Crossbreeding. Genotype as an integral system. Basic genetic concepts and symbols

Genetics, its tasks. Heredity and variability are the properties of organisms. Basic genetic concepts. Chromosomal theory of heredity. Genotype as an integral system. Development of knowledge about the genotype. The human genome.

Regularities of heredity, their cytological foundations. Mono- and dihybrid crossing. The laws of inheritance established by G. Mendel. Linked inheritance of traits, gene linkage disorder. T. Morgan's laws. Genetics of sex. Inheritance of sex-linked traits. Interaction of genes. Solving genetic problems. Drawing up crossing schemes.

Variability of traits in organisms: modification, mutational, combinative. Types of mutations and their causes. The significance of variability in the life of organisms and in evolution. Reaction rate. The harmful effect of mutagens, alcohol, drugs, nicotine on the genetic apparatus of the cell. Protection of the environment from contamination by mutagens. Identification of sources of mutagens in the environment (indirectly) and assessment of the possible consequences of their influence on one's own body. Hereditary human diseases, their causes, prevention.

Breeding, its tasks and practical significance. The teachings of N.I. Vavilov on the centers of diversity and origin of cultivated plants. The law of homologous series in hereditary variation. Methods for breeding new varieties of plants, animal breeds, strains of microorganisms. The value of genetics for breeding. Biological bases cultivation of cultivated plants and domestic animals.

Biotechnology, cell and genetic engineering, cloning. The role of cell theory in the formation and development of biotechnology. The importance of biotechnology for the development of breeding, Agriculture, microbiological industry, conservation of the planet's gene pool. Ethical aspects of the development of some research in biotechnology (human cloning, directed changes in the genome).

Among the tasks in genetics on the exam in biology, 6 main types can be distinguished. The first two - for determining the number of gamete types and monohybrid crossing - are found most often in part A of the exam (questions A7, A8 and A30).

Problems of types 3, 4 and 5 are devoted to dihybrid crossing, inheritance of blood groups and sex-linked traits. Such tasks make up most of the C6 questions in the exam.

The sixth type of problem is mixed. They consider the inheritance of two pairs of traits: one pair is linked to the X chromosome (or determines human blood groups), and the genes of the second pair of traits are located in autosomes. This class of problems is considered the most difficult for applicants.

This article sets out theoretical basis genetics necessary for successful preparation for task C6, as well as solutions of problems of all types are considered and examples for independent work are given.

Basic terms of genetics

Gene is a section of a DNA molecule that carries information about the primary structure of one protein. A gene is a structural and functional unit of inheritance.

Allelic genes (alleles)- different variants of the same gene, encoding an alternative manifestation of the same trait. Alternative signs are signs that cannot be in the body at the same time.

Homozygous organism- an organism that does not split in one way or another. Its allelic genes equally affect the development of this trait.

Heterozygous organism- an organism that splits according to one or another characteristic. Its allelic genes affect the development of this trait in different ways.

Dominant gene is responsible for the development of a trait that manifests itself in a heterozygous organism.

Recessive gene is responsible for the trait, the development of which is suppressed by the dominant gene. A recessive trait is manifested in a homozygous organism containing two recessive genes.

Genotype- a set of genes in a diploid set of an organism. The set of genes in a haploid set of chromosomes is called genome.

Phenotype- the totality of all the signs of the body.

G. Mendel's laws

Mendel's first law - the law of uniformity of hybrids

This law is derived from the results of monohybrid crossing. For the experiments, two varieties of peas were taken, differing from each other by one pair of signs - the color of the seeds: one variety had a yellow color, the second - green. The crossed plants were homozygous.

To record the results of crossing, Mendel proposed the following scheme:

Yellow seed color
- green color of seeds

(parents)
(gametes)
(first generation)	(all plants had yellow seeds)

Formulation of the law: when crossing organisms that differ in one pair of alternative traits, the first generation is uniform in phenotype and genotype.

Mendel's second law - the law of splitting

From seeds obtained by crossing a homozygous plant with a yellow seed color with a plant with a green seed color, plants were grown and obtained by self-pollination.

The wording of the law: in the offspring obtained from crossing the first generation hybrids, there is a splitting according to the phenotype in the ratio, and according to the genotype -.

Mendel's third law - the law of independent inheritance

This law was deduced from the data obtained from the dihybrid crossing. Mendel considered the inheritance of two pairs of traits in peas: color and shape of seeds.

Mendel used plants homozygous for both pairs of traits as parental forms: one variety had yellow seeds with smooth skin, the other had green and wrinkled seeds.

Yellow color of seeds, - green color of seeds,
- smooth shape, - wrinkled shape.

Then Mendel grew plants from seeds and, by self-pollination, obtained second-generation hybrids.

In there was a splitting into phenotypic class in the ratio. of all seeds had both dominant traits (yellow and smooth), - the first dominant and the second recessive (yellow and wrinkled), - the first recessive and the second dominant (green and smooth), - both recessive traits (green and wrinkled).

When analyzing the inheritance of each pair of traits, the following results are obtained. In parts of yellow seeds and parts of green seeds, i.e. ratio. Exactly the same ratio will be for the second pair of traits (seed shape).

Formulation of the law: when organisms are crossed that differ from each other by 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.

Mendel's third law is fulfilled only if the genes are in different couples ah homologous chromosomes.

The law (hypothesis) of "purity" of gametes

When analyzing the traits of hybrids of the first and second generations, Mendel found that the recessive gene does not disappear and does not mix with the dominant one. Both genes are manifested, which is possible only if the hybrids form two types of gametes: some carry a dominant gene, others a recessive one. This phenomenon is called the gamete purity hypothesis: each gamete carries only one gene from each allelic pair. The hypothesis of the purity of gametes was proved after studying the processes occurring in meiosis.

The hypothesis of "purity" of gametes is the cytological basis of Mendel's first and second laws. With its help, it is possible to explain the segregation by phenotype and genotype.

Analyzing cross

This method was proposed by Mendel to elucidate the genotypes of organisms with a dominant trait that have the same phenotype. For this, they were crossed with homozygous recessive forms.

If, as a result of crossing, the entire generation turned out to be the same and similar to the analyzed organism, then it could be concluded that the original organism is homozygous for the studied trait.

If, as a result of crossing in a generation, splitting in the ratio was observed, then the original organism contains genes in a heterozygous state.

Inheritance of blood groups (AB0 system)

The inheritance of blood groups in this system is an example of multiple allelism (this is the existence of more than two alleles of the same gene in a species). In the human population, there are three genes encoding erythrocyte antigen proteins that determine the blood groups of people. The genotype of each person contains only two genes that determine his blood group: the first group; second and; third and and fourth.

Inheritance of sex-linked traits

In most organisms, sex is determined during fertilization and depends on the set of chromosomes. This method is called chromosomal sex determination. Organisms with this type of sex determination have autosomes and sex chromosomes - and.

In mammals (including humans), the female sex has a set of sex chromosomes, the male sex. The female sex is called homogametic (forms one type of gametes); and the male is heterogametic (forms two types of gametes). In birds and butterflies, males are homogametic, and females are heterogametic.

The USE includes tasks only for characters linked to the -chromosome. Basically they relate to two signs of a person: blood clotting (- norm; - hemophilia), color vision (- norm, - color blindness). Much less common is the problem of inheriting sex-linked traits in birds.

