bibliography
- Mamontov S.G., Zakharov V.B., Agafonova I.B., Sonin N.I. Biology 11th grade. General biology. Profile level. - 5th edition, stereotypical. - Bustard, 2010.
- Belyaev D.K. General biology. A basic level of. - 11th edition, stereotypical. - M.: Education, 2012.
- Pasechnik V.V., Kamensky A.A., Kriksunov E.A. General biology, grades 10-11. - M.: Bustard, 2005.
- Agafonova I. B., Zakharova E. T., Sivoglazov V. I. Biology 10-11 grade. General biology. A basic level of. - 6th ed., add. - Bustard, 2010.
Homework
- Name the basic genetic concepts.
- According to what Mendelian laws do we receive traits from our parents?
- What is variability and what types does it consist of?
Mendel's laws
Diagram of Mendel's first and second laws. 1) A plant with white flowers (two copies of the recessive allele w) is crossed with a plant with red flowers (two copies of the dominant allele R). 2) All descendant plants have red flowers and the same genotype Rw. 3) When self-fertilization occurs, 3/4 of the plants of the second generation have red flowers (genotypes RR + 2Rw) and 1/4 have white flowers (ww).
Mendel's laws- these are the principles of transmission of hereditary characteristics from parent organisms to their descendants, resulting from the experiments of Gregor Mendel. These principles formed the basis for classical genetics and were subsequently explained as a consequence of the molecular mechanisms of heredity. Although three laws are usually described in Russian-language textbooks, the “first law” was not discovered by Mendel. Of particular importance among the patterns discovered by Mendel is the “hypothesis of gamete purity.”
Story
At the beginning of the 19th century, J. Goss, experimenting with peas, showed that when crossing plants with greenish-blue peas and yellowish-white peas in the first generation, yellow-white ones were obtained. However, during the second generation, the traits that did not appear in the first generation hybrids and later called recessive by Mendel appeared again, and plants with them did not split during self-pollination.
O. Sarge, conducting experiments on melons, compared them according to individual characteristics (pulp, peel, etc.) and also established the absence of confusion of characteristics that did not disappear in the descendants, but were only redistributed among them. C. Nodin, crossing various types of datura, discovered the predominance of the characteristics of datura Datula tatula above Datura stramonium, and this did not depend on which plant was the mother and which was the father.
Thus, by the middle of the 19th century, the phenomenon of dominance was discovered, the uniformity of hybrids in the first generation (all hybrids of the first generation are similar to each other), splitting and combinatorics of characters in the second generation. However, Mendel, highly appreciating the work of his predecessors, pointed out that they had not found a universal law for the formation and development of hybrids, and their experiments did not have sufficient reliability to determine numerical ratios. The discovery of such a reliable method and mathematical analysis of the results, which helped create the theory of heredity, is the main merit of Mendel.
Mendel's methods and progress of work
- Mendel studied how individual traits are inherited.
- Mendel chose from all the characteristics only alternative ones - those that had two clearly different options in his varieties (the seeds are either smooth or wrinkled; there are no intermediate options). Such a conscious narrowing of the research problem made it possible to clearly establish the general patterns of inheritance.
- Mendel planned and carried out a large-scale experiment. He received 34 varieties of peas from seed-growing companies, from which he selected 22 “pure” varieties (which do not produce segregation according to the studied characteristics during self-pollination). Then he carried out artificial hybridization of varieties, and crossed the resulting hybrids with each other. He studied the inheritance of seven traits, studying a total of about 20,000 second-generation hybrids. The experiment was facilitated by a successful choice of object: peas are normally self-pollinating, but artificial hybridization is easy to carry out.
- Mendel was one of the first in biology to use precise quantitative methods to analyze data. Based on his knowledge of probability theory, he realized the need to analyze a large number of crosses to eliminate the role of random deviations.
Mendel called the manifestation of the trait of only one of the parents in hybrids as dominance.
Law of Uniformity of First Generation Hybrids(Mendel’s first law) - when crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative manifestations of the trait, the entire first generation of hybrids (F1) will be uniform and will carry the manifestation of the trait of one of the parents.
This law is also known as the "law of trait dominance." Its formulation is based on the concept clean line relative to the characteristic being studied - on modern language this means that individuals are homozygous for this trait. Mendel formulated the purity of a character as the absence of manifestations of opposite characters in all descendants in several generations of a given individual during self-pollination.
When crossing pure lines of purple-flowered peas and white-flowered peas, Mendel noticed that the descendants of the plants that emerged were all purple-flowered, with not a single white one among them. Mendel repeated the experiment more than once and used other signs. If he crossed peas with yellow and green seeds, all the offspring would have yellow seeds. If he crossed peas with smooth and wrinkled seeds, the offspring would have smooth seeds. The offspring from tall and short plants were tall. So, first-generation hybrids are always uniform in this characteristic and acquire the characteristic of one of the parents. This sign (stronger, dominant), always suppressed the other ( recessive).
Codominance and incomplete dominance
Some opposing characters are not in the relation of complete dominance (when one always suppresses the other in heterozygous individuals), but in the relation incomplete dominance. For example, when pure snapdragon lines with purple and white flowers are crossed, the first generation individuals have pink flowers. When pure lines of black and white Andalusian chickens are crossed, gray chickens are born in the first generation. With incomplete dominance, heterozygotes have characteristics intermediate between those of recessive and dominant homozygotes.
The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation.
Explanation
Law of gamete purity: each gamete contains only one allele from a pair of alleles of a given gene of the parent individual.
Normally, the gamete is always pure from the second gene of the allelic pair. This fact, which could not be firmly established in Mendel's time, is also called the gamete purity hypothesis. This hypothesis was later confirmed by cytological observations. Of all the laws of inheritance established by Mendel, this “Law” is the most general in nature (it is fulfilled under the widest range of conditions).
Law of independent inheritance of characteristics
Illustration of independent inheritance of traits
Definition
Law of independent inheritance(Mendel’s third law) - when crossing two homozygous individuals that differ from each other in two (or more) pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations (as in monohybrid crossing). When plants differing in several characters, such as white and purple flowers and yellow or green peas, were crossed, the inheritance of each character followed the first two laws and in the offspring they were combined in such a way as if their inheritance occurred independently of each other. The first generation after crossing had a dominant phenotype for all traits. In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1, that is, 9:16 were with purple flowers and yellow peas, 3:16 were with white flowers and yellow peas, 3:16 were with purple flowers and green peas, 1 :16 with white flowers and green peas.
Explanation
Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes. During meiosis, homologous chromosomes of different pairs are randomly combined in gametes. If the paternal chromosome of the first pair gets into the gamete, then with equal probability both the paternal and maternal chromosomes of the second pair can get into this gamete. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. (It later turned out that of the seven pairs of characters studied by Mendel in the pea, which has a diploid number of chromosomes 2n=14, the genes responsible for one of the pairs of characters were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since as linkage between these genes was not observed due to the large distance between them).
Basic provisions of Mendel's theory of heredity
In modern interpretation, these provisions are as follows:
- Discrete (separate, non-mixable) hereditary factors - genes are responsible for hereditary traits (the term “gene” was proposed in 1909 by V. Johannsen)
- Each diploid organism contains a pair of alleles of a given gene responsible for a given trait; one of them is received from the father, the other from the mother.
- Hereditary factors are transmitted to descendants through germ cells. When gametes are formed, each of them contains only one allele from each pair (the gametes are “pure” in the sense that they do not contain the second allele).
Conditions for the fulfillment of Mendel's laws
According to Mendel's laws, only monogenic traits are inherited. If more than one gene is responsible for a phenotypic trait (and the absolute majority of such traits), it has a more complex pattern of inheritance.
Conditions for fulfilling the law of segregation during monohybrid crossing
Splitting 3:1 by phenotype and 1:2:1 by genotype is performed approximately and only under the following conditions.
Patterns of heredity. G. Mendel's laws, their statistical nature and cytological foundations
The basic laws of heredity were established by the outstanding Czech scientist Gregor Mendel. G. Mendel began his research with monohybrid crossing, in which the parent individuals differ in the state of one trait. The seed pea he chose is a self-propagating plant, so the descendants of each individual are pure lines. Together, peas can be artificially cross-pollinated, which makes it possible to hybridize and obtain heterozygous (hybrid) forms. How maternal (P) plants of a pure line were taken from yellow seeds, and the parent (P) - with a green color. As a result of such crossing, the seeds of plants (first generation hybrids - F1) turned out to be uniform - yellow in color. That is, only dominant traits appeared in the phenotype of F1 hybrids.
The uniformity of the first hybrid generation and the identification of only a dominant trait in hybrids is called the law of dominance or Mendel’s law.
Segregation is the phenomenon of manifestation of both states of traits in the second generation of hybrids (F2), due to the difference in the allelic genes that determine them.
There are self-pollinating F1 plants with yellow seeds that produce offspring with yellow and green seeds; the recessive trait does not disappear, but is only temporarily suppressed and reappears in F2 in the ratio of 1/4 green seeds and 3/4 yellow seeds. That is exactly - 3:1.
The manifestation of a recessive trait in the phenotype of a quarter of second-generation hybrids, and the manifestation of a dominant trait in three-quarters, is called the law of segregation, Mendel’s II law.
Subsequently, G. Mendel complicated the conditions in the experiments - he used plants that differed in different states of two (Dihybrid crossing) or more characters (polyhybrid crossing). When crossing pea plants with yellow smooth seeds and wrinkled green ones, all the first generation hybrids had smooth yellow seeds - a manifestation of Mendel's law - the uniformity of the first generation hybrids. But among the F2 hybrids there were four phenotypes.
Based on the results obtained, G. Mendel formulated the law of independent combination of character states (the law of independent inheritance of characters). This is Mendel's third law. In di- or polyhybrid crossing, the splitting of the states of each trait in the descendants occurs independently of the others. Dihybrid crosses are characterized by cleavage according to the 9:3:3:1 phenotype, and groups with new combinations of characteristics appear.
Incomplete dominance is the intermediate nature of inheritance. There are alleles that are only partially dominant over recessive ones. Then the hybrid individual has a degree of the trait in the phenotype, which distinguishes it from the parents. This phenomenon is called incomplete dominance.
Methods for checking the genotype of hybrid individuals
As is known, with complete dominance, individuals with a dominant and heterozygous set of chromosomes are phenotypically identical. It is possible to determine their genotype using analytical crossbreeding. It is based on the fact that individuals homozygous for a recessive trait are always similar phenotypically. This is the crossing of a recessive homozygous individual with an individual with a dominant trait but an unknown genotype.
When receiving a monotonous F1, each parent produces only one type of gamete. So, the dominant individual is homozygous for the genotype (AA).
If, when crossing an individual with a dominant trait with an individual with a recessive homozygous trait, the resulting offspring has a 1:1 split, then the individual with the dominant trait is heterozygous (Aa).
