The patterns of inheritance of characters during sexual reproduction were established by G. Mendel. It is necessary to have a clear understanding of genotype and phenotype, alleles, homo- and heterozygosity, dominance and its types, types of crosses, and also draw up diagrams.

Monohybrid called crossing, in which parent forms differ from each other in one pair of contrasting, 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. Examples of monohybrid crossings carried out by G. Mendel include crossings of peas with such clearly visible alternative characters as purple and white flowers, yellow and green coloring of unripe fruits (beans), smooth and wrinkled surface of seeds, yellow and green coloring, etc.

Uniformity of first generation hybrids (Mendel's first law). When crossing peas with purple (AA) and white (aa) flowers, Mendel found that all first generation hybrid plants (F 1) had purple flowers (Fig. 2).

Figure 2 – Monohybrid crossing scheme

At the same time, the white color of the flower did not appear. When crossing plants with smooth and wrinkled seeds, the hybrids will have smooth seeds. G. Mendel also established that all F 1 hybrids turned out to be uniform (homogeneous) in each of the seven characters he studied. Consequently, in first-generation hybrids, out of a pair of parental alternative traits, only one appears, and the trait of the other parent seems to disappear.

Alternative signs are mutually exclusive and contrasting signs.

Mendel called the phenomenon of predominance of traits of one of the parents in F 1 hybrids dominance, and the corresponding trait - dominant. He called traits that do not appear in F 1 hybrids recessive. Since all first-generation hybrids are uniform, this phenomenon was called Mendel's first laws, or the law of uniformity of first-generation hybrids, as well as the rule of dominance.

It can be formulated as follows: when crossing two organisms belonging to different pure lines (two homozygous organisms), differing from each other in one pair of alternative traits, the entire first generation of hybrids will be uniform and will carry the trait of one of the parents.

Each gene has two states - “A” and “a”, so they form one pair, and each member of the pair is called an allele. Genes located in the same loci (sections) of homologous chromosomes and determining the alternative development of the same trait are called allelic.

For example, the purple and white color of a pea flower are dominant and recessive traits, respectively, for two alleles (A and a) of one gene. Due to the presence of two alleles, two states of the body are possible: homo- and heterozygous. If an organism contains identical alleles of a particular gene (AA or aa), then it is called homozygous for this gene (or trait), and if different (Aa) it is called heterozygous. Therefore, an allele is a form of existence of a gene. An example of a triallelic gene is the gene that determines the ABO blood group system in humans. There are even more alleles: for the gene that controls the synthesis of human hemoglobin, many dozens of them are known.

From hybrid pea seeds, Mendel grew plants that self-pollinated, and sowed the resulting seeds again. As a result, the second generation of hybrids, or F 2 hybrids, was obtained. Among the latter, a split in each pair of alternative characters was found in a ratio of approximately 3:1, i.e., three quarters of the plants had dominant characters (purple flowers, yellow seeds, smooth seeds etc.) and one quarter are recessive (white flowers, green seeds, wrinkled seeds, etc.). Consequently, the recessive trait in the F 1 hybrid did not disappear, but was only suppressed and reappeared in the second generation. This generalization was later called Mendel's second law, or law of splitting.

Segregation is a phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some of which carry a recessive trait.

Mendel's second law: when two descendants of the first generation are crossed with each other (two heterozygous individuals), in the second generation a splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1 (Fig. 3).

Figure 3 – Character splitting scheme

when crossing F 1 hybrids

G. Mendel explained the splitting of characters in the offspring when crossing heterozygous individuals by the fact that gametes are genetically pure, that is, they 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 ends up in each gamete.

It should be borne in mind that the use of the hybridological method for analyzing the inheritance of traits in any species of animals or plants involves the following crosses:

1) crossing parental forms (P) that differ in one (monohybrid crossing) or several pairs (polyhybrid crossing) of alternative characters and obtaining first-generation hybrids (F 1);

2) crossing F 1 hybrids with each other and obtaining second generation hybrids (F 2);

3) mathematical analysis crossbreeding results.

Subsequently, Mendel moved on to the study of dihybrid crossing.

Dihybrid cross- this is a crossing in which two pairs of alleles are involved (paired genes are allelic and are located only on homologous chromosomes).

In dihybrid crossing, G. Mendel studied the inheritance of traits for which genes lying in different pairs of homologous chromosomes are responsible. In this regard, each gamete must contain one gene from each allelic pair.

Hybrids that are heterozygous for two genes are called diheterozygous, and if they differ in three or many genes, they are called tri- and polyheterozygous, respectively.