In humans, the female sex can be homozygous or heterozygous for these genes. Let's consider the possible genetic sets in a woman using hemophilia as an example (a similar picture is observed with color blindness): - healthy; - is healthy, but is a carrier; - sick. The male sex for these genes is homozygous, because - the chromosome does not have alleles of these genes: - healthy; - is ill. Therefore, men most often suffer from these diseases, and women are their carriers.

Typical USE tasks in genetics

Determination of the number of gamete types

Determination of the number of gamete types is carried out according to the formula:, where is the number of pairs of genes in a heterozygous state. For example, an organism with a genotype has no genes in a heterozygous state, i.e. , therefore, and it forms one type of gametes. An organism with a genotype has one pair of genes in a heterozygous state, i.e. therefore, it also forms two types of gametes. An organism with a genotype has three pairs of genes in a heterozygous state, i.e. therefore, it also forms eight types of gametes.

Problems for mono- and dihybrid crossing

For monohybrid crossing

Task: White rabbits were crossed with black rabbits (black is dominant). In whites and blacks. Determine the genotypes of the parents and offspring.

Solution: Since in the offspring splitting according to the studied trait is observed, therefore, the parent with the dominant trait is heterozygous.

	(black)	(White)
	(black): (white)

For dihybrid crossing

Dominant genes are known

Task: Tomatoes of normal growth with red fruits were crossed with dwarf tomatoes with red fruits. All plants were of normal growth; - with red fruits and - with yellow ones. Determine the genotypes of parents and offspring if it is known that in tomatoes, red fruit color dominates over yellow, and normal growth over dwarfism.

Solution: Let's designate dominant and recessive genes: - normal growth, - dwarfism; - red fruits, - yellow fruits.

Let's analyze the inheritance of each trait separately. All descendants are of normal height, i.e. splitting for this trait is not observed, therefore the original forms are homozygous. By the color of the fruit, splitting is observed, therefore the original forms are heterozygous.

Dominant genes unknown

Task: Two varieties of phlox were crossed: one has red saucer-shaped flowers, the second has red funnel-shaped flowers. In the offspring, red saucer-shaped, red funnel-shaped, white saucer-shaped and white funnel-shaped were obtained. Identify the dominant genes and genotypes of the parental forms, as well as their offspring.

Solution: Let's analyze the splitting for each attribute separately. Among the descendants, plants with red flowers make up, with white flowers - i.e. ... Therefore - red, - white, and the parental forms are heterozygous for this trait (because there is a splitting in the offspring).

Splitting is also observed in the shape of the flower: half of the offspring have saucer-shaped flowers, half are funnel-shaped. Based on these data, it is not possible to unambiguously determine the dominant feature. Therefore, we will assume that - saucer-shaped flowers, - funnel-shaped flowers.

Red saucer flowers,
- red funnel-shaped flowers,
- white saucer flowers,
- white funnel-shaped flowers.

Solving problems for blood groups (AB0 system)

Task: the mother has the second blood group (she is heterozygous), the father has the fourth. What blood types are possible in children?

Solution:

Solving problems on the inheritance of sex-linked traits

Such tasks may well be encountered both in part A and in part C of the exam.

Task: carrier of hemophilia married a healthy man. What kind of children can be born?

Solution:

Mixed problem solving

Task: A man with brown eyes and blood type married a woman with brown eyes and blood type. They had a blue-eyed baby with a blood group. Determine the genotypes of all persons indicated in the task.

Solution: Brown eye color dominates over blue, therefore - Brown eyes, - blue eyes... The child has blue eyes, so his father and mother are heterozygous for this trait. The third blood group can have a genotype or, the first - only. Since the child has the first blood group, therefore, he received the gene from both his father and mother, therefore his father has a genotype.

Task: The man is color blind, right-handed (his mother was left-handed), married to a woman with normal vision (her father and mother were completely healthy), left-handed. What kind of children can this couple have?

Solution: The person has the best possession right hand dominates over left-handedness, therefore - right-handed, - left-handed. Male genotype (since he received the gene from a left-handed mother), and women -.

A color-blind man has a genotype, and his wife has a genotype. her parents were completely healthy.

Tasks for independent solution

1. Determine the number of gamete types in an organism with a genotype.
2. Determine the number of gamete types in an organism with a genotype.
3. They crossed tall plants with low plants. B - all medium-sized plants. What will happen?
4. We crossed a white rabbit with a black rabbit. All rabbits are black. What will happen?
5. We crossed two rabbits with gray hair. B with black wool, - with gray and white. Identify genotypes and explain this splitting.
6. They crossed a black hornless bull with a white horned cow. We got black hornless, black horned, white horned and white hornless. Explain this cleavage if black color and absence of horns are dominant.
7. Drosophila with red eyes and normal wings were crossed with fruit flies with white eyes and defective wings. In the offspring, all flies with red eyes and defective wings. What will be the offspring from crossing these flies with both parents?
8. A blue-eyed brunette married a brown-eyed blonde. What kind of children can be born if both parents are heterozygous?
9. A right-handed man with a positive Rh factor married a left-handed woman with a negative rhesus factor. What children can be born if a man is heterozygous only for the second trait?
10. The mother and father have a blood type (both parents are heterozygous). What blood group is possible in children?
11. The mother has a blood group, the child has a blood group. What blood type is impossible for a father?
12. The father has the first blood group, the mother has the second. What is the probability of having a baby with the first blood group?
13. A blue-eyed woman with a blood group (her parents had a third blood group) married a brown-eyed man with a blood group (his father had blue eyes and a first blood group). What kind of children can be born?
14. A hemophilic man, right-handed (his mother was left-handed) married a left-handed woman with normal blood (her father and mother were healthy). What kind of children can be born from this marriage?
15. We crossed strawberry plants with red fruits and long-petiolate leaves with strawberry plants with white fruits and short-petiolized leaves. What offspring can there be if the red color and short-petiolate leaves are dominant, while both parent plants are heterozygous?
16. A man with brown eyes and a blood type married a woman with brown eyes and a blood type. They had a blue-eyed baby with a blood group. Determine the genotypes of all persons indicated in the task.
17. Melons with white oval fruits were crossed with plants that had white globular fruits. The offspring produced the following plants: with white oval, white globular, yellow oval and yellow globular fruits. Determine the genotypes of the original plants and offspring, if the white color of the melon dominates over the yellow, the oval shape of the fruit - over the spherical.

Answers

1. type of gametes.
2. types of gametes.
3. type of gametes.
4. high, medium and low (incomplete dominance).
5. black and white.
6. - black, - white, - gray. Incomplete dominance.
7. Bull:, cow -. Offspring: (black hornless), (black horned), (white horned), (white hornless).
8. - Red eyes, - white eyes; - defective wings, - normal. Initial forms - and, offspring.
   Crossing results:
   a)
9. - Brown eyes, - blue; - dark hair, - light. Father mother - .
   