- Features of the hybridological analysis method. Mendel's laws.
- Types of gene interactions.
- Linked inheritance of traits.
- Cytoplasmic inheritance.
Method hybridological analysis , which consists of crossing and subsequent accounting of splits (ratios of phenotypic and genotypic varieties of descendants), was developed by the Czech naturalist G. Mendel (1865). The features of this method include: 1) when crossing, taking into account not the entire diverse set of traits in parents and descendants, but analyzing the inheritance of individual alternative traits identified by the researcher; 2) quantitative accounting in a series of successive generations of hybrid plants that differ in individual characteristics; 3) individual analysis of the offspring from each plant.
Working with self-pollinating garden pea plants, G. Mendel chose for the experiment varieties (pure lines) that differed from each other in alternative manifestations of traits. Mendel processed the data obtained mathematically, as a result of which a clear pattern of inheritance of individual characteristics of parental forms by their descendants in a number of subsequent generations was revealed. Mendel formulated this pattern in the form of rules of heredity, later called Mendel's laws.
The crossing of two organisms is called hybridization. Monohybrid (monogenic) is the crossing of two organisms in which the inheritance of one pair of alternative manifestations of a trait is traced (the development of this trait is determined by a pair of alleles of the same gene). The first generation hybrids are uniform in the studied trait. In F1, only one of a pair of alternative variants of the seed color trait appears, called dominant. These results illustrate Mendel's first law - the law of uniformity of first-generation hybrids, as well as the rule of dominance.
Mendel's first law can be formulated as follows: when crossing homozygous individuals that differ in one or several pairs of alternative traits, all first-generation hybrids will be uniform in these traits. Hybrids will exhibit the dominant traits of their parents.
In the second generation, splitting according to the studied trait was discovered
The ratio of offspring with dominant and recessive manifestations of the trait turned out to be close to ¾ to ¼. Thus, Mendel's second law can be formulated as follows: during a monohybrid crossing of heterozygous individuals (F1 hybrids) in the second generation, splitting is observed in the variants of the analyzed trait in a ratio of 3:1 by phenotype and 1:2:1 by genotype. To explain the distribution of traits in hybrids of successive generations, G. Mendel suggested that each hereditary trait depends on the presence in somatic cells of two hereditary factors received from the father and mother. It has now been established that Mendelian hereditary factors correspond to genes - chromosome loci.
Homozygous plants with yellow seeds (AA) produce gametes of the same variety with the A allele; plants with green seeds (aa) produce gametes with a. Thus, using modern terminology, the hypothesis “ purity of gametes“can be formulated as follows: “In the process of formation of germ cells, only one gene from an allelic pair enters each gamete, because during the process of meiosis, one chromosome from a pair of homologous chromosomes enters the gamete.
Crossing in which inheritance of two pairs of alternative characters is traced is called dihybrid, according to several pairs of signs - polyhybrid. In Mendel's experiments, when crossing a pea variety that had yellow (A) and smooth (B) seeds with a pea variety with green (a) and wrinkled (b) seeds, the F1 hybrids had yellow and smooth seeds, i.e. Dominant characters appeared (the hybrids are uniform).
Hybrid seeds of the second generation (F2) were distributed into four phenotypic groups in the ratio: 315 - with smooth yellow seeds, 101 - with wrinkled yellow seeds, 108 - with smooth green seeds, 32 - with green wrinkled seeds. If the number of offspring in each group is divided by the number of offspring in the smallest group, then in F2 the ratio of phenotypic classes is approximately 9:3:3:1. So, according to Mendel's third law, genes of different allelic pairs and the characteristics corresponding to them are transmitted to the offspring regardless from each other combining in all sorts of combinations.
With complete dominance of one allele over the other, heterozygous individuals are phenotypically indistinguishable from those homozygous for the dominant allele and can only be distinguished using hybridological analysis, i.e. by the offspring that is obtained from a certain type of crossing, called analyzing. Analyzing is a type of crossing in which a test individual with a dominant trait is crossed with an individual homozygous for the recessive apple.
If the dominant individual is homozygous, the offspring from such a cross will be uniform and no segregation will occur. In the event that an individual with a dominant trait is heterozygous, splitting will occur in a 1:1 ratio in phenotype and genotype.
Gene interaction
In some cases, the action of different genes is relatively independent, but, as a rule, the manifestation of traits is the result of the interaction of products of different genes. These interactions may be related to both allelic, so with non-allelic genes.
Interaction between alleles genes are carried out in three forms: complete dominance, incomplete dominance and independent manifestation (co-dominance).
Previously, Mendel's experiments were reviewed, which revealed the complete dominance of one allele and the recessivity of the other. Incomplete dominance is observed when one gene from a pair of alleles does not provide the formation of its protein product sufficient for the normal manifestation of the trait. With this form of gene interaction, all heterozygotes and homozygotes differ significantly in phenotype from each other. At co-dominance in heterozygous organisms, each of the allelic genes causes the formation of a trait controlled by it in the phenotype. An example of this form of interaction of alleles is the inheritance of human blood groups according to the ABO system, determined by the I gene. There are three alleles of this gene, Io, Ia, Ib, which determine blood group antigens. The inheritance of blood groups also illustrates the phenomenon plural allelism: in the gene pools of human populations, gene I exists in the form of three different alleles, which are combined in individual individuals only in pairs.
Interaction of nonallelic genes. In some cases, one trait of an organism can be influenced by two (or more) pairs of non-allelic genes. This leads to significant numerical deviations of phenotypic (but not genotypic) classes from those established by Mendel during dihybrid crossing. The interaction of non-allelic genes is divided into main forms: complementarity, epistasis, polymerization.
At complementary interaction, the trait manifests itself only in the case of the simultaneous presence of two dominant non-allelic genes in the genotype of the organism. An example of a complementary interaction is crossing two different varieties of sweet peas with white flower petals.
The next type of interaction of non-allelic genes is epistasis, in which the gene of one allelic pair suppresses the effect of the gene of the other pair. A gene that suppresses the action of another is called epistatic genome(or suppressor). The suppressed gene is called hypostatic. Epistasis can be dominant or recessive. An example of dominant epistasis is the inheritance of plumage color in chickens. Gene C in a dominant form determines normal pigment production, but the dominant allele of another gene I is its suppressor. As a result of this, chickens that have a dominant allele of the color gene in their genotype turn out to be white in the presence of a suppressor. The epistatic effect of a recessive gene illustrates the inheritance of coat color in house mice. The color of agouti (reddish-gray coat color) is determined by the dominant gene A. Its recessive allele a, in the homozygous state, causes black coloration. The dominant gene of another pair C determines the development of pigment; homozygotes for the recessive allele c are albinos with white fur and red eyes (lack of pigment in the fur and iris of the eyes).
Inheritance of a trait, the transmission and development of which is determined, as a rule, by two alleles of one gene, is called monogenic. In addition, genes from different allelic pairs are known (they are called polymeric or polygenes), having approximately the same effect on the trait.
The phenomenon of simultaneous action on a trait of several non-allelic genes of the same type is called polymerization. Although polymer genes are not allelic, since they determine the development of one trait, they are usually designated by one letter A (a), with numbers indicating the number of allelic pairs. The action of polygenes is most often additive.
Chained inheritance
An analysis of the simultaneous inheritance of several traits in Drosophila, carried out by T. Morgan, showed that the results of analytical crossing of F1 hybrids sometimes differ from those expected in the case of their independent inheritance. In the descendants of such a cross, instead of freely combining the characteristics of different pairs, a tendency was observed to inherit predominantly parental combinations of characteristics. This inheritance of traits was called linked. Linked inheritance is explained by the location of the corresponding genes on the same chromosome. As part of the latter, they are transmitted from generation to generation of cells and organisms, preserving the combination of alleles of the parents.
The dependence of linked inheritance of traits on the localization of genes on one chromosome gives grounds to consider chromosomes as separate clutch groups. An analysis of the inheritance of the eye color trait in Drosophila in T. Morgan's laboratory revealed some features that forced us to distinguish it as a separate type of inheritance of traits. sex-linked inheritance.
The dependence of the experimental results on which parent was the carrier of the dominant variant of the trait allowed us to suggest that the gene that determines eye color in Drosophila is located on the X chromosome and does not have a homologue on the Y chromosome. All features of sex-linked inheritance are explained by the unequal dose of the corresponding genes in representatives of different sexes - homo- and heterogametic. The X chromosome is present in the karyotype of each individual, therefore the characteristics determined by the genes of this chromosome are formed in both female and male representatives. Individuals of the homogametic sex receive these genes from both parents and pass them on to all offspring through their gametes. Representatives of the heterogametic sex receive a single X chromosome from the homogametic parent and pass it on to their homogametic offspring. In mammals (including humans), the male sex receives X-linked genes from the mother and passes them on to daughters. At the same time, the male sex never inherits the paternal X-linked trait and does not pass it on to his sons
Actively functioning genes on the Y chromosome, which do not have alleles on the X chromosome, are present in the genotype only of the heterogametic sex, and in a hemizygous state. Therefore, they manifest themselves phenotypically and are transmitted from generation to generation only in representatives of the heterogametic sex. Thus, in humans, the sign of hypertrichosis of the auricle (“hairy ears”) is observed exclusively in men and is inherited from father to son.
We will start by presenting Mendel's laws, then we will talk about Morgan, and at the end we will say why genetics is needed today, how it helps and what its methods are.
In the 1860s, the monk Mendel began researching the inheritance of traits. This was done before him, and this is the first time it is mentioned in the Bible. The Old Testament says that if a livestock owner wanted to get a certain breed, then he fed some sheep with peeled branches if he wanted to get offspring with white wool, and unshelled ones if he wanted to get black cattle skins. That is, how traits are inherited worried people even before the Bible was written. Why, before Mendel, could they not find laws for the transmission of traits across generations?
The fact is that before him, researchers chose a set of characteristics of one individual, which were more difficult to deal with than with one trait. Before him, the transmission of characteristics was often considered as a single complex (for example, she has a grandmother’s face, although there are a lot of individual signs). And Mendel recorded the transmission of each trait separately, regardless of how other traits were transmitted to descendants.
It is important that Mendel chose for the study features whose registration was extremely simple. These are discrete and alternative signs:
- discrete (discontinuous) signs: a given sign is either present or absent. For example, a color attribute: a pea is either green or not green.
- alternative characteristics: one state of a trait excludes the presence of another state. For example, the state of such a feature as color: a pea is either green or yellow. Both states of a trait cannot appear in one organism.
Mendel had an approach to analyzing descendants that had not been used before. This is a quantitative, statistical method of analysis: all descendants with a given trait state (for example, green peas) were combined into one group and their number was calculated, which was compared with the number of descendants with a different trait state (yellow peas).