More complex dihybrid crossing schemes, recording of F 2 genotypes and phenotypes are carried out using the Punnett lattice. Let's look at an example of such a crossing. For crossing, two initial homozygous parental forms were taken: the first form had yellow and smooth seeds; the second form had green and wrinkled seeds (Fig. 4).

Figure 4 – Dihybrid crossing of pea plants,

seeds differing in shape and color

Yellow color and smooth seeds are dominant characteristics; green color and wrinkled seeds are recessive traits. First generation hybrids crossed with each other. In the second generation, phenotypic cleavage was observed in the ratio 9:3:3:1, or (3+1) 2 , after self-pollination of the F 1 hybrids, wrinkled and green seeds reappeared in accordance with the law of cleavage.

The parent plants in this case have the genotypes AABB and aabb, and the genotype of the F 1 hybrids is AaBb, i.e. it is diheterozygous.

Thus, when crossing heterozygous individuals that differ in several pairs of alternative traits, the offspring exhibit phenotypic cleavage in the ratio (3+1) n, where n is the number of pairs of alternative traits.

Genes that determine the development of different pairs of traits are called non-allelic.

The results of dihybrid and polyhybrid crossings depend on whether the genes that determine the traits under consideration are located on the same chromosome or on different chromosomes. Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes.

During meiosis homologous chromosomes different pairs are combined in gametes randomly. 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. Subsequently, it turned out that of the seven pairs of traits studied by Mendel in peas, which have a diploid chromosome number of 2n = 14, the genes responsible for one of the pairs of traits were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since linkage between these genes was not observed due to the large distance between them).

Based on his research, Mendel derived the third law - the law of independent inheritance of traits, or independent combination of genes.

Each pair of allelic genes (and the alternative traits controlled by them) is inherited independently of each other.

The law of independent combination of genes forms the basis of combinative variability observed during crossing in all living organisms. Note also that, unlike Mendel’s first law, which is always valid, the second law is valid only for genes localized in different pairs of homologous chromosomes. This is due to the fact that non-homologous chromosomes are combined in the cell independently of each other, which was proven not only by studying the nature of inheritance of traits, but also by direct cytological methods.

When studying the material, pay attention to cases of violations of regular phenotypic cleavages caused by the lethal effect of individual genes.

Heredity and variability. Heredity and variability are the most important properties characteristic of all living organisms.

Hereditary, or genotypic, variability is divided into combinative and mutational.

Combinative variation is called variability, which is based on the formation of recombinations, i.e., such combinations of genes that the parents did not have.

The basis of combinative variability is the sexual reproduction of organisms, as a result of which a huge variety of genotypes arises. Three processes serve as virtually unlimited sources of genetic variation:

1. Independent divergence of homologous chromosomes in the first meiotic division. It is the independent combination of chromosomes during meiosis that is the basis of G. Mendel’s third law. The appearance of green smooth and yellow wrinkled pea seeds in the second generation from crossing plants with yellow smooth and green wrinkled seeds is an example of combinative variability.

2. Mutual exchange of sections of homologous chromosomes, or crossing over. It creates new clutch groups, i.e. it serves important source genetic recombination of alleles. Recombinant chromosomes, once in the zygote, contribute to the appearance of characteristics that are atypical for each of the parents.

3. Random combination of gametes during fertilization.

These sources of combinative variability act independently and simultaneously, ensuring a constant “shuffling” of genes, which leads to the emergence of organisms with a different genotype and phenotype (the genes themselves do not change). However, new gene combinations break down quite easily when passed on from generation to generation.

An example of combinative variability. The night beauty flower has a gene for red petals A and a gene for white petals A. Organism Aa has pink petals. Thus, the night beauty does not have a gene for pink color, pink color arises from the combination (combination) of red and white genes.

The person has the hereditary disease sickle cell anemia. AA is the norm, aa is death, Aa is SKA. With SCD, a person cannot tolerate increased physical activity, and he does not suffer from malaria, i.e., the causative agent of malaria, Plasmodium falciparum, cannot feed on the wrong hemoglobin. This sign is useful in the equatorial zone; There is no gene for it, it arises from a combination of genes A and a.