   	- brown eyes, dark hair - brown eyes, blonde hair - blue eyes, dark hair - blue eyes, blonde hair

10. - right-handed, - left-handed; - Rh positive, - negative. Father mother - . Children: (right-handed, Rh positive) and (right-handed, Rh negative).
11. Father and mother - . Children may have a third blood group (probability of birth -) or first blood group (probability of birth -).
12. Mother, child; from his mother he received the gene, and from his father -. The following blood groups are impossible for the father: second, third, first, fourth.
13. A child with the first blood group can be born only if his mother is heterozygous. In this case, the probability of birth is.
14. - Brown eyes, - blue. Female Male . Children: (brown eyes, fourth group), (brown eyes, third group), (blue eyes, fourth group), (blue eyes, third group).
15. - right-handed, - left-handed. Man Woman . Children (healthy boy, right-handed), (healthy girl, carrier, right-handed), (healthy boy, left-handed), (healthy girl, carrier, left-handed).
16. - red fruits, - white; - short petiolate, - long petiolate.
    Parents: and. Offspring: (red fruits, short petiolate), (red fruits, long petiolate), (white fruits, short petiolate), (white fruits, long petiolate).
    We crossed strawberry plants with red fruits and long-petiolate leaves with strawberry plants with white fruits and short-petiolized leaves. What kind of offspring can there be if the red color and short-petiolate leaves are dominant, while both parent plants are heterozygous?
17. - Brown eyes, - blue. Female Male . Child:
18. - white color, - yellow; - oval fruits, - round. Source plants: and. Offspring:
    with white oval fruits,
    with white globular fruits,
    with yellow oval fruits,
    with yellow spherical fruits.

Regularities of heredity, their cytological foundations. Regularities of inheritance, established by G. Mendel, their cytological basis (mono- and dihybrid crossing). T. Morgan's laws: linked inheritance of traits, gene linkage disorder. Genetics of sex. Inheritance of sex-linked traits. Interaction of genes. Genotype as an integral system. Human genetics. Methods for studying human genetics. Solving genetic problems. Drawing up crossing schemes

Regularities of heredity, their cytological foundations

According to the chromosomal theory of heredity, each pair of genes is localized in a pair of homologous chromosomes, and each of the chromosomes carries only one of these factors. If we imagine that genes are point objects on straight lines - chromosomes, then schematically homozygous individuals can be written as A || A or a || a, while heterozygous - A || a. When gametes are formed during meiosis, each of the genes of a heterozygote pair will be in one of the germ cells.

For example, if you cross two heterozygous individuals, then provided that only a pair of gametes is formed in each of them, it is possible to obtain only four daughter organisms, three of which will carry at least one dominant gene A, and only one will be homozygous for the recessive gene a, that is, the patterns of heredity are statistical in nature.

In cases where genes are located in different chromosomes, then during the formation of gametes, the distribution of alleles from a given pair of homologous chromosomes between them occurs completely independently of the distribution of alleles from other pairs. It is the random arrangement of homologous chromosomes at the spindle equator in metaphase I of meiosis and their subsequent divergence in anaphase I that leads to a variety of allele recombinations in gametes.

The number of possible combinations of alleles in male or female gametes can be determined by the general formula 2 n, where n is the number of chromosomes characteristic of the haploid set. In humans, n = 23, and the possible number of combinations is 2 23 = 8388608. The subsequent union of gametes during fertilization is also random, and therefore in the offspring it is possible to record an independent splitting for each pair of characters.

However, the number of signs in each organism is many times greater than the number of its chromosomes, which can be distinguished under a microscope, therefore, each chromosome must contain many factors. If we imagine that gametes are formed in some individual heterozygous for two pairs of genes located in homologous chromosomes, then one should take into account not only the probability of the formation of gametes with the original chromosomes, but also gametes that have received chromosomes changed as a result of crossing over in prophase I of meiosis. Consequently, new combinations of traits will appear in the offspring. The data obtained in experiments on Drosophila formed the basis chromosomal theory of heredity.

Another fundamental confirmation of the cytological basis of heredity was obtained in the study various diseases... So, in humans, one of the forms of cancer is caused by the loss of a small section of one of the chromosomes.

Regularities of inheritance established by G. Mendel, their cytological basis (mono- and dihybrid crossing)

The main regularities of independent inheritance of traits were discovered by G. Mendel, who achieved success by applying in his studies a new hybridological method at that time.

G. Mendel's success was due to the following factors:

• a good choice of the object of study (sowing pea), which has a short growing season, is a self-pollinated plant, gives a significant amount of seeds and is represented by a large number of varieties with well-distinguishable characteristics;
• using only pure lines of peas, which for several generations did not give splitting of traits in the offspring;
• concentration on only one or two signs;
• planning the experiment and drawing up clear crossing schemes;
• accurate quantitative calculation of the resulting offspring.

For the study, G. Mendel selected only seven signs with alternative (contrasting) manifestations. Already in the first crosses, he noticed that in the offspring of the first generation, when crossing plants with yellow and green seeds, all the offspring had yellow seeds. Similar results were obtained in the study of other signs. The signs that prevailed in the first generation were named by G. Mendel dominant... The same ones that did not appear in the first generation were named recessive.

Individuals that gave splitting in the offspring were named heterozygous, and individuals that did not split, - homozygous.

Pea traits, the inheritance of which was studied by G. Mendel

Crossing, in which the manifestation of only one trait is investigated, is called monohybrid... In this case, the patterns of inheritance of only two variants of one trait are traced, the development of which is due to a pair of allelic genes. For example, the trait “color of the flower corolla” in peas has only two manifestations - red and white. All other signs characteristic of these organisms are not taken into account and are not taken into account in the calculations.

The monohybrid crossing scheme is as follows:

Having crossed two pea plants, one of which had yellow seeds, and the other - green, in the first generation G. Mendel obtained plants exclusively with yellow seeds, regardless of which plant was chosen as the mother and which was the father. The same results were obtained in crosses on other grounds, which gave G. Mendel reason to formulate the law of uniformity of the first generation hybrids also called Mendel's first law and the law of domination.

Mendel's first law:

When crossing homozygous parental forms that differ in one pair of alternative traits, all hybrids of the first generation will be uniform both in genotype and phenotype.

A - yellow seeds; a- green seeds.

When self-pollinating (crossing) hybrids of the first generation, it turned out that 6022 seeds are yellow, and 2001 - green, which approximately corresponds to a ratio of 3: 1. The discovered pattern was named splitting law, or Mendel's second law.

Mendel's second law:

When crossing heterozygous hybrids of the first generation in the offspring, there will be a predominance of one of the traits in a ratio of 3: 1 by phenotype (1: 2: 1 by genotype).

However, by the phenotype of an individual, it is far from always possible to establish its genotype, since as homozygotes for the dominant gene ( AA) and heterozygotes ( Aa) will have the manifestation of a dominant gene in the phenotype. Therefore, for organisms with cross fertilization, they use analyzing cross- crossing, in which an organism with an unknown genotype is crossed with a homozygote for a recessive gene to check the genotype. At the same time, homozygous individuals for the dominant gene do not split in the offspring, while in the offspring of heterozygous individuals there is an equal number of individuals with both dominant and recessive traits:

Based on the results of his own experiments, G. Mendel suggested that hereditary factors during the formation of hybrids do not mix, but remain unchanged. Since the connection between generations is carried out through gametes, he admitted that in the process of their formation, only one factor from a pair gets into each of the gametes (that is, the gametes are genetically pure), and during fertilization the pair is restored. These assumptions are called rules of gamete purity.

Gamete purity rule:

During gametogenesis, the genes of one pair are separated, that is, each gamete carries only one variant of the gene.

However, organisms differ from each other in many ways, therefore, it is possible to establish the patterns of their inheritance only by analyzing two or more traits in the offspring.

Crossbreeding, in which inheritance is considered and an accurate quantitative account of the offspring is made according to two pairs of traits, is called dihybrid... If the manifestation of a larger number of hereditary traits is analyzed, then this is already polyhybrid crossing.