As a trait, Mendel chose the color of the seeds of peas, the state of which was mutually exclusive: the color was either yellow or green. Another sign is the shape of the seeds. Alternative states of the trait are the shape is either wrinkled or smooth. It turned out that these characteristics are stably reproduced over generations, and appear either in one state or in another. In total, Mendel studied 7 pairs of traits, monitoring each one separately.
In crossing, Mendel studied the transmission of traits from parents to their offspring. And that's what he got. During self-pollination, one of the parents produced only wrinkled seeds in a succession of generations, the other parent - only smooth seeds.
Peas are self-pollinators. In order to get offspring from two different parents (hybrids), he had to make sure that the plants did not self-pollinate. To do this, he removed the stamens from one parent plant and transferred pollen from another plant to it. In this case, the resulting seeds were hybrid. All hybrid seeds in the first generation turned out to be the same. They all turned out to be smooth. We call the manifested state of the trait dominant (the meaning of the root of this word is dominant). Another state of the trait (wrinkled seeds) was not detected in the hybrids. We call this state of the trait recessive (inferior).
Mendel crossed the first generation of plants within himself and looked at the shape of the resulting peas (this was the second generation of the offspring of the cross). The main part of the seeds turned out to be smooth. But part of it was wrinkled, exactly the same as that of the original parent (if we were talking about our own family, we would say that the grandson was exactly like his grandfather, although his father and mother did not have this sign at all). He conducted a quantitative study of what proportion of offspring belonged to one class (smooth - dominant), and what to another class (wrinkled - recessive). It turned out that about a quarter of the seeds were wrinkled, and three quarters were smooth.
Mendel carried out the same crossings of first-generation hybrids for all other characteristics: seed color, flower color, etc. He saw that the 3:1 ratio was maintained.
Mendel crossed in one direction (father with a dominant trait, mother with a recessive one) and in the other (father with a recessive trait, mother with a dominant one). At the same time, the qualitative and quantitative results of the transmission of traits across generations were the same. From this we can conclude that both the female and paternal inclinations of the trait make an equal contribution to the inheritance of the trait in the offspring.
The fact that in the first generation the trait of only one parent appears, we call the law of uniformity of first-generation hybrids or the law of dominance.
The fact that in the second generation the characteristics of both one parent (dominant) and the other (recessive) reappear allowed Mendel to suggest that it is not the trait as such that is inherited, but the inclination of its development (what we now call the gene). He also suggested that each organism contains a pair of such inclinations for each trait. Only one of the two inclinations passes from parent to descendant. The deposit of each type (dominant or recessive) passes to the descendant with equal probability. When a descendant combines two different inclinations (dominant and recessive), only one of them appears (dominant, denoted by a capital letter A). The recessive inclination (denoted by the small letter a) does not disappear in the hybrid, since it manifests itself as a trait in the next generation.
Since in the second generation exactly the same organism as the parent one appeared, Mendel decided that the deposit of one trait “is not obscured”; when combined with another, it remains just as pure. Subsequently, it was found out that only half of its inclinations are transmitted from a given organism - sex cells, they are called gametes, carry only one of two alternative characteristics.
Humans have about 5 thousand morphological and biochemical characteristics that are inherited quite clearly according to Mendel. Judging by the splitting in the second generation, the alternative inclinations of one trait were combined with each other independently. That is, a dominant trait could appear in combinations like Ahh, aA And AA, and recessive only in combination ahh.
Let us repeat that Mendel suggested that it is not the trait that is inherited, but the inclinations of the trait (genes) and that these inclinations do not mix, therefore this law is called the law of purity of gametes. Through the study of the inheritance process, it was possible to draw conclusions about some characteristics of the inherited material, that is, that the inclinations are stable over generations, retain their properties, that the inclinations are discrete, that is, only one state of the trait is determined, that there are two of them, they are combined randomly, etc. d.
At the time of Mendel, nothing was known about meiosis, although they already knew about the nuclear structure of cells. The fact that the nucleus contains a substance called nuclein became known only a couple of years after the discovery of Mendel’s laws, and this discovery was in no way connected with him.
All the conclusions of the above material can be formulated as follows:
1) Each hereditary characteristic is determined by a separate hereditary factor, a deposit; in the modern view, these inclinations correspond to genes;
2) Genes are preserved in their pure form over a number of generations, without losing their individuality: this was proof of the main point of genetics: the gene is relatively constant;
3) Both sexes participate equally in the transmission of their hereditary properties to offspring;
4) Reduplication of an equal number of genes and their reduction in male and female germ cells; this position was a genetic prediction of the existence of meiosis;
5) Hereditary inclinations are paired, one is maternal, the other is paternal; one of them may be dominant, the other recessive; This position corresponds to the discovery of the principle of allelism: a gene is represented by at least two alleles.
The laws of inheritance include the law of splitting hereditary characteristics in the offspring of a hybrid and the law of independent combination of hereditary characteristics. These two laws reflect the process of transmission of hereditary information in cellular generations during sexual reproduction. Their discovery was the first actual evidence of the existence of heredity as a phenomenon.
The laws of heredity have a different content, and they are formulated as follows:
- The first law is the law of discrete (genetic) hereditary determination of traits; it underlies the gene theory.
- The second law is the law of the relative constancy of the hereditary unit - the gene.
- The third law is the law of the allelic state of the gene (dominance and recessivity).
The fact that Mendel's laws are related to the behavior of chromosomes during meiosis was discovered at the beginning of the twentieth century during the rediscovery of Mendel's laws by three groups of scientists independently of each other. As you already know, the peculiarity of meiosis is that the number of chromosomes in a cell is halved; chromosomes can change their parts during meiosis. This feature characterizes the life cycle situation in all eukaryotes.
In order to test the assumption about the inheritance of inclinations in this form, as we have already said, Mendel also crossed the descendants of the first generation, which had yellow seeds with parental green (recessive). He called crossbreeding into a recessive organism analyzing. As a result, he got a one-to-one split: ( Ahh X ahh = Ahh + Ahh + ahh + ahh). Thus, Mendel confirmed the assumption that in the body of the first generation there are the makings of the traits of each of the parents in a ratio of 1 to 1. Mendel called the state when both the makings of the traits are the same homozygous, and when they are different - heterozygous.
Mendel took into account the results obtained on thousands of seeds, that is, he conducted statistical studies that reflect a biological pattern. The very laws he discovered will also apply to other eukaryotes, such as fungi. Shown here are fungi in which the four spores produced by a single meiosis remain in a common shell. Analyzing crossing in such fungi leads to the fact that in one shell there are 2 spores with a characteristic of one parent and two with a characteristic of the other. Thus, 1:1 splitting in an analyzing cross reflects the biological pattern of splitting the inclinations of one character in each meiosis, which will look like a statistical pattern if all the spores are mixed.
The fact that the parents had different states of one trait suggests that the inclinations for the development of the trait may somehow change. These changes are called mutations. Mutations can be neutral: hair shape, eye color, etc. Some mutations lead to changes that disrupt the normal functioning of the body. These are short legs in animals (cattle, sheep, etc.), eyelessness and winglessness in insects, hairlessness in mammals, gigantism and dwarfism.
Some mutations may be harmless, such as hairlessness in humans, although all primates have hair. But sometimes there are changes in the intensity of hair on the body and in people. N.I. Vavilov called this phenomenon the law of homological series of hereditary variability: that is, a trait typical only for one of two related species can be found with some frequency in individuals of a related species.
This slide shows that mutations can be quite noticeable; we see a black family into which a white albino black man was born. His children are likely to be pigmented, since this mutation is recessive and its frequency of occurrence is low.
We talked before about signs that fully manifest themselves. But this is not true for all signs. For example, the phenotype of heterozygotes may be intermediate between the dominant and recessive traits of the parents. Thus, the color of the fruit of eggplants in the first generation changes from dark blue to a less intense purple. At the same time, in the second generation, the splitting according to the presence of color remained 3:1, but if we take into account the color intensity, the splitting became 1:2:1 (color dark blue - AA, purple - 2 Ahh and white - ahh, respectively) In this case, it is clear that the manifestation of the trait depends on the dose of the dominant allele. Phenotype splitting corresponds to genotype splitting: classes AA, Ahh And ahh, in a ratio of 1:2:1.
Let us once again highlight the role of Mendel in the development of science. No one before him had thought that the makings of signs could exist at all. It was believed that inside each of us there is a little man, inside him there is another little man, etc. Conception has something to do with its appearance, but according to the mechanism, the ready little man is already present from the very beginning of his growth. These were the dominant ideas, which, of course, had a drawback - according to this theory, with a large number of generations, the homunculus should have turned out to be smaller in size than an elementary particle, but then they did not yet know about particles J.
How did Mendel know which trait was dominant and which was recessive? He didn’t know anything like that, he just took some principle of organizing experience. Conveniently, the signs he observed were different: height, size, flower color, bean color, etc. He did not have an a priori model of the mechanism of inheritance; he derived it from observation of the transmission of a trait over generations. Another feature of his method. He found that the proportion of individuals with a recessive trait in the second generation is a quarter of all offspring. That is, the probability that a given pea is green is 1/4. Let's say we got an average of 4 peas in one pod. Will each pod (the offspring of two and only two parents) contain 1 green pea and 3 yellow peas? No. For example, the probability that there will be 2 green peas is 1/4 x 1/4 = 1/16, and that all four are green is 1/256. That is, if you take a bunch of beans with four peas in each, then every 256th pea will have recessive traits, that is, green. Mendel analyzed the offspring of many identical pairs of parents. Crossing was talked about because they show that Mendel's laws appear as statistical ones, but are based on a biological pattern - 1:1. That is, gametes different types in EACH meiosis, heterozygotes are formed in an equal ratio - 1:1, and the patterns manifest themselves statistically, since the descendants of hundreds of meioses are analyzed - Mendel analyzed more than 1000 descendants in each type of cross.
Mendel first studied the inheritance of one pair of traits. He then wondered what would happen if two pairs of signs were observed simultaneously. The figure above, on the right side, illustrates such a study on thought pairs of signs - the color of peas and the shape of peas.
Parents of one type produced yellow and round peas during self-pollination. Parents of another type produced green and wrinkled peas during self-pollination. In the first generation, all the peas were yellow and round in shape. The resulting splitting in the second generation can be conveniently examined using the Penet lattice. We obtained a split according to the characteristics 9:3:3:1 (yellow and round: yellow and wrinkled: green and round: green and wrinkled). Splitting for each pair of characteristics occurs independently of each other. The ratio 9zhk + 3zhm + 3zk + 1zm corresponds to an independent combination of the results of two crosses (3zh + 1z) x (3k + 1m). That is, the makings of the characteristics of these pairs (color and shape) are combined independently.