Thus, hereditary variability is enhanced by combinative variability. Having arisen, individual mutations find themselves in the vicinity of other mutations and become part of new genotypes, i.e., many combinations of alleles arise. Any individual is genetically unique (with the exception of identical twins and individuals that arose due to asexual reproduction clone having one cell as its ancestor). So, if we assume that in each pair of homologous chromosomes there is only one pair of allelic genes, then for a person who has a haploid set of chromosomes equal to 23, the number of possible genotypes will be 3 to the 23 power. Such a huge number of genotypes is 20 times greater than the number of all people on Earth. However, in reality, homologous chromosomes differ in several genes and the phenomenon of crossing over is not taken into account in the calculation . Therefore, the number of possible genotypes is expressed in an astronomical number, and it can be confidently stated that the emergence of two identical people is almost impossible (with the exception of identical twins arising from one fertilized egg). This, in particular, implies the possibility of reliably determining identity from the remains of living tissue, confirming or excluding paternity.

Thus, the exchange of genes due to the crossing of chromosomes in the first division of meiosis, the independent and random recombination of chromosomes in meiosis and the randomness of the fusion of gametes during the sexual process are three factors that ensure the existence of combinative variability. Mutational variability of the genotype itself.

Mutations are sudden, inherited changes in genetic material that lead to changes in certain characteristics of an organism.

The main provisions of mutation theory were developed by the scientist G. De Vries in 1901 – 1903 and boil down to the following:

Mutations arise suddenly, spasmodically, as discrete changes in characteristics;

Distinguished from non-hereditary changes, mutations are qualitative changes that are passed on from generation to generation;

Mutations manifest themselves in different ways and can be both beneficial and harmful, both dominant and recessive;

The probability of detecting mutations depends on the number of individuals examined;

Similar mutations may occur repeatedly;

Mutations are undirected (spontaneous), i.e., any part of the chromosome can mutate, causing changes in both minor and vital signs.

Almost any change in the structure or number of chromosomes, in which the cell retains the ability to reproduce itself, causes a hereditary change in the characteristics of the organism.

Based on the nature of the change in the genome, i.e., the set of genes contained in a haploid set of chromosomes, gene, chromosomal and genomic mutations are distinguished.

Gene, or point, mutations are the result of changes in the nucleotide sequence in a DNA molecule within one gene.

Such a change in the gene is reproduced during transcription in the structure of the mRNA; it leads to a change in the sequence of amino acids in the polypeptide chain formed during translation on ribosomes. As a result, another protein is synthesized, which leads to a change in the corresponding characteristic of the body. This is the most common type of mutation and the most important source of hereditary variability in organisms.

Chromosomal mutations(rearrangements, or aberrations) are changes in the structure of chromosomes that can be identified and studied under a light microscope.

Various types of rearrangements are known:

a lack of – loss of the terminal sections of a chromosome;

Deletion – loss of a section of a chromosome in its middle part;

Duplication – double or multiple repetition of genes localized in a specific region of the chromosome;

Inversion – rotation of a section of a chromosome by 180°, as a result of which genes in this section are located in the reverse sequence compared to the usual one;

Translocation – change in the position of any part of a chromosome in the chromosome set. The most common type of translocations are reciprocal, in which regions are exchanged between two non-homologous chromosomes. A section of a chromosome can change its position without reciprocal exchange, remaining in the same chromosome or being included in some other one.

Genomic mutations are changes in the number of chromosomes in the genome of body cells. This phenomenon occurs in two directions: towards an increase in the number of entire haploid sets (polyploidy) and towards the loss or inclusion of individual chromosomes (aneuploidy).

Polyploidy – multiple increase in the haploid set of chromosomes. Cells with different numbers haploid sets of chromosomes are called triploid (3 n), tetraploid (4 n), hexaploid (6 n), octaploid (8 n), etc. Most often, polyploids are formed when the order of chromosome divergence to the cell poles is disrupted during meiosis or mitosis. Polyploidy results in changes in the characteristics of an organism and is therefore an important source of variation in evolution and selection, especially in plants. This is due to the fact that hermaphroditism (self-pollination), apomixis (parthenogenesis) and vegetative propagation. Therefore, about a third of plant species common on our planet – polyploids, and in the sharply continental conditions of the high-mountain Pamirs, up to 85% of polyploids grow. Almost all cultivated plants are also polyploids, which, unlike their wild relatives, have larger flowers, fruits and seeds, and more accumulate in storage organs (stems, tubers). nutrients. Polyploids adapt more easily to unfavorable conditions life, tolerate low temperatures and drought more easily. That is why they are widespread in the northern and high mountain regions.