Dihybrid crossing scheme:

With a greater variety of gametes, the determination of the genotypes of offspring becomes difficult, therefore, for analysis, the Pennett lattice is widely used, into which male gametes are entered horizontally, and female gametes are entered vertically. The genotypes of the offspring are determined by the combination of genes in columns and rows.

For the dihybrid crossing, G. Mendel chose two traits: the color of the seeds (yellow and green) and their shape (smooth and wrinkled). In the first generation, the law of uniformity of hybrids of the first generation was observed, and in the second generation there were 315 yellow smooth seeds, 108 green smooth seeds, 101 yellow wrinkled and 32 green wrinkled seeds. The calculation showed that the cleavage was close to 9: 3: 3: 1, but for each of the signs the ratio of 3: 1 was maintained (yellow - green, smooth - wrinkled). This pattern was named the law of independent splitting of features, or Mendel's third law.

Mendel's third law:

When crossing homozygous parental forms that differ in two or more pairs of traits, in the second generation, an independent splitting of these traits will occur in a ratio of 3: 1 (9: 3: 3: 1 with a dihybrid crossing).

$ F_2 (9A_B_) ↙ (\ text "smooth yellow"): (3A_bb) ↙ (\ text "wrinkled yellow"): (3aaB_) ↙ (\ text "smooth green"): (1aabb) ↙ (\ text "green wrinkled ") $

Mendel's third law applies only to cases of independent inheritance, when genes are located in different pairs of homologous chromosomes. In cases where genes are located in one pair of homologous chromosomes, the laws of linked inheritance are valid. The patterns of independent inheritance of traits, established by G. Mendel, are also often violated in the interaction of genes.

T. Morgan's laws: linked inheritance of traits, disruption of gene linkage

The new organism receives from the parents not a scattering of genes, but whole chromosomes, while the number of traits and, accordingly, the genes that determine them are much larger than the number of chromosomes. According to the chromosomal theory of heredity, genes located on the same chromosome are inherited linked. As a consequence, in dihybrid crossing, they do not give the expected splitting of 9: 3: 3: 1 and do not obey Mendel's third law. One would expect that the linkage of genes is complete, and when crossing individuals homozygous for these genes in the second generation, it gives the initial phenotypes in a ratio of 3: 1, and when analyzing crossing of hybrids of the first generation, the cleavage should be 1: 1.

To test this assumption, the American geneticist T. Morgan chose a pair of genes in Drosophila that control body color (gray - black) and wing shape (long - rudimentary), which are located in one pair of homologous chromosomes. A gray body and long wings are dominant features. When crossing a homozygous fly with a gray body and long wings and a homozygous fly with a black body and rudimentary wings in the second generation, mainly parental phenotypes were actually obtained in a ratio close to 3: 1, but there were also a small number of individuals with new combinations of these traits. ... These individuals are called recombinant.

However, after analyzing the crossing of first-generation hybrids with homozygotes for recessive genes, T. Morgan found that 41.5% of individuals had a gray body and long wings, 41.5% had a black body and rudimentary wings, 8.5% had a gray body. and rudimentary wings, and 8.5% - black body and rudimentary wings. He associated the resulting cleavage with crossing over, which occurs in prophase I of meiosis, and suggested that the unit of distance between genes in the chromosome is 1% crossing over, which was later named after him. morganida.

The patterns of linked inheritance, established during experiments on Drosophila, were called T. Morgan's law.

Morgan's Law:

Genes localized on one chromosome occupy a certain place, called a locus, and are inherited linked, and the strength of the link is inversely proportional to the distance between the genes.

Genes located in the chromosome directly one after another (the probability of crossing over is extremely small) are called fully linked, and if there is at least one more gene between them, then they are not fully linked and their linking is disrupted during crossing over as a result of the exchange of regions of homologous chromosomes.

The phenomena of gene linkage and crossing over make it possible to construct maps of chromosomes with the order of genes placed on them. Genetic maps of chromosomes have been created for many genetically well-studied objects: Drosophila, mice, humans, corn, wheat, peas, etc. The study of genetic maps allows you to compare the structure of the genome in different types organisms that have essential for genetics and breeding, and evolutionary research.

Genetics of gender

Floor- This is a set of morphological and physiological characteristics of an organism that provide sexual reproduction, the essence of which is reduced to fertilization, that is, the fusion of male and female germ cells into a zygote, from which a new organism develops.

The signs by which one sex differs from another are divided into primary and secondary. The primary sexual characteristics include the genitals, and all the rest are secondary.

In humans, secondary sexual characteristics are body type, tone of voice, predominance of muscle or adipose tissue, the presence of hair on the face, Adam's apple, and mammary glands. So, in women, the pelvis is usually wider than the shoulders, adipose tissue predominates, the mammary glands are expressed, the voice is high. Men differ from them in wider shoulders, a predominance of muscle tissue, the presence of hair on the face and Adam's apple, as well as a low voice. Humanity has long been interested in the question of why males and females are born in a ratio of approximately 1: 1. The explanation for this was obtained by studying the karyotypes of insects. It turned out that females of some bugs, grasshoppers and butterflies have one more chromosome than males. In turn, males produce gametes that differ in the number of chromosomes, thereby determining in advance the sex of the offspring. However, later it was found that in most organisms the number of chromosomes in males and females still does not differ, but one of the sexes has a pair of chromosomes that do not fit each other in size, while the other has all the chromosomes in pairs.

A similar difference was also found in the human karyotype: males have two unpaired chromosomes. In shape, these chromosomes at the beginning of division resemble the Latin letters X and Y, and therefore were named X and Y chromosomes. A man's sperm can carry one of these chromosomes and determine the sex of the unborn child. In this regard, the chromosomes of humans and many other organisms are divided into two groups: autosomes and heterochromosomes, or sex chromosomes.

TO autosomes include chromosomes that are the same for both sexes, while sex chromosomes- These are chromosomes that differ in different sexes and carry information about sex characteristics. In cases where the sex carries the same sex chromosomes, for example XX, it is said that he homozygous, or homogametic(forms the same gametes). The other sex, which has different sex chromosomes (XY), is called hemizygous(not having a full allelic gene equivalent), or heterogametic... In humans, most mammals, the Drosophila fly and other organisms, the female sex is homogametic (XX), and the male is heterogametic (XY), while in birds the male sex is homogametic (ZZ, or XX), and the female is heterogametic (ZW, or XY) ...

The X chromosome is a large unequal chromosome that carries over 1,500 genes, and many of their mutant alleles cause severe hereditary diseases such as hemophilia and color blindness in humans. The Y chromosome, on the other hand, is very small, it contains only about a dozen genes, including specific genes responsible for male development.

The male karyotype is written as $ ♂ $ 46, XY, and the female karyotype is $ ♀ $ 46, XX.

Since gametes with sex chromosomes are produced in males with equal probability, the expected sex ratio in the offspring is 1: 1, which coincides with the actually observed one.

Bees differ from other organisms in that females develop from fertilized eggs, and males from unfertilized ones. The sex ratio in them differs from that indicated above, since the fertilization process is regulated by the uterus, in the genital tract of which spermatozoa are stored from spring for the whole year.

In a number of organisms, sex can be determined in a different way: before fertilization or after it, depending on the environmental conditions.