Let's count how many different phenotypic classes we got. We had 2 phenotypic classes: yellow and green; and according to another characteristic, 2 phenotypic classes: round and wrinkled. And in total there will be 2*2=4 phenotypic classes, which is what we got above. If we consider three characteristics, then there will be 2 3 =8 phenotypic classes. Mendel went as far as dihybrid crosses. The makings of all the traits, fortunately for Mendel, were located on different chromosomes in peas, and there are 7 pairs of chromosomes in peas. Therefore, it turned out that he took on traits that combined independently in the offspring.
Humans have 23 pairs of chromosomes. If we consider one heterozygous trait for each chromosome, a person may have 2 23 ~ 8 * 10 6 phenotypic classes in the offspring of one married couple. As mentioned in the first lecture, each of us contains about 1 difference between our father's and mother's chromosomes per 1000 positions, that is, a total of about a million differences between our father's and mother's chromosomes. That is, each of us is a descendant of a million-hybrid cross, in which the number of phenotypic classes is 2 1000000. In practice, this number of phenotypic classes is not realized in the offspring of one pair, because we have only 23 chromosomes, not a million. It turns out that 8 * 10 6 is the lower limit of the possible diversity in the offspring of a given married couple. Based on this, you can understand that there cannot be two absolutely identical people. The probability of mutation of a given nucleotide in DNA in one generation is about 10 -7 - 10 -8, that is, for the entire genome (3 * 10 9) there will be about 100 de novo changes between parent and child. And the total differences in the father's half of your genome from the mother's half are about 1,000,000. This means that old mutations in your genome are much more frequent than newly emerged ones (10,000 times).
Mendel also carried out analytical crossing - crossing with a recessive homozygote. In a descendant of the first generation, the combination of genes has the form AaB b. If you cross it with a representative with a completely recessive set of genes ( aabb), then we will get four possible classes, which will be in the ratio 1:1:1:1, in contrast to the crossing discussed above, when we got a splitting of 9:3:3:1.
Below are some statistical criteria - what ratios of numbers should be considered consistent with what is expected, say 3:1. For example, for 3:1 - out of four hundred peas it is unlikely that exactly 300 to 100 will turn out. If it turns out, for example, 301 to 99, then this ratio can probably be considered equal to 3 to 1. But 350 to 50 is probably not equal to 3 to 1.
The chi-square test (χ 2) statistic is used to test the hypothesis that the observed distribution matches the expected one. This Greek letter is pronounced in Russian as “chi”, and in English as “chi” (chi).
The value of χ 2 is calculated as the sum of the squared deviations of the observed values from the expected value, divided by the expected value. Then, using a special table for a given value of χ 2, find the probability that such a difference between the observed and expected value is random. If the probability is less than 5%, then the deviation is considered not random (the figure of five percent was chosen by agreement).
Will any hereditarily predetermined trait always manifest itself? After all, this default assumption underlies the interpretation of the data obtained by Mendel.
It turns out that this can depend on many reasons. There is such an inherited trait in humans - six-fingeredness. Although we, like all vertebrates, normally have five fingers.
The probability of manifestation of the inclination of a trait in the form of an observable trait (here - six-fingeredness) may be less than 100%. In the photograph, the man has 6 toes on both feet. And his twin will not necessarily show this sign. The proportion of individuals with a given genotype who exhibit the corresponding phenotype was called penetrance (this term was introduced by the Russian geneticist Timofeev-Resovsky).
In some cases, the sixth digit may simply be marked by some skin growth. Timofeev-Resovsky proposed to call the degree of expression of a trait in an individual expressiveness.
The not 100% connection between genotype and phenotype is especially clear when studying identical twins. Their genetic constitution is the same, and their characteristics coincide to varying degrees. Below is a table showing the coincidence of characteristics for identical and non-identical twins. Various diseases are taken as signs in this table.
The trait that is present in the majority of individuals in natural habitats is called the wild type. The most common trait is often dominant. Such a connection may have adaptive significance beneficial for the species. In humans, dominant characteristics are, for example, black hair, dark eyes, curly hair. By the way, since the corresponding genes are on different chromosomes, you can get a curly-haired black man who will be blond - nothing prohibits this.
Why does it happen that in a monohybrid cross, three genotypic classes in the second generation offspring correspond in some cases to three phenotypic classes (blue purple and white eggplants), and in another case to two classes (yellow or green peas)? Why is the manifestation of a dominant trait incomplete in one case, and complete in the other? An analogy can be drawn with photographic film. Depending on the amount of light, the frame can turn out completely transparent, gray or completely black. It's the same with genes. For example, corn has a gene Y, which determines the formation of vitamin A. When the dose of the Y gene per cell increases from one to three, the activity of the enzyme that it encodes changes linearly and, in this case, the formation of vitamin A and the color of the grain increase. (In corn, the main part of the grain is the endosperm. Each endosperm cell has three genomes - two from mom and one from dad). That is, many traits depend quantitatively on the allele dose. The more copies of an allele of the desired type, the greater the value of the trait it controls. This connection is constantly used in biotechnology.
Mendel could have successfully not discovered his laws. Research on peas allowed Mendel to discover his laws, because peas are a self-pollinating plant, and therefore homozygous without coercion. During self-pollination, the proportion of heterozygotes decreases in proportion to two to the power of the generation number. This was Mendel's luck - if the proportion of heterozygotes had been large, then no patterns would have been observed. When he then took cross-pollinators, the patterns broke down, which greatly upset Mendel because he thought he had discovered something private. It turned out not.
Above we talked about the inheritance of qualitative traits, but usually most traits are quantitative. Their genetic control is quite complex. Quantitative characteristics are described through average value the value of the characteristic and the range of variation, which is called the reaction norm. Both the average value and the reaction norm are species-specific indicators that depend on both the genotype and environmental conditions. For example, human life expectancy. Although the Bible says that prophets lived for 800 years, it is now clear that no one lives more than 120-150 years. And, a mouse, for example, lives for two years, although it is also a mammal. Our height, our weight - these are all quantitative signs. There are no people 3-4 meters tall, although there are elephants, for example. Each species has its own average for each quantitative trait and its own range of variation.
Patterns of inheritance are discovered in the study of qualitative traits.
Most of our traits are quantitative.
The values of traits in a representative sample of individuals of a given species are characterized by a certain average and breadth of variation, which is called the reaction norm and depends on both the genotype and the conditions for the formation of the trait.
Topic 4.2 Basic patterns
heredity
Terminology 1.Alternative– contrasting signs. 2. Clean lines– plants in a row of which no splitting is observed during self-pollination. 3. Hybridological method– obtaining hybrid offspring and its analysis. 4. Parents– R. 5. Males – ♂. 6. Females – ♀. 7. Crossbreeding– X. 8. Hybrids F 1, F 2, F n. 9. Monohybrid– crossing of individuals with one contrasting trait. Patterns of inheritance of traits Quantitative patterns of inheritance of traits were discovered by the Czech amateur botanist G. Mendel. Having set the goal of finding out the patterns of inheritance of traits, he, first of all, paid attention to the choice of the object of study. For his experiments, G. Mendel chose peas - those varieties that clearly differed from each other in a number of characteristics. One of the most significant points in all the work was determining the number of characteristics by which the crossed plants should be distinguished. G. Mendel first realized that by starting with the simplest case - differences between parents on one single trait and gradually complicating the task, one can hope to unravel the whole tangle of patterns of transmission of traits from generation to generation, i.e. their inheritance. Here the strict mathematical nature of his thinking was revealed. It was this approach that allowed G. Mendel to clearly plan the further complication of experiments. In this respect, Mendel stood above all the biologists of his time. Another important feature of his research was that he chose for experiments organisms belonging to pure lines, i.e. such plants, in a number of generations of which, during self-pollination, no segregation according to the studied trait was observed. No less important is that he observed the inheritance of alternatives, i.e. contrasting features. For example, the flowers of one plant were purple, and the flowers of another were white, the growth of the plant was tall or short, the beans were smooth or wrinkled, etc. Comparing the results of experiments and theoretical calculations, G. Mendel especially emphasized the average statistical nature of the patterns he discovered. Thus, the method of crossing individuals that differ in alternative characteristics, i.e. hybridization, followed by strict consideration of the distribution of parental characteristics in offspring, is called hybridiological. The patterns of inheritance of traits, identified by G. Mendel and confirmed by many biologists on a variety of objects, are currently formulated in the form of laws that are universal in nature. Law of Uniformity of the First Generation of Hybrids Monohybrid crossing. To illustrate the law of uniformity of the first generation - Mendel's first law, let us reproduce his experiments on monohybrid crossing of pea plants. Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative characteristics. Consequently, with such crossing, patterns of inheritance of only two variants of the trait can be traced, the development of which is determined by a pair of allelic genes. For example, a sign is the color of the seeds, options are yellow or green. All other characteristics characteristic of these organisms are not taken into account. If you cross pea plants with yellow and green seeds, then all the resulting hybrid offspring will have yellow seeds. The same picture is observed when crossing plants with smooth and wrinkled seeds - all the seeds of the hybrids will be smooth. Consequently, in a first-generation hybrid, only one of each pair of alternative characters appears. The second sign seems to disappear and does not appear. Mendel called the predominance of the trait of one of the parents in a hybrid dominance. A trait that appears in a first-generation hybrid and suppresses the development of another trait was called dominant, opposite, i.e. the suppressed trait is recessive. A dominant trait is usually denoted by a capital letter (A), and a recessive trait by a lowercase letter (a). Mendel used in his experiments plants belonging to different pure lines, or varieties, the descendants of which were similar to their parents over a long series of generations. Therefore, in these plants both allelic genes are the same. Thus, if the genotype of an organism contains two identical allelic genes, i.e. two absolutely identical gene nucleotide sequences, such an organism is called homozygous. An organism can be homozygous for dominant (AA) or recessive (aa) genes. If allelic genes differ from each other in nucleotide sequence, for example, one is dominant and the other is recessive (Aa), such an organism is called heterozygous. Mendel's first law is also called the law of dominance or uniformity, since all individuals of the first generation have the same manifestation of a trait inherent in one of the parents. It is formulated like this: When crossing two organisms belonging to different pure lines (two homozygotes), differing from each other in a pair of alternative traits, the entire first generation of hybrids (F 1) will be uniform and will carry the trait of one parent. In terms of color, Mendel established that red or black would dominate over white, with pink and gray of varying intensities being intermediate colors. Mendel proposed graphic designations for characters: P – parents, ♂ – male, ♀ – female,, - gametes, X - crossing, F 1, F 2, F n - offspring. Mendel's first law is presented in Figure 1.
Figure 1. Mendel's first law
All offspring have the same intermediate color, which does not contradict Mendel’s first law.
Control questions
1. Biological material Mendel. 2. Alternative signs in Mendel’s experiments. 3. Clean lines and their definition. 4. The essence of the hybridiological method. 5. Monohybrid crossing. 6. Dominant and recessive traits. 7. Allelic genes. 8. Mendel's first law. Law of Uniformity.