Having worked through these topics, you should be able to:

  1. Give definitions: gene, dominant trait; recessive trait; allele; homologous chromosomes; monohybrid crossing, crossing over, homozygous and heterozygous organism, independent distribution, complete and incomplete dominance, genotype, phenotype.
  2. Using the Punnett grid, illustrate crossbreeding for one or two traits and indicate what numerical ratios of genotypes and phenotypes should be expected in the offspring from these crosses.
  3. Explain the rules of inheritance, segregation, and independent distribution of characters, the discovery of which was Mendel's contribution to genetics.
  4. Explain how mutations can affect the protein encoded by a particular gene.
  5. Indicate the possible genotypes of people with blood groups A; IN; AB; ABOUT.
  6. Give examples of polygenic traits.
  7. Indicate the chromosomal mechanism of sex determination and types of inheritance of sex-linked genes in mammals, and use this information when solving problems.
  8. Explain the difference between sex-linked traits and sex-dependent traits; give examples.
  9. Explain how human genetic diseases such as hemophilia, color blindness, and sickle cell anemia are inherited.
  10. Name the features of methods of selection of plants and animals.
  11. Indicate the main directions of biotechnology.
  12. Be able to solve simple genetic problems using this algorithm:

    Algorithm for solving problems

    • Determine the dominant and recessive traits based on the results of crossing the first generation (F1) and the second (F2) (according to the conditions of the problem). Enter letter designations: A - dominant and - recessive.
    • Write down the genotype of an individual with a recessive trait or an individual with a known genotype and gametes based on the conditions of the problem.
    • Record the genotype of the F1 hybrids.
    • Draw up a scheme for the second crossing. Record the gametes of F1 hybrids in a Punnett grid horizontally and vertically.
    • Record the genotypes of the offspring in the gamete intersection cells. Determine the ratios of phenotypes in F1.

Task design scheme.

Letter designations:
a) dominant trait _______________
b) recessive trait _______________

Gametes

F1(first generation genotype)

gametes
? ?

Punnett lattice

F2
gametes ? ?
?
?

Phenotype ratio in F2: _____________________________
Answer:_________________________

Examples of solving monohybrid crossing problems.

Task.“There are two children in the Ivanov family: a brown-eyed daughter and a blue-eyed son. The mother of these children is blue-eyed, but her parents had brown eyes. How is eye color inherited in humans? What are the genotypes of all family members? Eye color is a monogenic autosomal trait.”

The eye color trait is controlled by one gene (by condition). The mother of these children is blue-eyed, and her parents had brown eyes. This is only possible if both parents were heterozygous, therefore, brown eyes dominate over blue ones. Thus, grandparents, father and daughter had the genotype (Aa), and mother and son had the genotype aa.

Task."A rooster with a rose-shaped comb was crossed with two hens, also having a rose-shaped comb. The first gave 14 chickens, all with a rose-shaped comb, and the second gave 9 chickens, of which 7 with a rose-shaped and 2 with a leaf-shaped comb. The shape of the comb is a monogenic autosomal trait. What are genotypes of all three parents?

Before determining the genotypes of the parents, it is necessary to find out the nature of inheritance of the comb shape in chickens. When a rooster was crossed with a second hen, 2 chicks with leaf combs were produced. This is possible if the parents are heterozygous; therefore, it can be assumed that the rose-shaped comb in chickens is dominant over the leaf-shaped one. Thus, the genotypes of the rooster and the second hen are Aa.

When crossing the same rooster with the first hen, no splitting was observed, therefore, the first hen was homozygous - AA.

Task.“In a family of brown-eyed, right-handed parents, fraternal twins were born, one of whom is brown-eyed, left-handed, and the other blue-eyed, right-handed. What is the probability of the next child being born similar to his parents?”

The birth of a blue-eyed child to brown-eyed parents indicates the recessiveness of blue eye color, respectively, the birth of a left-handed child to right-handed parents indicates the recessivity of better control of the left hand compared to the right. Let's introduce allele designations: A - brown eyes, a - blue eyes, B - right-handed, c - left-handed. Let's determine the genotypes of parents and children:

RAaBv x AaBv
F,A_bb, aaB_

A_bb is a phenotypic radical, which shows that this child left-handed with brown eyes. The genotype of this child may be Aavv, AAvv.

Further solution of this problem is carried out in the traditional way, by constructing a Punnett lattice.

ABAvaBAv
ABAABBAAVvAaBBAaVv
AvAAVvAAbbAaVvAaww
aBAaBBAaVvaaBBAaVv
awAaVvAawwaaVvAaww

9 variants of descendants that interest us are underlined. Total possible options 16, so the probability of a child being born similar to its parents is 9/16.