Inheritance of sex-linked traits

Since some genes are found on sex chromosomes that are not the same in representatives of opposite sexes, the nature of inheritance of the traits encoded by these genes differs from the general one. This type of inheritance is called kris-cross inheritance, since men inherit the characteristics of the mother and women inherit the characteristics of the father. Traits determined by genes that are located on the sex chromosomes are called sex-linked. Examples of signs sex-linked, are recessive signs of hemophilia and color blindness, which are mainly manifested in men, since there are no allelic genes on the Y chromosome. Women suffer from such diseases only if they received such symptoms from both their father and mother.

For example, if a mother was a heterozygous carrier of hemophilia, then half of her sons will have impaired blood clotting:

X H - normal blood clotting

X h - incoagulability of blood (hemophilia)

Traits encoded in the genes of the Y chromosome are transmitted exclusively through the male line and are called Dutch(the presence of a membrane between the toes, increased hairiness of the edge of the auricle).

Interaction of genes

Checking the patterns of independent inheritance on various objects already at the beginning of the 20th century showed that, for example, in a night beauty when crossing plants with a red and white corolla in hybrids of the first generation, corollas are colored in pink color, while in the second generation there are individuals with red, pink and white flowers in a ratio of 1: 2: 1. This led researchers to the idea that allelic genes can have a certain effect on each other. Subsequently, it was also found that non-allelic genes contribute to the manifestation of traits of other genes or suppress them. These observations became the basis for the concept of the genotype as a system of interacting genes. Currently, the interaction of allelic and non-allelic genes is distinguished.

The interaction of allelic genes includes complete and incomplete dominance, codominance and overdominance. Complete dominance consider all cases of interaction of allelic genes, in which the manifestation of an exclusively dominant trait is observed in the heterozygote, such as, for example, the color and shape of the seed in peas.

Incomplete dominance- this is a type of interaction of allelic genes in which the manifestation of a recessive allele to a greater or lesser extent weakens the manifestation of the dominant one, as in the case of the color of the corolla of a night beauty (white + red = pink) and wool in cattle.

Codominating call this type of interaction of allelic genes in which both alleles are manifested without weakening the effects of each other. A typical example of codominance is the inheritance of blood groups according to the AB0 system.

As can be seen from the table, blood groups I, II and III are inherited according to the type of complete dominance, while group IV (AB) (genotype - I A I B) is a case of codominance.

Overdominance- this is a phenomenon in which in a heterozygous state the dominant trait manifests itself much stronger than in a homozygous one; overdominance is often used in breeding and is considered to be the cause heterosis- phenomena of hybrid power.

A special case of the interaction of allelic genes can be considered the so-called lethal genes, which in a homozygous state lead to the death of the organism most often in the embryonic period. The reason for the death of offspring is the pleiotropic effect of genes for gray coat color in astrakhan sheep, platinum color in foxes, and the absence of scales in mirror carps. When two individuals heterozygous for these genes are crossed, the splitting according to the studied trait in the offspring will be equal to 2: 1 due to the death of 1/4 of the offspring.

The main types of interaction between non-allelic genes are complementarity, epistasis, and polymeria. Complementarity- This is a type of interaction of non-allelic genes, in which the presence of at least two dominant alleles of different pairs is necessary for the manifestation of a certain state of a trait. For example, when a pumpkin is crossed with spherical (AAbb) and long (aaBB) fruits, plants with disc-shaped fruits (AaBb) appear in the first generation.

TO epistasis include such phenomena of interaction of non-allelic genes, in which one non-allelic gene suppresses the development of a trait of another. For example, in chickens, plumage color is determined by one dominant gene, while another dominant gene suppresses color development, as a result of which most chickens have white plumage.

Polymer is called a phenomenon in which non-allelic genes have the same effect on the development of a trait. In this way, quantitative characteristics are most often coded. For example, a person's skin color is determined by at least four pairs of non-allelic genes - the more dominant alleles in a genotype, the darker the skin.

Genotype as an integral system

The genotype is not a mechanical sum of genes, since the possibility of a gene manifestation and the form of its manifestation depend on environmental conditions. V this case by the environment is meant not only the environment, but also the genotypic environment - other genes.

The manifestation of qualitative signs rarely depends on the conditions. the environment, although if an ermine rabbit shaves a part of the body with white hair and applies an ice bubble to it, then over time, black wool will grow in this place.

The development of quantitative traits is much more dependent on environmental conditions. For example, if modern wheat varieties are cultivated without the use of mineral fertilizers, then its yield will differ significantly from the genetically programmed 100 or more centners per hectare.

Thus, only the "abilities" of the organism are recorded in the genotype, but they are manifested only in interaction with environmental conditions.

In addition, genes interact with each other and, being in the same genotype, can strongly influence the manifestation of the action of neighboring genes. Thus, for each individual gene, there is a genotypic environment. It is possible that the development of any trait is associated with the action of many genes. In addition, the dependence of several traits on one gene was revealed. For example, in oats, the color of the flower scales and the length of their awn are determined by one gene. In Drosophila, the gene for white eye coloration simultaneously affects body color and internal organs, wing length, decreased fertility and decreased life span. It is possible that each gene is simultaneously a gene of the main action for "its" trait and a modifier for other traits. Thus, the phenotype is the result of the interaction of genes of the entire genotype with the environment in the ontogeny of an individual.

In this regard, the famous Russian geneticist M.E. Lobashev defined the genotype as system of interacting genes... This integral system has developed in the process of evolution. organic world, while only those organisms survived in which the interaction of genes gave the most favorable reaction in ontogenesis.

Human genetics

For humans as a biological species, the genetic laws of heredity and variability established for plants and animals are fully valid. At the same time, human genetics, which studies the patterns of heredity and variability in humans at all levels of its organization and existence, occupies a special place among other branches of genetics.

Human genetics is both a fundamental and applied science, since it is engaged in the study of hereditary human diseases, of which more than 4 thousand have already been described. It stimulates the development of modern trends in general and molecular genetics, molecular biology and clinical medicine. Depending on the subject matter, human genetics is divided into several areas that have developed into independent sciences: genetics normal signs human, medical genetics, genetics of behavior and intelligence, human population genetics. In this regard, in our time, a person as a genetic object has been studied almost better than the main model objects of genetics: Drosophila, Arabidopsis, etc.

The biosocial nature of man leaves a significant imprint on research in the field of his genetics due to late puberty and large time gaps between generations, the small number of offspring, the impossibility of directed crosses for genetic analysis, the lack of pure lines, insufficient accuracy of registration of hereditary traits and small pedigrees, the impossibility of creating identical and strictly controlled conditions for the development of descendants from different marriages, comparatively a large number poorly distinguished chromosomes and the impossibility of experimental obtaining mutations.

Methods for studying human genetics

The methods used in human genetics do not fundamentally differ from those generally accepted for other objects - these are genealogical, twin, cytogenetic, dermatoglyphic, molecular biological and population statistical methods, somatic cell hybridization method and modeling method... Their use in human genetics takes into account the specifics of a person as a genetic object.

Twin method helps to determine the contribution of heredity and the influence of environmental conditions on the manifestation of a trait based on the analysis of the coincidence of these traits in identical and fraternal twins. So, most identical twins have the same blood types, eye and hair color, as well as a number of other signs, while both types of twins get sick at the same time.

Dermatoglyphic method based on the study of the individual characteristics of the skin patterns of the fingers (fingerprinting), palms and feet. Based on these features, it often makes it possible to timely identify hereditary diseases, in particular chromosomal abnormalities, such as Down syndrome, Shereshevsky-Turner syndrome, etc.