Topic 4.2.1 Incomplete dominance of genes
Terminology 1. Allelic genes– genes located in identical loci of homologous chromosomes. 2. Dominant trait– suppressing the development of another. 3. Recessive trait- suppressed. 4. Homozygote- a zygote that has the same genes. 5. Heterozygote- a zygote with different genes. 6. Split– divergence of traits in the offspring. 7. Crossing over- chromosome overlap. In the heterozygous state, the dominant gene does not always completely suppress the manifestation of the recessive gene. In some cases, the F 1 hybrid does not completely reproduce one of the parental characteristics and the expression of the trait is intermediate in nature with a greater or lesser bias towards a dominant or recessive state. But all individuals of this generation show uniformity in this trait. The intermediate nature of inheritance in the previous scheme does not contradict Mendel's first law, since all descendants of F 1 are uniform. Incomplete dominance- a widespread phenomenon. It was discovered when studying the inheritance of flower color in snapdragons, the structure of bird feathers, the color of the wool of cattle and sheep, biochemical characteristics in humans, etc. Multiple allelism. Until now, we have analyzed examples in which the same gene was represented by two alleles - dominant (A) and recessive (a). These two gene conditions arise due to mutation. A gene can mutate repeatedly. As a result, several variants of allelic genes arise. The set of these allelic genes, which determine the diversity of variants of the trait, is called a series of allelic genes. The occurrence of such a series due to repeated mutation of one gene is called multiple allelism or multiple allelomorphism. Gene A can mutate into the state a 1, a 2, a 3, and n. Gene B, located in another locus, is in the state b 1, b 2, b 3, b n. For example, in the Drosophila fly, a series of alleles for the eye color gene is known, consisting of 12 members: red, coral, cherry, apricot, etc. to white, determined by a recessive gene. Rabbits have a series of multiple alleles for coat color. This causes the development of solid color or lack of pigmentation (albinism). Members of the same series of alleles may be in different dominant-recessive relationships with each other. It should be remembered that the genotype of diploid organisms can contain only two genes from a series of alleles. The remaining alleles of this gene in different combinations are included in pairs in the genotypes of other individuals of this species. Thus, multiple allelism characterizes the diversity of the gene pool, i.e. the totality of all genes that make up the genotypes of a certain group of individuals or an entire species. In other words, multiple allelism is a species trait, not an individual trait. Mendel's Second Law - Law of Segregation If the descendants of the first generation, identical in the studied trait, are crossed with each other, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 of the individuals will have a dominant trait, 1/4 will have a recessive one. According to the genotype in F 2, there will be 25% of individuals homozygous for dominant alleles, 50% of organisms will be heterozygous, and 25% of the offspring will be organisms homozygous for recessive alleles. The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation. Thus, Mendel’s second law (see Fig. 2) can be formulated as follows: when two descendants of the first generation are crossed with each other (two heterozygotes), in the second generation a splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1: 2:1.
Figure 2. Mendel's second law
With incomplete dominance in the offspring of F 2 hybrids, the segregation of genotype and phenotype coincides (1: 2: 1). Law of gamete purity This law reflects the essence of the process of gamete formation in meiosis. Mendel suggested that hereditary factors (genes) do not mix during the formation of hybrids, but are preserved unchanged. In the body of the F hybrid, from crossing parents that differ in alternative characteristics, both factors are present - dominant and recessive. The dominant hereditary factor is manifested in the form of a trait, while the recessive one is suppressed. The connection between generations during sexual reproduction is carried out through germ cells - gametes. Therefore, it must be assumed that each gamete carries only one factor from a pair. Then, during fertilization, the fusion of two gametes, each of which carries a recessive hereditary factor, will lead to the formation of an organism with a recessive trait that manifests itself phenotypically. The fusion of gametes carrying a dominant factor, or two gametes, one of which contains a dominant and the other a recessive factor, will lead to the development of an organism with a dominant trait. Thus, the appearance in the second generation (F 2) of a recessive trait of one of the parents (P) can only occur if two conditions are met: 1. If in hybrids the hereditary factors remain unchanged. 2. If the germ cells contain only one hereditary factor from an allelic pair. Mendel explained the splitting of characteristics in the offspring when crossing heterozygous individuals by the fact that the gametes are genetically pure, i.e. carry only one gene from an allelic pair. The law of gamete purity can be formulated as follows: during the formation of germ cells, only one gene from an allelic pair (from each allelic pair) gets into each gamete. Cytological proof of the law of gamete purity is the behavior of the chromosome in meiosis: in the first meiotic division, homologous chromosomes end up in different cells, and in the anaphase of the second, daughter chromosomes, which, due to crossing over, may contain different alleles of the same gene. It is known that every cell of the body has exactly the same diploid set of chromosomes. Two homologous chromosomes contain two identical allelic genes. The formation of genetically “pure” gametes is shown in the diagram in Figure 3.
Figure 3. Formation of “pure” gametes
When male and female gametes merge, a hybrid is formed that has a diploid set of chromosomes (see Fig. 4).
Figure 4. Hybrid formation
As can be seen from the diagram, the zygote receives half of the chromosomes from the father’s body, and half from the mother’s. During the formation of gametes in a hybrid, homologous chromosomes also end up in different cells during the first meiotic division (see Fig. 5).
Figure 5. Formation of two types of gametes
Two varieties of gametes are formed according to a given allelic pair. Thus, the cytological basis of the law of gamete purity, as well as the splitting of characteristics in offspring during monohybrid crossing, is the divergence of homologous chromosomes and the formation of haploid cells in meiosis. Analysis cross The hybridological method of studying heredity developed by Mendel makes it possible to establish whether an organism that has a dominant phenotype for the gene being studied is homozygous or heterozygous. Is the breed pure? To do this, an individual with an unknown genotype is crossed with an organism that is homozygous for the recessive allele and has a recessive phenotype. If the dominant individual is homozygous, the offspring from such a cross will be uniform and splitting will not occur (see Fig. 6).
Figure 6. Crossing dominant individuals.
A different picture will turn out if the organism under study is heterozygous (see Fig. 7).
Figure 7. Crossing of heterozygous individuals.
Cleavage will occur in a 1:1 ratio according to phenotype. This result of crossing is proof of the formation of two varieties of gametes in one of the parents, i.e. his heterozygosity is not a pure breed (see Fig. 8).
Figure 8. Segregation will occur in a 1:1 ratio according to phenotype.
Control questions
1. Incomplete dominance and its manifestation in nature. 2. The essence of multiple allelism. 3. II-Mendel's law. The law of splitting. 4. The law of gamete purity. 5. Cytological evidence of the law of gamete purity. 6. Analyzing crossing, its essence and significance.
Topic 4.2.2 III Mendel's law - the law of independent
combining features
Terminology 1. Dihybrid crossing– crossing for two contrasting characteristics. 2. Diheterozygous organisms– organisms heterozygous for two pairs of allelic genes. 3. Pannet grille– graphical method for calculating the results of crossing. 4. Recombination– recombination of features. 5. Crossing over– the appearance of new characteristics when chromosomes overlap. 6. Morganida– distance between genes. Dihybrid and polyhybrid crossing Organisms differ from each other in many ways. The patterns of inheritance of two or more pairs of alternative traits can be established by dihybrid or polyhybrid crossing. For dihybrid crossing, Mendel used homozygous pea plants that differed in two pairs of characteristics - seed color (yellow and green) and seed shape (smooth and wrinkled). The dominant ones were yellow color (A) and smooth seed shape (B). Each plant produces one variety of gametes according to the alleles studied. When gametes merge, all offspring will be uniform (see Fig. 9).
Figure 9. Fusion of gametes
Organisms that are heterozygous for two pairs of allelic genes are called diheterozygous. When gametes are formed in a hybrid, from each pair of allelic genes, only one gets into the gamete, and due to the randomness of the divergence of the paternal and maternal chromosomes in the first division of meiosis, gene A can end up in the same gamete with gene B or gene b, just like gene a can combine in one gamete with gene B or with gene b (see Fig. 10).
Figure 10. Gamete formation in a hybrid
Table 1.
Processing the results of dihybrid crosses
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
G. Mendel's laws of inheritance of characteristics describe the primary principles of the transmission of hereditary characteristics from parent organisms to their children; these principles underlie classical genetics. These laws were discovered by Mendel as a result of crossing organisms (in this case, plants) with different genotypes. Usually one rule and two laws are described.
First generation hybrid uniformity rule
When crossing seed peas with stable traits - purple and white flowers, Mendel noticed that the hybrids that emerged were all with purple flowers, there was not a single white one among them. Mendel repeated his experiments more than once and used other signs. For example, if he crossed peas with yellow and green seeds, the offspring had yellow seeds; when he crossed peas with smooth and wrinkled seeds, the offspring had smooth seeds. The offspring from tall and short plants were tall.
So, first generation hybrids always acquire one of the parental characteristics. One trait (stronger, dominant) always suppresses another (weaker, recessive). This phenomenon is called complete dominance.
If we apply the above rule to a person, say, using the example of brown and blue eyes, then it is explained as follows. If in one homozygous parent both genes in the genome determine brown eye color (we denote such a genotype as AA), and in another, also homozygous, both genes determine blue eye color (we denote this genotype as ahh), then the haploid gametes produced by them will always carry either the gene A, or A(see diagram below).
Scheme of transmission of characteristics when crossing homozygous organisms
Then all children will have the genotype Ahh, but everyone will have brown eyes because the gene for brown eyes is dominant over the gene for blue eyes.
Now let's consider what happens if heterozygous organisms (or first-generation hybrids) are crossed. In this case it will happen character splitting in certain quantitative relationships.
The law of segregation of characters, or Mendel's First Law
If heterozygous descendants of the first generation, identical in the studied trait, are crossed with each other, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 of the individuals will have a dominant trait, 1/4 will have a recessive trait(see diagram below).
Pattern of inheritance of traits when crossing heterozygous organisms
The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called splitting. As we understand, the recessive trait did not disappear in the first generation hybrids, but was only suppressed and appeared in the second hybrid generation. Mendel was the first to understand that during the formation of hybrids, hereditary factors are not mixed or “eroded,” but are preserved unchanged. In a hybrid organism, both factors (genes) are present, but only the dominant hereditary factor manifests itself as a trait.
The connection between generations during sexual reproduction is carried out through germ cells, each gamete carries only one factor from the pair. The fusion of two gametes, each of which carries one recessive hereditary factor, will lead to the appearance of an organism with a recessive trait. The fusion of gametes, each of which carries a dominant factor, or two gametes, one of which contains a dominant and the other a recessive factor, leads to the development of an organism with a dominant trait.