Ivanova T.V., Kalinova G.S., Myagkova A.N. " General biology". Moscow, "Enlightenment", 2000

  • Topic 10. "Monohybrid and dihybrid crossing." §23-24 pp. 63-67
  • Topic 11. "Genetics of sex." §28-29 pp. 71-85
  • Topic 12. "Mutational and modification variability." §30-31 pp. 85-90
  • Topic 13. "Selection." §32-34 pp. 90-97

Question 1. What is hybridization?

The crossing of two organisms is called hybridization.

Question 2. Which cross is called monohybrid?

Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative (mutually exclusive) characteristics.

Question 3. What phenomenon is called dominance?

G. Mendel called the predominance of the trait of one of the parents in a hybrid dominance.

Question 4. Which trait is called dominant and which is called recessive?

A trait that appears in a first-generation hybrid and suppresses the development of another trait was called dominant (from the Latin dominus - master), and the opposite, i.e. suppressed, was called recessive (from the Latin recess - retreat, removal).

Question 5. Tell us about Mendel’s experiments on monohybrid crossing of pea plants.

When Mendel crossed purple-flowered and white-flowered peas, he discovered that all of the first generation (F1) hybrid plants had purple flowers. At the same time, the white color of the flower did not appear.

Mendel also established that all F1 hybrids turned out to be uniform (homogeneous) in each of the seven characters he studied.

Consequently, in first-generation hybrids, out of a pair of parental alternative traits, only one appears, and the trait of the other parent seems to disappear. Mendel called the phenomenon of predominance of the traits of one of the parents in F1 hybrids dominance, and the corresponding trait - dominant. He called traits that do not appear in F1 hybrids recessive.

Question 6. Which organism is called homozygous; heterozygous?

If the genotype of an organism (zygote) contains two identical allelic genes, absolutely identical in nucleotide sequence, such an organism is called homozygous for this gene. An organism can be homozygous for dominant (AA or BB) or recessive (aa or bb) genes. If allelic genes differ from each other (one of them is dominant and the other is recessive (Aa, Bb), such an organism is called heterozygous.

Question 7. Formulate Mendel’s first law. Why is this law called the law of dominance?

This law can be formulated as follows: when crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative traits, the entire first generation of hybrids (F1) will be uniform and will carry the trait of one of the parents.

Since all first-generation hybrids are uniform, this phenomenon was called Mendel's first laws by K. Correns, or the law of uniformity of first-generation hybrids, as well as the rule of dominance.

Question 8. Using additional sources of information, give examples of incomplete dominance of traits in humans.

In humans, incomplete dominance manifests itself when the hair structure is inherited. The gene for curly hair does not fully dominate over the gene for straight hair, and heterozygotes exhibit an intermediate manifestation of the trait - wavy hair.

Another example is sickle cell anemia, which is based on a gene mutation that leads to the replacement of one of the 287 amino acids in the hemoglobin protein - valine - with glutamic acid. As a result, the structure of hemoglobin changes and red blood cells take on the shape of a sickle, which leads to oxygen deficiency. Homozygous organisms die in early age, and heterozygotes are viable, but suffer from shortness of breath on exertion.

Question 9. Which night beauty plants should be crossed with each other so that the offspring will be half plants with pink flowers and half with white flowers?

Heredity is the body’s ability to ensure material and functional continuity over a number of generations, as well as characteristic type individual development.

The hybridological method is a method of crossing pure lines to obtain hybrids, which are then crossed with each other. The pattern of inheritance of traits is analyzed quantitatively from each parental pair in each generation. Within the framework of the hybridological method, Mendel formulated that the crossing of two genetically different organisms is called hybridization, offspring from such a cross - hybrid or hybrid. Splitting concerning one pair of alternative characteristics, i.e. one locus is called monohybrid; from 2 pairs of characters – dihybrid; from more than 2 pairs of alleles – polyhybrid.

Mendel's first law – law of uniformity of first generation hybrids.

When pure lines that differ in one pair of alternative traits are crossed, the first generation hybrids exhibit the traits of one of the parents. The second sign seems to disappear and does not appear. Mendel called the phenomenon of predominance of a trait of one of the parents dominance, and a trait that appears in first-generation hybrids and suppresses the development of the second trait is dominant. A trait that is suppressed by the dominant one and does not appear in the first generation hybrids is called recessive. According to this law, in first-generation hybrids a dominant trait appears and a recessive trait does not appear if dominance is complete. If dominance is incomplete, then the manifestation of the trait is intermediate, and the splits in genotype and phenotype coincide.