Genealogical method- This is a method of compiling genealogies, with the help of which the character of inheritance of the studied traits, including hereditary diseases, is determined, and the birth of offspring with the corresponding traits is predicted. It made it possible to reveal the hereditary nature of such diseases as hemophilia, color blindness, Huntington's chorea, etc. even before the discovery of the basic laws of heredity. When drawing up pedigrees, they keep records of each of the family members and take into account the degree of kinship between them. Further, based on the data obtained, a family tree is built with the help of special symbols.

The genealogical method can be used on one family if there is information about a sufficient number of direct relatives of a person whose pedigree is compiled - proband, - on the paternal and maternal lines, otherwise they collect information about several families in which this sign is manifested. The genealogical method makes it possible to establish not only the heritability of the trait, but also the nature of inheritance: dominant or recessive, autosomal or sex-linked, etc. Thus, according to the portraits of the Austrian monarchs of the Habsburgs, the inheritance of prognathia (strongly protruding lower lip) and "royal hemophilia" was established from the descendants of the British Queen Victoria.

Solving genetic problems. Drawing up crossing schemes

The whole variety of genetic problems can be reduced to three types:

1. Computational tasks.
2. Genotype determination tasks.
3. Tasks to establish the type of inheritance of a trait.

Feature calculation tasks is the availability of information about the inheritance of the trait and the phenotypes of the parents, by which it is easy to establish the genotypes of the parents. They need to establish the genotypes and phenotypes of the offspring.

Objective 1. What color will the seeds of sorghum, obtained by crossing pure lines of this plant with dark and light color of seeds, have if it is known that dark color dominates over light color? What color will the seeds of plants obtained from self-pollination of these hybrids have?

Solution.

1. Let's designate genes:

A - dark color of seeds, a- light color of seeds.

2. We draw up a crossing scheme:

a) first, we write down the genotypes of the parents, who are homozygous by the condition of the problem:

$ Р (♀АА) ↙ (\ text "dark seeds") × (♂аа) ↙ (\ text "light seeds") $

b) then we write the gametes in accordance with the rule of gamete purity:

Gametes A a

c) merge gametes in pairs and write down the genotypes of the descendants:

F 1 A a

d) according to the law of dominance, all hybrids of the first generation will have a dark color, therefore, we sign the phenotype under the genotype.

Phenotype dark seeds

3. We write down the scheme of the following crossing:

Answer: in the first generation, all plants will have a dark seed color, and in the second, 3/4 of the plants will have dark seeds, and 1/4 will have light seeds.

Objective 2. In rats, the black color of the coat dominates over the brown, and the normal tail length over the shortened tail. How many offspring in the second generation from crossing homozygous rats with black hair and a normal tail with homozygous rats with brown hair and a shortened tail had black hair and a shortened tail, if a total of 80 pups were born?

Solution.

1. We write down the condition of the problem:

A - black wool, a- brown wool;

B is the normal length of the tail, b- a shortened tail.

F 2 A_ bb ?

2. We write down the crossing scheme:

Note. It should be remembered that the letter designations of genes are written in alphabetical order, while in genotypes, the uppercase letter will always come before the lowercase: A - before a, Forward b etc.

It follows from the Pennett lattice that the proportion of rat pups with black hair and a shortened tail was 3/16.

3. We calculate the number of pups with the indicated phenotype in the offspring of the second generation:

80 × 3/16 × 15.

Answer: 15 pups had black hair and a shortened tail.

V tasks to determine the genotype the nature of the inheritance of the trait is also given and the task is to determine the genotypes of the offspring by the genotypes of the parents or vice versa.

Objective 3. In a family where the father had the III (B) blood group according to the AB0 system, and the mother had the II (A) group, a child with the I (0) blood group was born. Determine the genotypes of the parents.

Solution.

1. We recall the nature of the inheritance of blood groups:

Inheritance of blood groups according to the AB0 system

2. Since two variants of genotypes with II and III blood groups are possible, we write down the crossing scheme as follows:

3. From the above crossing scheme, we see that the child received recessive alleles i from each of the parents, therefore, the parents were heterozygous for the genes of the blood group.

4. We supplement the crossing scheme and check our assumptions:

Thus, our assumptions were confirmed.

Answer: parents are heterozygous for genes of blood groups: mother's genotype - I A i, father's genotype - I B i.

Task 4. Color blindness (color blindness) is inherited as a sex-linked recessive trait. What kind of children can be born to a man and a woman who can distinguish colors normally, although their parents were color blind, and the mothers and their relatives are healthy?

Solution.

1. Let's designate genes:

X D - normal color vision;

X d - color blindness.

2. We establish the genotypes of men and women, whose fathers were color blind.

3. We write down the crossing scheme to determine the possible genotypes of children:

Answer: all girls will have normal color vision (but 1/2 of the girls will carry the gene for color blindness), 1/2 of the boys will be healthy, and 1/2 will be color blind.

V tasks to determine the nature of the inheritance of a trait only the phenotypes of parents and offspring are given. Questions of such tasks are precisely the clarification of the nature of the inheritance of a trait.

Task 5. From crossing chickens with short legs, 240 chickens were obtained, 161 of which were short-legged, and the rest were long-legged. How is this trait inherited?

Solution.

1. Determine the splitting in the offspring:

161: 79 $≈$ 2: 1.

This splitting is typical for crosses in the case of lethal genes.

2. Since there were twice as many chickens with short legs as with long ones, let us assume that this is a dominant trait, and it is this allele that has a lethal effect. Then the original chickens were heterozygous. We denote genes:

C - short legs, c - long legs.

3. We write down the crossing scheme:

Our assumptions were confirmed.

Answer: short-legged dominates over long-legged, this allele has a lethal effect.

Basic terms of genetics

• Gene is a section of a DNA molecule that carries information about the primary structure of one protein. A gene is a structural and functional unit of inheritance.
• Allelic genes (alleles)- different variants of the same gene, encoding an alternative manifestation of the same trait. Alternative signs are signs that cannot be in the body at the same time.
• Homozygous organism- an organism that does not split in one way or another. Its allelic genes equally affect the development of this trait.
• Heterozygous organism- an organism that splits according to one or another characteristic. Its allelic genes affect the development of this trait in different ways.
• Dominant gene is responsible for the development of a trait that manifests itself in a heterozygous organism.
• Recessive gene is responsible for the trait, the development of which is suppressed by the dominant gene. A recessive trait is manifested in a homozygous organism containing two recessive genes.
• Genotype- a set of genes in a diploid set of an organism. The collection of genes in a haploid set of chromosomes is called a genome.
• Phenotype- the totality of all the signs of the body.

When solving problems in genetics, it is necessary:

1. Determine the types of crossing and interactions of allelic and non-alelic genes (determine the nature of the crossing).
2. Determine the dominant and recessive trait (s) by the condition of the problem, picture, scheme or by the results of crossing F 1 and F 2.
3. Enter the letter designations of the dominant (capital letter) and recessive ( capital letter) signs if they are not given in the problem statement.
4. Record the phenotypes and genotypes of the parental forms.
5. Record the phenotypes and genotypes of the offspring.
6. Draw up a crossing scheme, be sure to indicate the gametes that form the parental forms.
7. Record your answer.