Mendel explained splitting when crossing heterozygous individuals by the fact that gametes carry only one gene from an allelic pair ( law of gamete purity). Indeed, this is only possible if the genes remain unchanged and the gametes contain only one gene from a pair. It is convenient to study the relationships between features using the so-called Punnett lattice:
| A (0.5) | a (0.5) | |
| A (0.5) | AA (0.25) | Aa (0.25) |
| a (0.5) | Aa (0.25) | aa (0.25) |
Due to statistical probability at sufficiently large quantities of gametes in the offspring, 25% of the genotypes will be homozygous dominant, 50% - heterozygous, 25% - homozygous recessive, i.e. a mathematical ratio is established: 1 AA:2Ahh:1ahh. Accordingly, according to the phenotype, the offspring of the second generation during a monohybrid cross are distributed in a ratio of 3:1 - 3 parts of individuals with a dominant trait, 1 part of individuals with a recessive one.
We should not forget that the distribution of genes and their entry into gametes is probabilistic. Mendel's approach to the analysis of descendants was quantitative, statistical: all descendants with a given trait state (for example, smooth or wrinkled peas) were combined into one group, their number was calculated, which was compared with the number of descendants with a different trait state (wrinkled peas). This pairwise analysis ensured the success of his observations. In the case of a person, it can be very difficult to observe such a distribution - it is necessary that one pair of parents have at least a dozen children, which is quite a rare occurrence in modern society. So it may well happen that brown-eyed parents give birth to one and only child, and that child is blue-eyed, which, at first glance, violates all the laws of genetics. At the same time, if you experiment with Drosophila or laboratory mice, Mendelian laws are quite easy to observe.
It should be said that in a certain sense, Mendel was lucky - from the very beginning he chose a suitable plant as an object - colored peas. If he came across, for example, plants such as night beauty or snapdragon, the result would be unpredictable. The fact is that in snapdragons, heterozygous plants obtained by crossing homozygous plants with red and white flowers have pink flowers. Moreover, none of the alleles can be called either dominant or recessive. This phenomenon can be explained by the fact that complex biochemical processes caused by various jobs alleles do not necessarily lead to alternative, mutually exclusive outcomes. The result may be intermediate, depending on the characteristics of metabolism in a given organism, in which there are always many options, shunt mechanisms or parallel existing processes with different external manifestations.
This phenomenon is called incomplete dominance or codominance, it occurs quite often, including in humans. An example is the human blood group system MN (note in passing that this is only one of the systems; there are many classifications of blood groups). At one time, Landsteiner and Levin explained this phenomenon by the fact that red blood cells can carry on their surface either one antigen (M), or another (N), or both together (MN). If in the first two cases we are dealing with homozygotes (MM and NN), then in the heterozygous state (MN) both alleles manifest themselves, and both appear (dominate), hence the name - codominance.
The Law of Independent Inheritance of Characters, or Mendel's Second Law
This law describes the distribution of characteristics under the so-called dihybrid And polyhybrid crossing, i.e. when the individuals being crossed differ in two or more characteristics. In Mendel's experiments, plants were crossed that differed in several pairs of traits, such as: 1) white and purple flowers, and 2) yellow or green seeds. Moreover, the inheritance of each trait followed the first two laws, and the traits were combined independently of each other. As expected, the first generation after crossing had a dominant phenotype in all respects. The second generation followed the formula 9:3:3:1, that is, 9/16 copies were with purple flowers and yellow peas, 3/16 were with white flowers and yellow peas, another 3/16 were with purple flowers and green peas and, finally, 1/16 - with white flowers and green peas. This happened because Mendel successfully selected traits whose genes were located on different pea chromosomes. Mendel's second law is true only in cases where the analyzed gene pairs are located on different chromosomes. According to the rule of gamete frequency, characters are combined independently of each other, and if they are on different chromosomes, then the inheritance of characters occurs independently.
Mendel's 1st and 2nd laws are universal, but there are always exceptions to the 3rd law. The reason for this becomes clear if we remember that there are many genes on one chromosome (in humans, from several hundred to a thousand or more). If the genes are on the same chromosome, then there may be chained inheritance. In this case, the characteristics are transmitted in pairs or groups. Genes located on the same chromosome are called in genetics clutch groups. Most often, traits determined by genes located close to each other on the chromosome are transmitted together. Such genes are called closely interlocked. At the same time, sometimes genes located far from each other are inherited in a linked manner. The reason for this different behavior of genes is a special phenomenon exchange of material between chromosomes during gamete formation, in particular, at the prophase stage of the first meiotic division.
This phenomenon was studied in detail by Barbara McClintock (Nobel Prize in Physiology or Medicine in 1983) and was called crossing over. Crossing over- this is nothing more than the exchange of homologous regions between chromosomes. It turns out that each specific chromosome, when transmitted from generation to generation, does not remain unchanged; it can “take with it” a homologous section from its paired chromosome, giving it, in turn, a section of its DNA.
In the case of humans, it can be quite difficult to establish the linkage of genes, as well as to identify crossing over due to the impossibility of arbitrary crossings (you cannot force people to produce offspring in accordance with some scientific tasks!), therefore such data were obtained mainly on plants, insects and animals . However, thanks to the study of large families in which several generations are present, examples of autosomal linkage (i.e., joint transmission of genes located on autosomes) are known in humans. For example, there is close linkage between the genes that control the Rh factor (Rh) and the MNS blood group antigen system. In humans, cases of linkage of certain characteristics with sex are better known, that is, in connection with sex chromosomes.
Crossing over generally enhances combinative variability, i.e., it contributes to a greater diversity of human genotypes. Due to this, this process has great importance For. Using the fact that the further apart genes are located on the same chromosome, the more susceptible they are to crossing over, Alfred Sturtevant constructed the first maps of Drosophila chromosomes. Completed received today physical cards of all human chromosomes, i.e. it is known in what sequence and what genes are located on them.
Topic: Basic laws of inheritance of traits. Mendel's laws.
Lesson objectives:
Educational:
To form ideas about monohybrid crossing, the first and second laws of G. Mendel.
To consolidate knowledge of the terms and symbols used in genetics.
To contribute to the development of students’ skills in finding cause-and-effect relationships between genotype and phenotype, to continue the formation of a biological picture of the world.
Educational: To develop in students the ability to highlight the main thing, compare, and contrast.
Educational:
To promote the development of interest in genetics as a science.
Cultivate a tolerant attitude towards people of different races.
Methods: Explanatory and motivating, partially search, method of self-organization of cognitive work.
Lesson type: combined
Equipment:
Portrait of G. Mendel,
multimedia equipment,
Handout,
dynamic manual “Monohybrid crossing”.
During the classes
1. Organizational moment. Examination. D/z.
2. Updating knowledge.
3. Goal setting 1 min.
Teacher. Today we begin studying the science of genetics, we will get acquainted with new concepts, terms, symbols; Let's learn to solve genetic problems.(slide 1 - topic and goals)
Before we begin studying the topic, we will remember the definitions that we are already familiar with.
What is heredity?
Heredity is the property of all living organisms to transmit their characteristics and properties from generation to generation.
What is variability?
Variability is the property of all living organisms to acquire new characteristics and properties in the process of individual development.
What is a hybrid?
Organisms resulting from crossing.
Genotype is the totality of genes, the totality of all hereditary properties of an individual.
Genetics -
, studying the patterns of heredity and variability of living organisms.
The patterns by which traits are passed on from generation to generation were first discovered by the great Czech scientist Gregor Mendel (1822-1884) (slide 2) portrait
2. Preparation for learning new material: 1 min.
In the old film "Circus" the actress, a light-skinned woman, gave birth to a child - a dark-skinned baby. Why?
Let's turn to the teachings of the founder of genetics Gregor Mendel (portrait, slide 2)
The student will become familiar with the biography of G. Mendel.
Student:
Johann Mendel was born in 1822 into a poor peasant family in a small village in the Austrian Empire (today it is the territory of the Czech Republic. Having taken monastic orders, Johann Mendel received his middle name - Gregor. Gregor Mendel became a monk at the age of 25, after which he took a course mathematics and natural sciences at the University of Vienna.Later, from 1868, he was rector of the Augustinian monastery in the Czech city of Brno and at the same time taught at school natural history and physics. For many years Mendel -an amateur conducted experiments in the monastery garden, he begged for a small fenced area for the garden and in 1865 published the work “Experiments on Plant Hybrids,” in which he outlined the basic laws of heredity.
He devoted many years of his life to the study of genetics.
3. Studying new material.
Genetics has its own terminology and symbolism.
Let's turn to the reminders that are on your table. Lay them out carefully.
Now let's get acquainted with the symbolism with which the crossing of hybrids is depicted (icons on cards, on each desk):
Sly.3
P - parents (from the Latin "parenta" - parents)
♀ - “mirror of Venus” - female,
♂ - “Shield and Spear of Mars” - male
X - crossing.
F - from Lat hybrid offspring, if the index is signed 1,2, etc., the numbers correspond serial number generations (F1).
AA-dominant-homozygous
Aa-dominant-heterozygote
aa- recessive - homozygous
Mendel used peas for his research(slide 4)
4. In his work, Mendel used the so-called hybridological method. The essence of this method is the crossing (hybridization) of organisms that differ from each other in some characteristics, and the subsequent analysis of the nature of inheritance of these characteristics in the offspring. The hybridological method still underlies the research of all geneticists.
When conducting experiments, Mendel adhered to several rules.
Firstly, Working with garden peas, he used plants that belonged to different varieties for crossing. So, for example, one variety of peas was always yellow, while another was always green.(magnets on the board) Since peas are a self-pollinating plant, natural conditions these varieties do not mix. Such varieties are called pure lines.
Secondly, to get more material for analyzing the laws of heredity, Mendel not with one, but with several parent pairs of peas.
Third, Mendel deliberately simplified the problem by observing the inheritance of not all pea traits at once, but only one pair of them. For his experiments, he initially chose the color of pea seeds - peas. In cases where the parent organisms differ in only one characteristic (for example, only in the color of the seeds or only in the shape of the seeds), crossing is calledmonohybrid. Slide 5.
Fourthly, Having a mathematical education, Mendel used quantitative methods to process data: he not only noticed the color of pea seeds in the offspring, but also accurately counted how many such seeds appeared
It should be added that Mendel very successfully chose peas for his experiments.
Why do you think this particular plant? ? (Working with the textbook p. 101). Students' answer.Slide 4.
Peas are easy to grow, in the conditions of the Czech Republic they reproduce several times a year, pea varieties differ from each other in a number of clearly visible characteristics, and, finally, in nature peas are self-pollinating, but in an experiment this self-pollination can be easily prevented, and the experimenter can pollinate the plant with pollen from another plants, i.e. cross.
If we use terms that appeared many years after Mendel’s work, we can say that pea plants of one variety contain two genes only for yellow coloring, and genes of plants of another variety contain two genes only for green coloring.
Genes responsible for the development of one trait (for example, seed color) are called allelic genes. Slide 6.