Mendel's second law - the law of splitting, which states that when crossing first-generation hybrids, the offspring are split into alternative characteristics in a ratio of 3:1, respectively, of individuals with a dominant and recessive phenotype.

The cytological basis of monohybrid crosses is the behavior of chromosomes during meiosis and fertilization.

Mendel's third law is the law of independent inheritance of traits controlled by non-allelic genes. Analysis of segregation during dihybrid crossing using the Punnett grid shows that each of the characters is inherited independently of the other, because the phenotypic splitting for each of them is 3:1, as in a monohybrid cross.

Based on Mendel's laws, the following types of monogenic inheritance are distinguished: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive and Y-linked.

Variability – This is the ability of organisms to acquire new properties during ontogenesis. Variation is distinguished as hereditary, or genotypic, and non-hereditary, or phenotypic.

Phenotypic variability occurs under the influence of environmental factors and is inherent large groups individuals. It is reversible if the environmental factor ceases to act. Varieties non-hereditary variability are ontogenetic and modification.

Ontogenetic variability lies in the fact that the phenotype of an organism changes throughout life, while the genotype does not change, but only a switching of gene activity occurs.

Modification variability occurs under the influence of environmental factors, but its scope is determined by the genotype, i.e. genetically determined reaction norm.

Hereditary variability is associated with changes in the genotype and can be inherited as a combination or mutation.

Combinative variability is associated with recombination of parental genes and can be the cause of monogenic and multifactorial pathology (for example, schizophrenia, epilepsy).

Mutational variability occurs due to mutations that disrupt genetic material suddenly and spasmodically. In terms of their effect, mutations may beneficial, harmful, neutral, according to the method of occurrence - spontaneous and induced. Spontaneous mutations are a rare phenomenon, but induced ones occur under the influence of various mutagens: physical (irradiation), chemical (drugs), biological (bacteria and viruses).

Depending on the cell type, somatic (they do not affect the offspring, but can lead to neoplasms) and gametic (they lead to fetal malformations) mutations are distinguished. Zygotic mutations are also possible, which lead to mosaicism, when some cells of the body have a normal karyotype, while others have an abnormal one.

According to the nature of the changes, gene, chromosomal and genomic mutations are distinguished.

The fundamental laws of inheritance were discovered in the second half of the 19th century by Gregor Mendel. In his famous experiments, G. Mendel crossed different varieties of peas, differing in seven stably inherited morphological characters, mainly related to the shape and color of seeds or flowers. Then, over the course of several subsequent generations, he carried out quantitative records of plants separately for each of these characteristics. It turned out that under these conditions, all first-generation hybrids were similar to one of the parents. These observations were the basis for the formulation of Mendel's first law - law of uniformity of first generation hybrids. The trait that appeared in the first generation hybrids was named by Mendel dominant, and not a manifested sign - recessive. Later it was shown that this law belongs to the category of general biological laws . It should be emphasized that dominance is not always absolute. For example, when crossing plants with red and white flowers, the hybrids may exhibit an intermediate, pink color of the flowers. In this case they talk about incomplete dominance.

In the second generation, when hybrids self-pollinated, plants appeared with both dominant and recessive traits, on average, in a ratio of 3:1. This is Mendel's second law - law of character splitting. Of course, this law is implemented only in large samples. If we limit ourselves to the analysis of one family, that is, we study the offspring from the crossing of just two hybrid plants, then the ratios by traits can turn out to be anything. To obtain this pattern, it is necessary to summarize the results of the analysis of the offspring from the crossing of many hybrid plants, and the larger the studied sample of descendants, the more accurately the actual distribution by traits will approach the hypothetical value of 3:1. To discover this pattern, Mendel had to count more than 10,000 plants. Note that with incomplete dominance in the second generation, all forms of plants will be observed: the first parent, hybrid and second parent, on average, in a ratio of 1: 2: 1, respectively.

The observed patterns allowed Mendel to hypothesize the existence of two discrete hereditary factors responsible for each of the studied traits - the dominant one, which he designated capital letter A, and recessive – A. The next assumption was that only one of these factors enters the germ cells or gametes. Thus, the original pea varieties carry two identical hereditary factors responsible for the trait being studied; in one variety it is AA, and in the other - ah. First generation hybrids carry both hereditary factors - Ahh. And although the recessive factor ( A) does not appear in the presence of a dominant ( A), but it doesn't disappear. These two discrete hereditary factors do not merge with each other, and each of them is equally likely to end up in different germ cells. Moreover, gametes with both dominant and recessive factors are equally involved in fertilization. As a result, three types of plants are formed: AA,Ahh And ahh in a ratio of 1:2:1.