When solving problems on the interaction of non-allelic genes, it is necessary:

1. Make a short note of the task.
2. Analyze each feature separately, making a corresponding entry for each feature.
3. Apply monohybrid crossing formulas, if none of them fit, then
   • Add the weight of the numerical indicators in the offspring, divide the amount by 16, find one part and express all the numerical indicators in parts.
   • Based on the fact that the cleavage in F 2 of a dihybrid crossing follows the formula 9A_B_: 3A_bb: 3 aaB_: l aabb, find the genotypes Fr
   • Find genotypes F by F 2
   • Find the genotypes of the parents using F.

Formulas for determining the nature of crossing:

where n is the number of alleles, pairs of features

• Genotype cleavage - (3: 1) n
• Cleavage by phenotype - (1: 2: 1) n
• Number of gamete types - 2 n
• The number of phenotypic classes - 2 n
• The number of genotypic classes - 3 n
• The number of possible combinations, combinations of gametes - 4 n

Basic rules for solving genetic problems:

1. If, when two phenotypically identical individuals are crossed, a splitting of traits is observed in their offspring, then these individuals are heterozygous.
2. If, as a result of crossing of individuals that differ feiotypically in one pair of traits, offspring is obtained in which splitting according to the same pair of traits is observed, then one of the parent individuals was heterozygous, and the other was homozygous for a recessive trait.
3. If, when crossing feiotypically identical (one pair of traits) individuals in the first generation of hybrids, traits are split into three phenotypic groups in a ratio of 1: 2: 1, then this indicates incomplete dominance and that the parent individuals are heterozygous.
4. If, when crossing two feiotypically identical individuals in the offspring, a splitting of characters occurs in a ratio of 9: 3: 3: 1, then the original individuals were diheterozygous.

Genetics- a science that studies heredity and variability of organisms.
Heredity- the ability of organisms to transmit their characteristics (features of structure, functions, development) from generation to generation.
Variability- the ability of organisms to acquire new characteristics. Heredity and variability are two opposite but interrelated properties of an organism.

Heredity

Basic concepts
Gene and alleles. The unit of hereditary information is the gene.
Gene(from the point of view of genetics) - a section of a chromosome that determines the development of one or more traits in an organism.
Alleles- different states of the same gene, located at a certain locus (site) of homologous chromosomes and determining the development of one particular trait. Homologous chromosomes are found only in cells containing a diploid set of chromosomes. They are absent in the germ cells (gametes) of eukaryotes and prokaryotes.

Sign (hair dryer)- some quality or property by which one can distinguish one organism from another.
Domination- the phenomenon of the predominance of the trait of one of the parents in the hybrid.
Dominant feature- a trait that appears in the first generation of hybrids.
Recessive trait- a trait that externally disappears in the first generation of hybrids.

Dominant and recessive traits in humans

Dominant allele - the allele that determines the dominant trait. It is designated by a Latin capital letter: A, B, C,….
Recessive allele - an allele that determines a recessive trait. It is designated by a Latin lowercase letter: a, b, c,….
The dominant allele ensures the development of the trait in both homozygous and heterozygous states, the recessive allele manifests itself only in the homozygous state.
Homozygote and heterozygote. Organisms (zygotes) can be homozygous and heterozygous.
Homozygous organisms have two identical alleles in their genotype - both dominant or both recessive (AA or aa).
Heterozygous organisms have one of the alleles in the dominant form, and the other in the recessive form (Aa).
Homozygous individuals do not split in the next generation, while heterozygous individuals do splitting.
Different allelic forms of genes result from mutations. A gene can mutate multiple times to form many alleles.
Multiple allelism - the phenomenon of the existence of more than two alternative allelic forms of a gene, which have different manifestations in the phenotype. Two or more states of a gene result from mutations. A number of mutations cause the appearance of a series of alleles (A, a1, a2, ..., an, etc.), which are in different dominant-recessive relationships to each other.
Genotype - the set of all genes of the body.
Phenotype - the totality of all the signs of the body. These include morphological (external) signs (eye color, color of flowers), biochemical (form of a molecule of a structural protein or enzyme), histological (shape and size of cells), anatomical, etc. On the other hand, signs can be divided into qualitative ( eye color) and quantitative (body weight). The phenotype depends on the genotype and environmental conditions. It develops as a result of the interaction of the genotype and environmental conditions. The latter to a lesser extent affect the qualitative characteristics and to a greater extent - on the quantitative ones.
Crossing (hybridization). One of the main methods of genetics is crossing, or hybridization.
Hybridological method - crossing (hybridization) of organisms that differ from each other in one or more characteristics.
Hybrids - descendants from crosses of organisms that differ from each other in one or more characteristics.
Depending on the number of signs by which parents differ among themselves, they distinguish different types crossbreeding.
Monohybrid crossing - crossing, in which the parents differ in only one sign.
Dihybrid crossing - crossing, in which the parents differ in two ways.
Polyhybrid crossing - crossing, in which the parents differ in several ways.
To record the results of crosses, the following generally accepted designations are used:
P - parents (from lat. parental- parent);
F - offspring (from lat. filial- offspring): F 1 - hybrids of the first generation - direct descendants of parents P; F 2 - hybrids of the second generation - descendants from crossing between F 1 hybrids, etc.
♂ - male (shield and spear - the sign of Mars);
♀ - female (mirror with a handle - the sign of Venus);
X - cross icon;
: - splitting of hybrids, separates digital ratios of different (by phenotype or genotype) classes of offspring.
The hybridological method was developed by the Austrian naturalist G. Mendel (1865). He used self-pollinated garden pea plants. Mendel crossed pure lines (homozygous individuals) that differ from each other in one, two or more traits. He obtained hybrids of the first, second, etc. generations. Mendel processed the data obtained mathematically. The results obtained were formulated in the form of laws of heredity.

G. Mendel's laws

Mendel's first law. G. Mendel crossed pea plants with yellow seeds and pea plants with green seeds. Both were pure lines, that is, homozygotes.

Mendel's first law is the law of uniformity for first-generation hybrids (dominance law): when crossing pure lines, all hybrids of the first generation show one trait (dominant).
Mendel's second law. After that G. Mendel crossed the first generation hybrids with each other.

Mendel's second law is the law of feature splitting: hybrids of the first generation, when crossed, are split in a certain numerical ratio: individuals with a recessive manifestation of a trait make up 1/4 of the total number of offspring.

Splitting is a phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which are dominant, and some are recessive. In the case of a monohybrid crossing, this ratio is as follows: 1AA: 2Aa: 1aa, that is, 3: 1 (in case of complete dominance) or 1: 2: 1 (in case of incomplete dominance). In the case of a dihybrid crossing - 9: 3: 3: 1 or (3: 1) 2. With polyhybrid - (3: 1) n.
Incomplete dominance. The dominant gene does not always completely suppress the recessive gene. This phenomenon is called incomplete dominance ... An example of incomplete dominance is the inheritance of the color of the flowers of a night beauty.