If an organism contains two identical allelic genes (for example, both genes for green seeds or, conversely, both genes for yellow seeds), then such organismsare called homozygous. If the allelic genes are different (i.e., one of them determines the yellow and the other the green color of the seeds), then such organismsare called heterozygous. Slide 7.
Pure lines are formed by homozygous plants, so when self-pollinating they always reproduce one variant of the trait. In Mendel's experiments, it was one of two possible colors of pea seeds - either always yellow or always green.
(Let's not forget that in those years when Mendel carried out his experiments about genes, chromosomes, and nothing was known about meiosis!)
(Uniformity of first generation hybrids.) (Slide 8). By artificially crossing pea plants withyellow peas with plants havinggreen peas (i.e., by conducting a monohybrid cross), Mendel made sure that all the seeds of the hybrid descendants would beyellow colors. (I place magnets on the board).
Students work with cards on their desks.
He observed the same phenomenon in experiments when crossing plants with smooth and wrinkled seeds - all hybrid plants had smooth seeds.
Mendel called the characteristic that appears in hybrids (the yellowness of the seeds or the smoothness of the seeds)dominant , and the suppressed trait (i.e. green color of seeds or wrinkling of seeds) -recessive .
A dominant trait is usually denoted by a capital letter (A, B, C), and a recessive trait by a small letter (a, b, c).Slide 9.
Based on these datax Mendel formulated the rule of uniformity of first generation hybrids
: when crossing two homozygous organisms that differ from each other in one trait, all hybrids of the first generation will have the trait of one of the parents, and the generation for this trait will be uniform.
From the seeds obtained in the first generation, Mendel grew pea plants and crossed them again. In the second generation plants, most of the peas were yellow, but there were also green peas. From just a few crossed pairs of plants, Mendel received 6,022 yellow and 2,001 green peas. It is easy to calculate that 3/4 of the hybrid seeds were yellow and ¼ green. The phenomenon in which crossing leads to the formation of offspring partly with dominant and partly with recessive traits is called segregation.
Experiments with other traits confirmed these results, and Mendel formulatedsplitting rule slide 10, 11: when two descendants (hybrids) of the first generation are crossed with each other, in the second generation a split is observed and individuals with recessive characteristics appear again; these individuals make up one fourth of the total number of descendants of the second generation.
Law of purity of gametes. To explain the facts that formed the basis of the rule of uniformity of first-generation hybrids and the rule of segregation, G. Mendel suggested that there are two “elements of heredity” (genes) in each somatic cell. In the cells of the first generation hybrid, although they have only yellow peas, both “elements” (both yellow and green) must be present, otherwise the second generation hybrids cannot produce green peas. Communication between generations is ensured through sex cells - gametes. This means that each gamete receives only one “element of heredity” (gene) out of two possible ones - “yellow” or green.” Mendel's hypothesis that during the formation of gametes only one of two allelic genes enters each of them is calledlaw of gamete purity . Slide 12.
From G. Mendel’s experiments on monohybrid crossing, in addition to the law of gamete purity, it also follows that genes are passed on from generation to generation without changing. Otherwise it is impossible to explain the fact that in the first generation after crossing homozygotes with yellow and green peas, all the seeds were yellow, and in the second generation green peas appeared again. Consequently, the “pea green color” gene did not disappear and did not turn into the “pea yellow color” gene, but simply did not appear in the first generation, suppressed by the dominant yellow gene.
How to explain the laws of genetics from the standpoint modern science?
Cytological basis of inheritance patterns in monohybrid crossings.
Let's depict a monohybrid cross in the form of a diagram . The symbol ♀ - “mirror of Venus” - denotes a female individual, the symbol ♂ is male, x - crossing, P - parent generation, F1 - first generation of descendants, F2 - second generation of descendants, A - gene responsible for the dominant yellow color, a - gene , responsible for the recessive green color of pea seeds (Fig. 50).
The figure shows that each gamete of the parental individuals will contain one gene (remember meiosis): in one case A, in the other - a. Thus, in the first generation all somatic cells will be heterozygous - Aa. In turn, first-generation hybrids can produce gametes A or a with equal probability.
Random combinations of these gametes during the sexual process can give the following options: AA, Aa, aA, aa. The first three plants containing gene A, according to the dominance rule, will have yellow peas, and the fourth - recessive homozygote aa - will have green peas.Slide 13.
Solution to the problem: gray dominant rabbits were crossed with white recessive ones.(working with rabbit magnets).
- How did the rabbits turn out?
- Why?
Now let’s try to explain the birth of a dark-skinned baby to a light-skinned woman.
We studied crossing by one trait: color - yellow and green in peas, and the color of fur and rabbits, i.e., by one pair of traits; G. Mendel called such a crossing monohybrid.
4. Control of acquired knowledge. 4 min.
There is a crossword puzzle on the tables (4 min.) Remember the definitions. Write the correct answer directly into the crossword puzzle. I wish you success.
1. The totality of all signs of an organism.
2. Dominance, in which the dominant gene does not always completely suppress the manifestation of the recessive gene.
3. Crossing, in which one pair of alternative characteristics is traced.
4. Distribution of dominant and recessive traits among the offspring in the same numerical ratio.
5. Sex cells.
Peer review based on the answers on the slide.
Summary: Now share your answers with each other. We will conduct a mutual check, correct answers and evaluation criteria on the slide(slide 14)
They changed back.
Raise your hands who has 6 correct answers, who has 4 correct answers. Well done.
5. Consolidation of acquired knowledge. 4 min.
Frontal work. The solution of the problem:
Task 1.Slide 15.
The smooth shape of pea seeds dominates over the wrinkled one. Homozygous plants were crossed.
How many plants in the first generation will be heterozygous?
How many seeds in the second generation will be homozygous for the dominant trait?
How many seeds will be heterozygous in the second generation?
How many wrinkled seeds will there be in the second generation?
6. News of genetic science. (one of the students gives a message)
Project: “Human Genome”
The international project was started in 1988. Several thousand from more than 20 countries are working in the project. Since 1989, Russia has also participated in it. All chromosomes are divided between the participating countries, and Russia received chromosomes 3, 13, 19. The main goal of the project is to determine the localization of all genes in the DNA molecule. By 1998, approximately half of human genetic information had been deciphered.
Today it has been established that a predisposition to alcoholism and/or drug addiction can also have a genetic basis.
Today, based on genes, you can recognize a person by trace amounts of blood, skin flakes, and so on.
Currently, the problem of the dependence of a person’s abilities and talents on his genes is being intensively studied.
The main task of future research is to identify differences between people on genetic level. This will make it possible to create genetic portraits of people and more effectively treat diseases, assess the abilities and capabilities of each person, and assess the degree of adaptability of a particular person to a particular environmental situation.
Are there any of you who want to become a geneticist?
7. Reflection
So, have we achieved the goal of the lesson? Proveproblem solving.
Problem 4
The gene for brown eyes in humans is dominant over the gene for blue eyes. A blue-eyed homozygous man married a brown-eyed woman whose father had brown eyes and whose mother had blue eyes. Determine the genotypes of each of the individuals mentioned, write down how the trait is inherited. We write the problem condition:
Gene trait
And the brown ones
and blue
We record genotypes together. What kind of offspring will you get? Those. half of the children of these parents will have brown eyes, half will have blue eyes.
8. Summing up.
1. During the lesson I worked
2. Through my work in class I
3. The lesson seemed to me
4. For the lesson I
5.My mood
6.The lesson material was for me
9. D/Z
p.104 answer the questions in the workbook. Learn terms and concepts.
1
5
4
2
The laws of inheritance were formulated in 1865 by Gregory Mendel in his work “Experiments on Plant Hybrids.” In his experiments, he crossed different varieties of peas (Czech Republic / Austria-Hungary). In 1900, the patterns of inheritance were rediscovered by Correns, Chermak and Gogo de Vries.
Mendel's first and second laws are based on monohybrid crosses, and the third - on di and polyhybrid crosses. Monohybrid crossing is carried out according to one pair of alternative characteristics, dihybrid in two pairs, polyhybrid - more than two. Mendel's success is due to the peculiarities of the hybridological method used:
The analysis begins with crossing pure lines: homozygous individuals.
Separate alternative mutually exclusive features are analyzed.
Accurate quantitative accounting of descendants with different combinations of traits
The inheritance of the analyzed traits can be traced over a number of generations.
The rule for writing out gametes according to the formula 2n , where n is the number of heterozygotes: for monohybrids - 2 varieties of gametes, for dihybrids - 4, for trihybrids - 8.
Mendel's 1st law: "Law of uniformity of hybrids of the 1st generation"
When crossing homozygous individuals analyzed for one pair of alternative traits, the 1st generation hybrids exhibit only dominant traits and uniformity in phenotype and genotype is observed.
In his experiments, Mendel crossed pure lines of pea plants with yellow (AA) and green (aa) seeds. It turned out that all descendants in the first generation are identical in genotype (heterozygous) and phenotype (yellow).
Mendel's 2nd law: "Law of splitting"
When crossing heterozygous hybrids of the 1st generation, analyzed according to one pair of alternative characters, in the second generation hybrids a 3:1 cleavage is observed in the phenotype, and 1:2:1 in the genotype
In his experiments, Mendel crossed the hybrids (Aa) obtained in the first experiment with each other. It turned out that in the second generation the suppressed recessive trait reappeared. The data from this experiment indicate the elimination of the recessive trait: it is not lost, but appears again in the next generation.
Cytological basis of Mendel's 2nd law
The cytological basis of Mendel's 2nd law is revealed in the "gamete purity" hypothesis . From the crossing patterns it is clear that each trait is determined by a combination of two allelic genes. When heterozygous hybrids are formed, the allelic genes are not mixed, but remain unchanged. As a result meiosis In gametogenesis, only 1 of a pair of homologous chromosomes ends up in each gamete. Consequently, only one of a pair of allelic genes, i.e. the gamete is pure relative to another allelic gene.
Mendel's 3rd law: "The law of independent combination of characteristics"
When crossing homozygous organisms analyzed for two or more pairs of alternative traits, in hybrids of the 3rd generation (obtained by crossing hybrids of the 2nd generation) an independent combination of traits and the corresponding genes of different allelic pairs is observed.
To study patterns of inheritance plants , differing in one pair of alternative characters, Mendel used monohybrid cross . Next, he moved on to experiments on crossing plants that differed in two pairs of alternative traits: dihybrid cross , where he used homozygous pea plants that differed in color and seed shape. As a result of crossing smooth (B) and yellow (A) with wrinkled (c) and green (a), in the first generation all plants had yellow smooth seeds.
Thus, the law of uniformity of the first generation manifests itself not only in mono, but also in polyhybrid crossing, if the parent individuals are homozygous.