Subsequently, to simplify the understanding of inheritance patterns, it was proposed to use the so-called Punnett grid– a table in the first row and first column of which the types of female and male gametes are recorded, and at their intersection the types of descendants formed. In our case, this table looks like this:

Table 1.

Punnett grid for monohybrid crosses

Gametes ♀/♂

Since a recessive hereditary factor does not manifest itself in the presence of a dominant plant AA And Ahh will be externally identical to each other, and a 3:1 split will be observed according to the trait. These ingenious assumptions of Mendel were called gamete purity hypotheses. Note that in case of incomplete dominance, the distributions by hereditary factors and by traits coincide.

When describing the crossing scheme in genetics, the following designations are used: parents - P (from the Latin parentes - parent), female individuals - ♀ (mirror of Venus), male - ♂ (shield and spear of Mars), crossing - x (multiplication sign), offspring from crossing - F (from Latin filialis - filial) with a digital index: F 1 - first generation, F 2 - second, etc. The dash to the right of the dominant factor ( A _) means that this place can be occupied by both a dominant and a recessive factor. Let us write in these notations the crossing scheme used by Mendel, which was later called monohybrid cross- rice. 1.

P: A.A. X aa

gametes: A a

offspring F 1 : Aa

F 1: Aa X Aa

gametes: A a A a

F2, splitting by genotype:

1A.A.: 2Aa: 1aa

F2, splitting according to:

3A_ : 1aa

Figure 1. Monohybrid cross

Let us emphasize once again that the success of these experiments was largely predetermined by the fact that Mendel kept quantitative records of plants separately for each of the characteristics. A similar crossbreeding methodology that makes it possible to obtain and analyze hybrids is called hybridological analysis. When studying the inheritance of two traits at once (dihybrid crossing), it turned out that each of them behaves independently of each other. This leads to the fact that in the second generation 4 groups of plants are observed: those with both dominant traits at the same time, or only one of the two dominant traits, or those without dominant traits, in a ratio of 9:3:3:1. Let's look at this situation in more detail. Let us designate the dominant and recessive hereditary factor responsible for the first trait - A And A, and for the second sign – B And b, respectively. In these notations, the original parent varieties will have hereditary factors AABB And aabb, and the dihybrid crossing scheme will look like this:

P: AABB X aabb

gametes: AB ab

offspring F 1: AaBb

F 1: AaBb X AaBb

gametes: AB Ab аB ab АB Ab аB ab

F2, splitting by genotype:

1AABB: 2AABb: 2AaBB: 4AaBb: 1AAbb: 2Aabb: 1aaBB: 2aaBb:1aabb

F2, splitting according to characteristics:

9A_ B _ : 3A_ bb : 3aa B _ : 1aabb

Figure 2. Dihybrid cross

Table 2.

Punnett grid for dihybrid crosses

Gametes ♀/♂

With a trihybrid cross, the number of different combinations of traits in F 2 increases to 8, and the ratios become even more complex (27: 9: 9: 9: 3: 3: 3: 1). Try to draw a trihybrid crossing scheme and a Punnett grid for this scheme, and you will be convinced of the validity of these relationships.

Based on his observations, Mendel formulated law of independent combination of characteristics. However, this law turned out to be true not for all traits determined by one hereditary factor. It is observed only if these hereditary factors are located on different chromosomes. But this will be discussed later.

Unfortunately, the work of Gregor Mendel went unnoticed by his contemporaries, and his laws were independently rediscovered at the very beginning of the 20th century by three researchers, one of whom V. Johansen proposed to call the hereditary factors postulated by Mendel genes, a set of genes – genotype, and the set of characteristics of an organism is phenotype. Variants of hereditary factors or alternative states of genes (dominant, recessive) are called alleles. The genotype may be homozygous in the presence of two identical alleles ( AA or ahh) or heterozygous, if the alleles are different ( Ahh). In some cases there are no relationships of dominance and recessivity and both alleles are manifested in the phenotype. This type of allele relationship is called co-dominance.