Cytological bases of uniformity of the first generation and splitting of traits in the second generation consist in the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis.
Hypothesis (law) of gamete purity states: 1) during the formation of germ cells, only one allele from the allelic pair gets into each gamete, that is, the gametes are genetically pure; 2) in a hybrid organism, genes do not hybridize (do not mix) and are in a pure allelic state.
The statistical nature of the splitting phenomena. From the hypothesis of the purity of gametes, it follows that the law of segregation is the result of a random combination of gametes carrying different genes. With the random nature of the connection of gametes, the overall result turns out to be natural. Hence it follows that for monohybrid crossing the ratio 3: 1 (in case of complete dominance) or 1: 2: 1 (in case of incomplete dominance) should be considered as a pattern based on statistical phenomena. This also applies to the case of polyhybrid crossing. Exact fulfillment of numerical ratios during splitting is possible only with a large number of studied hybrid individuals. Thus, the laws of genetics are statistical in nature.
Analysis of offspring. Analyzing cross allows you to establish whether an organism is homozygous or heterozygous for the dominant gene. For this, an individual is crossed, the genotype of which should be determined, with an individual homozygous for the recessive gene. Often one of the parents is crossed with one of the offspring. This crossing is called returnable .
In the case of homozygosity of the dominant individual, splitting will not occur:

In the case of heterozygosity of the dominant individual, splitting will occur:

Mendel's third law. G. Mendel carried out a dihybrid crossing of pea plants with yellow and smooth seeds and pea plants with green and wrinkled seeds (both are pure lines), and then crossed their descendants. As a result, he found that each pair of traits during splitting in the offspring behaves in the same way as during monohybrid crossing (split 3: 1), that is, regardless of the other pair of traits.

Mendel's third law- the law of independent combination (inheritance) of traits: splitting for each trait occurs independently of other traits.

The cytological basis of independent combination is the random nature of the divergence of homologous chromosomes of each pair to different poles of the cell during meiosis, regardless of other pairs of homologous chromosomes. This law is valid only when the genes responsible for the development of different traits are located on different chromosomes. The exceptions are cases of chained inheritance.

Concatenated inheritance. Loss of adhesion

The development of genetics has shown that not all traits are inherited in accordance with Mendel's laws. Thus, the law of independent gene inheritance is valid only for genes located on different chromosomes.
The patterns of linked gene inheritance were studied by T. Morgan and his students in the early 1920s. XX century. The object of their research was the fruit fly Drosophila (its life span is short, and several tens of generations can be obtained in a year, its karyotype consists of only four pairs of chromosomes).
Morgan's Law: genes localized on one chromosome are inherited predominantly together.
Linked genes - genes that lie on the same chromosome.
Clutch group - all genes of one chromosome.
In a certain percentage of cases, the adhesion may be broken. The cause of the violation of adhesion is crossing over (crossing of chromosomes) - the exchange of chromosomes in the prophase of meiotic division. Crossing over leads to genetic recombination... The farther apart the genes are, the more often crossing over occurs between them. This phenomenon is based on the construction genetic maps- determination of the sequence of the location of genes in the chromosome and the approximate distance between them.

Genetics of gender

Autosomes - chromosomes, the same in both sexes.
Sex chromosomes (heterochromosomes) - chromosomes by which the male and female sex differ from each other.
A human cell contains 46 chromosomes, or 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. The sex chromosomes are referred to as the X and Y chromosomes. Women have two X chromosomes, while men have one X and one Y chromosome.
There are 5 types of chromosomal sex determination.

Types of chromosome sex determination

Gender-linked inheritance - inheritance of traits, the genes of which are located on the X- and Y-chromosomes. The sex chromosomes may contain genes that are not related to the development of sexual characteristics.
When XY is combined, most genes on the X chromosome do not have an allelic pair on the Y chromosome. Also, genes located on the Y chromosome do not have alleles on the X chromosome. Such organisms are called hemizygous ... In this case, a recessive gene appears, which is present in the genotype in singular... So the X chromosome may contain a gene that causes hemophilia (reduced blood clotting). Then all males who received this chromosome will suffer from this disease, since the Y chromosome does not contain a dominant allele.

Blood genetics

According to the AB0 system, people have 4 blood groups. The blood group is determined by gene I. In humans, the blood group is provided by three genes IA, IB, I0. The first two are codominant in relation to each other, and both are dominant in relation to the third. As a result, a person has 6 blood groups in genetics, and 4 in physiology.

The ratio of blood groups in the population is different for different peoples.

Distribution of blood groups according to the AB0 system in different nations,%

Also the blood different people may differ in Rh factor. Blood can be Rh-positive (Rh +) or Rh-negative (Rh -). This ratio is different for different peoples.

Distribution of the Rh factor among different peoples,%

The Rh factor in blood determines the R gene. R + gives information about protein production (Rh-positive protein), but the R gene does not. The first gene is dominant over the second. If Rh + blood is transfused into a person with Rh - blood, then specific agglutinins are formed in him, and repeated administration of such blood will cause agglutination. When an Rh woman develops a fetus that inherits a positive Rh from her father, a Rh conflict may occur. The first pregnancy, as a rule, ends well, and the second pregnancy ends with a child's illness or stillbirth.

Interaction of genes

A genotype is not just a mechanical set of genes. This is a historically developed system of genes interacting with each other. More precisely, it is not the genes themselves (sections of DNA molecules) that interact, but the products formed on their basis (RNA and proteins).
Both allelic genes and non-allelic genes can interact.
Interaction of allelic genes: complete dominance, incomplete dominance, codominance.
Complete domination - a phenomenon when a dominant gene completely suppresses the work of a recessive gene, as a result of which a dominant trait develops.
Incomplete dominance - the phenomenon when the dominant gene does not completely suppress the work of the recessive gene, as a result of which an intermediate trait develops.
Codominance (independent manifestation) - a phenomenon when both alleles are involved in the formation of a trait in a heterozygous organism. In humans, a gene that determines a blood group is represented by a series of multiple alleles. In this case, the genes that determine the blood groups A and B are codominant in relation to each other, and both are dominant in relation to the gene that determines the blood group 0.
Interaction of non-allelic genes: cooperation, complementarity, epistasis and polymerization.
Cooperation - a phenomenon when, with the mutual action of two dominant non-allelic genes, each of which has its own phenotypic manifestation, a new trait is formed.
Complementarity - a phenomenon when a trait develops only with the mutual action of two dominant non-allelic genes, each of which separately does not cause the development of a trait.
Epistasis - the phenomenon when one gene (both dominant and recessive) suppresses the action of another (non-allelic) gene (both dominant and recessive). A suppressor gene (suppressor) can be dominant (dominant epistasis) or recessive (recessive epistasis).
Polymerism - a phenomenon when several non-allelic dominant genes are responsible for a similar effect on the development of the same trait. The more such genes are present in the genotype, the more clearly the trait is manifested. The phenomenon of polymerization is observed when quantitative traits are inherited (skin color, body weight, milk yield of cows).
In contrast to polymerization, there is such a phenomenon as pleiotropy - multiple gene action, when one gene is responsible for the development of several traits.

Chromosomal theory of heredity

The main provisions of the chromosomal theory of heredity:

• chromosomes play a leading role in heredity;
• genes are located on the chromosome in a certain linear sequence;
• each gene is located at a certain place (locus) of the chromosome; allelic genes occupy the same loci in homologous chromosomes;
• genes of homologous chromosomes form a linkage group; their number is equal to the haploid set of chromosomes;
• the exchange of allelic genes (crossing over) is possible between homologous chromosomes;
• the frequency of crossing over between genes is proportional to the distance between them.

Nonchromosomal inheritance

According to the chromosomal theory of heredity, the leading role in heredity is played by the DNA of the chromosomes. However, DNA is also found in mitochondria, chloroplasts and cytoplasm. Nonchromosomal DNA is called plasmids ... Cells do not have special mechanisms for the uniform distribution of plasmids in the process of division, so one daughter cell can receive one genetic information, and the second - completely different. The inheritance of genes contained in plasmids does not obey the Mendelian laws of inheritance, and their role in the formation of the genotype is still poorly understood.