During fertilization, a diploid zygote is formed due to the fusion of different types of gametes. English geneticist Bennett to facilitate the calculation of options for their combination, he proposed an entry in the form gratings - tables with the number of rows and columns according to the number of types of gametes formed by crossing individuals.
Analysis cross
Since individuals with a dominant trait in the phenotype may have different genotypes (Aa and AA), Mendel proposed crossing this organism with recessive homozygote .
A homozygous individual will give uniform generation,
and heterozygous - split by phenotype and genotype 1:1.
Mogran's chromosome theory. Chained inheritance
Establishing patterns of inheritance, Mendel crossed pea plants. Thus, his experiments were carried out at the organismal level. The development of the microscope at the beginning of the 20th century made it possible to identify cells - the material carrier of hereditary information, transferring research to the cellular level. Based on the results of numerous experiments with fruit flies, in 1911 Thomas Morgan formulated the main provisions of the chromosomal theory of heredity .
Genes on a chromosome are arranged linearly in certain loci . Allelic genes occupy identical loci on homologous chromosomes.
Genes located on the same chromosome form clutch group and are inherited predominantly together. The number of linkage groups is equal to n set of chromosomes.
Between homologous chromosomes it is possible crossing over - exchange of sites that can disrupt the linkage of genes. The probability that genes will remain linked is directly proportional to the distance between them: the closer the genes are located on a chromosome, the higher the probability of their linkage. This distance is calculated in morganids: 1 morganid corresponds to 1% of the formation of crossover gametes.
For his experiments, Morgan used fruit flies that differed in 2 pairs of characteristics: color gray (B) and black (b); wing length is normal (V) and short (v).
1) Dihybrid crossing – first, homozygous individuals AABB and aabb were crossed. Thus, results similar to Mendel were obtained: all individuals with a gray body and normal wings.
2) Analyzing crossing
was carried out with the aim of breeding the genotype of 1st generation hybrids. A diheterozygous male was crossed with a recessive dihomozygous female. According to Mendel's 3rd law, one could expect the appearance of 4 phenotypes due to an independent combination of traits: sn (BbVv), chk (bbvv), ck (Bbvv), chn (bbVv) in a ratio of 1:1:1:1. However, only 2 combinations were obtained: sn (BbVv) chk (bbvv).
Thus, in the second generation we observed only original phenotypes in a 1:1 ratio.
This deviation from the free combination of characters is due to the fact that the genes that determine body color and wing length in Drosophila flies are located in one chromosome and are inherited linked . It turns out that a diheterozygous male produces only 2 varieties of non-crossover gametes, and not 4, as in the case of dihybrid crossing of organisms with unlinked traits.
3) Analyzing receptive crossing - a system of crosses in which genotypically different parents are used once as the maternal form, another time as the paternal form.
This time Morgan used a diheterozygous female and a homozygous recessive male. In this way, 4 phenotypes were obtained, but their ratio did not correspond to that observed in Mendel with an independent combination of traits. The number of sn and chk was 83% of all offspring, and the number of sc and chn was only 17%.
The linkage between genes localized on the same chromosome is disrupted as a result crossing over . If the chromosome breakpoint lies between linked genes, then the linkage is broken, and one of them passes into a homologous chromosome. So, in addition to two varieties non-crossover gametes , two more varieties are formed crossover gametes , in which chromosomes have exchanged homologous regions. From them, upon merging, crossover individuals develop. According to the chromosomal theory, the distance between the genes that determine body color and wing length in Drosophila is 17 morganids - 17% crossover gametes and 83% non-crossover ones.
Allelic interaction of genes
1) Incomplete dominance: When crossing homozygous sweet pea plants with red and white flowers, all offspring in the first generation have pink flowers - an intermediate form. In the second generation, splitting by phenotype corresponds to splitting by genotype in the ratio 1cr: 2rose: 1white.
2) Overdominance : at the dominant allele in heterozygote the trait is more pronounced than in the homozygote. At the same time, the heterozygous organism Aa has better fitness than both types of homozygotes.
Sickle cell anemia is caused by a mutant s allele. In areas where malaria is common, Ss heterozygotes are more resistant to it than SS homozygotes.
3) Codominance : in the heterozygote phenotype, both allelic genes are manifested, resulting in the formation of a new trait. But it is impossible to call one allele dominant and the other recessive, since they equally influence the phenotype.
Formation of the 4th blood group in humans. Allele Ia determines the presence of antigen a on erythrocytes, allele Ib determines the presence of antigen b. The presence of both alleles in the genotype causes the formation of both antigens on erythrocytes.
4) Multiple alleles: there are more than two allelic genes in a population. Such genes arise as a result of mutation of the same locus of the chromosome. In addition to dominant and recessive genes, there are intermediate alleles , which behave as recessive in relation to the dominant, and as dominant in relation to the recessive. Each diploid individual can have no more than two allelic genes, but in a population their number is not limited. The more allelic genes, the more options for their combinations. All alleles of one gene are designated by one letter with different indices: A1, A2, a3, etc.
In guinea pigs, coat color is determined by 5 alleys of one locus, which in various combinations give 11 color options. In humans, blood groups are inherited according to the ABO system due to multiple alleles. Three genes Io, Ia, Ib determine the inheritance of 4 human blood groups (genes Ia Ib are dominant in relation to Io).
Non-allelic gene interaction
1) Complementarity or complementary gene interaction is a phenomenon in which two non-allelic dominant or recessive genes give new sign . This interaction of genes is observed during the inheritance of comb shapes in chickens:
A pisiform (A-bb); B- rose-shaped (aaB-); AB nut-shaped; aavv leaf-shaped.
When crossing chickens with a pea-shaped and rose-shaped comb, all hybrids of the 1st generation will have a nut-shaped comb. When crossing dihybrids of the 1st generation with nut-shaped combs, in the 2nd generation individuals with all types of combs appear in the ratio 9or: 3rose: 3hor: 1leaf. However, unlike segregation under Mendel's 3rd law, there is no segregation of each allele in a 3:1 ratio. In other cases of complementarity, 9:7 and 9:6:1 are possible.
2) Epistasis or epistatic interaction of genes - suppression the actions of the genes of one allele by the genes of another. A suppressive gene is a suppressor or inhibitor.
Dominant epistasis - dominant suppressor gene: inheritance of feather color in chickens. C - pigment synthesis, I - suppressor gene. Chickens with genotype C-ii will be colored. The remaining individuals will be white, since in the presence of a dominant suppressor gene the suppressed color gene does not appear, or the gene responsible for pigment synthesis (ccii) is absent. In the case of crossing dihybrids, the splitting in the second generation will be 13:3 or 12:3:1.
Recessive epistasis - a suppressor gene is a recessive gene, for example, the inheritance of color in mice. B - synthesis of gray pigment, b - black; A promotes the manifestation of color, and suppresses it. Epistasis will manifest itself only in cases where the genotype contains two aa suppressor genes. When crossing dihybrid individuals with recessive epistasis, the splitting in the second generation is 9:3:4.
Bombay phenomenon manifests itself in the inheritance of blood groups according to the ABO system. A woman with blood group 1 (IoIo), who married a man with blood group 2 (IaIo), gave birth to two girls with blood groups 4 (IaIb) and 1 (IoIo). This is explained by the fact that their mother possessed the Ib allele, but its effect was suppressed by a rare recessive gene, which in the homozygous state exerted its epistatic effect. As a result, the woman phenotypically exhibited group 1.
3) Polymerism - the same sign is determined by several alleys. In this case, dominant genes from different allelic pairs influence the degree of manifestation of one trait. It depends on the number of dominant genes in the genotype (the more dominant genes, the more pronounced the trait) and on the influence of environmental conditions.
Polymer genes are usually denoted by one letter of the Latin alphabet with numerical indices A 1 A 2 a 3, etc. They determine polygenic traits . This is how many quantitative and some qualitative characteristics are inherited in animals and humans: height, weight, skin color. Inheritance of the color of wheat grains: each of the dominant genes determines the red color, recessive genes determine the white color. With an increase in the number of dominant genes, the color intensity increases. And only if the organism is homozygous for all pairs of recessive genes, the grains are not colored. So, when crossing dihybrids, the splitting is in the ratio 15 okr: 1 bel.
4) Pleiotropy- one gene affects several traits. The phenomenon was described by Mendel, who discovered that hereditary factors in pea plants can determine several characteristics: the red color of the flowers, the gray color of the seeds and the pink spot at the base of the leaves. Often extends to evolutionarily important traits: fertility, life expectancy, ability to survive in extreme environmental conditions.
In some cases, a pleietropic gene is dominant in relation to one trait, and recessive in relation to another. If a pleietropic gene is only dominant or only recessive in relation to all the characteristics it determines, then the nature of inheritance is similar to the patterns of Mendel’s laws.
A peculiar splitting is observed when one of the traits is recessive or lethal (homozygote leads to death). For example, the black wool of Karakul sheep and the development of the rumen are determined by one gene, while the gray wool and underdeveloped rumen are determined by the gene allelic to it. Gray dominates over black, normal over anomaly. Homozygous individuals for the gene for underdevelopment of the scar and gray color die, therefore, when crossing heterozygous individuals, a quarter of the offspring (gray homozygotes) are not viable. Splitting in a ratio of 2:1.
Penetrance and expressivity
The genotype of an individual determines only the potential development of a trait: the implementation of a gene into a trait depends on the influence of other genes and environmental conditions, therefore the same hereditary information manifests itself differently in different conditions. Consequently, it is not a ready-made trait that is inherited, but the type of reaction to the action of the environment.
Penetrance - penetration of a gene into a trait. Expressed as a percentage of the number of individuals carrying the trait, to total number carriers of a gene potentially capable of being translated into this trait. Full penetrance (100%) - all carriers of the gene have a phenotypic manifestation of the trait. Incomplete - the effect of the gene does not manifest itself in all carriers.
If a gene has become a trait, it is penetrant, but it can manifest itself in different ways. Expressiveness - degree of expression of the sign. The gene that causes a decrease in the number of eye facets in Drosophila has varying expressivity. Homozygotes have a different number of facets, up to their complete absence.
Penetrance and expressivity depend on the influence of other genes and the external environment.
Variability
Variability is the ability to acquire new characteristics under the influence of external and internal environmental factors (morphological, physiological, biochemical). Variability is associated with the diversity of individuals of the same species, which serves as material for evolutionary processes. The unity of heredity and variability is a condition for continuous biological evolution. There are several types:
1) Hereditary, genotypic, uncertain, individual
It is hereditary in nature, and is caused by the recombination of genes in the genotype and mutations, and is inherited. There are combinative and mutational
2) Non-hereditary, modification, phenotypic, group, specific
Modification variability is an evolutionarily fixed adaptive reaction of the body in response to changing environmental conditions, a consequence of the interaction of the environment and the genotype. It is not inherited, as it does not lead to a change in the genotype. Unlike mutations, many modifications are reversible: tanning, cow milk yield, etc.
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