Alleles or states of genes influence the pattern of development of traits, which serves as the basis for phenotypic variability. Of course, factors play an equally important role in this environment. If this variability does not go beyond the norm, then the corresponding alleles are called normal or wild type alleles. Normal alleles are usually widespread, but their frequencies can vary significantly between populations and ethnic groups. Those alleles whose frequencies in a population exceed a certain level, for example 5%, are called polymorphic alleles or polymorphisms. Alleles that lead to pathological development of a trait are called mutant alleles or mutations. In populations they are much less common, as they have bad influence on overall viability and are therefore subject to pressure natural selection. Mutations of various genes are associated with hereditary human diseases. Combinations of normal and mutant alleles of various genes determine individual hereditary constitution each person. Thus, people differ from each other not in their sets of genes, but in their states, that is, in their hereditary constitution.

Mendel's laws are valid for monogenic traits, which are also called Mendelian. Most often, their manifestations are of a qualitative alternative nature: brown or blue eye color, dark or light skin color, the presence or absence of some hereditary disease, etc. In the formation of other characteristics, such as height, weight, body type or type behavior, dozens or even hundreds of genes may be involved. The degree of expression of such characteristics in individual individuals can often be measured quantitatively, and therefore such characteristics are called quantitative.

Whether a trait is monogenic and whether the nature of its hereditary transmission in a number of generations obeys Mendel's laws can be easily established experimentally by carrying out certain crossing schemes between plants or animals. But the term crossing is not applicable to humans, since marriages between people are concluded on a voluntary basis. We can only study the consequences of these marriages, that is, compile human pedigrees, the analysis of which gives us the opportunity to judge whether a particular trait is monogenic and whether it obeys Mendel’s laws.

Let us give an example of such an analysis. As an alternative trait, we will choose brown and blue eye colors. In a Russian village, for several generations, children in all families have been blue-eyed, while in an Ossetian village they have been brown-eyed. A Russian young man married an Ossetian woman, and her fellow villager married a blue-eyed Russian girl. Each of these two families had five children, and all of them turned out to be brown-eyed. At this stage, we can say with great confidence that brown eye color is dominant over blue. The children of these two families were raised together, and two brothers from the first family married two sisters from the second family. The first brother had six children, and all of them turned out to be brown-eyed. The second brother had seven children, of which one boy and one girl turned out to be blue-eyed. Obviously, the first brother or his wife are homozygous for brown eye color. And the second brother and his wife are both heterozygous for the gene that controls eye color. The first brother's two sons married blue-eyed girls. The first son had five children with brown eyes, and the second son had three out of six children with blue eyes. It is obvious that either the father or the mother of these two sons is heterozygous for eye color, the first son is homozygous for brown coloring, and the second is heterozygous.

In order to facilitate the analysis of the hereditary transmission of a trait in a family, its pedigree is built. In this case, use the symbols presented in Fig. 3. All relatives belonging to the same generation should be placed on the same line. Generations are designated by Roman numerals, and individual members of each generation are designated by Arabic numerals. In this case, each family member will have his own individual number consisting of one Roman and one Arabic numeral. Let's draw a pedigree of our family and try to determine the possible genotypes of its members (Fig. 4):

I 1 ( AA) – Ossetian girl, I 2 ( ahh) – Russian youth (first family);

I 3 ( ahh) – Russian girl, I 4 ( AA) – Ossetian youth (second family).

II 1-3 ( A_)– unmarried children of the first family;

II 4 ( Ahh) and II 9 ( Ahh) – family of the first brother;

II 5 and II 6 – family of the second brother, one of the spouses AA, and the other Aa;

II 7, 8, 10 ( A_) – unmarried children of the second family.

III 1 ( ahh) - blue-eyed girl who married her first son - III 2 ( AA);

III 3-6 ( A_) – unmarried children of the first son;

III 7 ( Ahh) – second son who married a blue-eyed girl – III 8 ( ahh);

III 9 ( ahh) – blue-eyed son of the first brother;

III 10-13.15 ( A_) – brown-eyed children of the first brother;

III 9 ( ahh) – blue-eyed daughter of the first brother.

Thus, blue-eyed people are recessive homozygotes ( ahh), and brown-eyed people can be either dominant homozygotes ( AA), or heterozygotes ( Ahh). Two blue-eyed parents always have blue-eyed children. And two brown-eyed parents can have blue-eyed children with a 25% probability if they are both heterozygous ( Ahh). If at least one of the parents is homozygous for brown eye color ( AA), all children will be brown-eyed, but one of the grandchildren may turn out to be blue-eyed. If in a marriage between a brown-eyed spouse and a blue-eyed spouse, some of the children turn out to be blue-eyed, then the brown-eyed spouse is heterozygous for the gene that controls eye color ( Ahh).