1st Mendel's law formulation. Mendel's third law

Formulation 1 of Mendel's law The law of uniformity of the first generation of hybrids, or Mendel's first law. 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

Formulation of the 2nd law of Mendel The law of segregation, or the second law of Mendel Mendel When two heterozygous descendants of the first generation are crossed with each other in the second generation, segregation is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1.

Formulation 3 of Mendel's law Law of independent inheritance (Mendel's third law) When crossing two homozygous individuals that differ from each other in two (or more) pairs of alternative characteristics, genes and their corresponding characteristics are inherited independently of each other and are combined in all possible combinations (as and with monohybrid crossing). (The first generation after crossing had a dominant phenotype for all characteristics. In the second generation, a splitting of phenotypes was observed according to the formula 9: 3: 3: 1)

P AA BB aa bb x yellow, smooth seeds green, wrinkled seeds G (gametes) ABabab F1F1 Aa Bb yellow, smooth seeds 100% Mendel’s 3rd law DIHYBRID CROSSING. For the experiments, peas with smooth yellow seeds were taken as the mother plant, and peas with green wrinkled seeds were taken as the father plant. In the first plant both characters were dominant (AB), and in the second plant both were recessive (ab

The first generation after crossing had a dominant phenotype for all traits. (yellow and smooth peas) In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1. 9/16 yellow smooth peas, 3/16 yellow wrinkled peas, 3/16 green smooth peas, 1/16 green wrinkled peas.

Task 1. In spaniels, black coat color dominates over coffee, and short hair dominates over long hair. The hunter bought a black dog with short hair and, to be sure that it was purebred, he carried out an analytical crossbreeding. 4 puppies were born: 2 short-haired black, 2 short-haired coffee. What is the genotype of the dog purchased by the hunter? Dihybrid crossing problems.

Problem 2. In a tomato, the red color of the fruit dominates over the yellow color, and the high stem dominates over the low stem. By crossing a variety with red fruits and a high stem and a variety with yellow fruits and a low stem, 28 hybrids were obtained in the second generation. The first generation hybrids were crossed with each other, resulting in 160 second generation hybrid plants. How many types of gametes does a first generation plant produce? How many plants in the first generation have red fruit and a tall stem? How many different genotypes are there among the second generation plants with red fruit color and tall stem? How many plants in the second generation have yellow fruit and a tall stem? How many plants in the second generation have yellow fruit and a low stem?

Task 3 In humans, brown eye color dominates over blue color, and the ability to use the left hand is recessive in relation to right-handedness. From the marriage of a blue-eyed, right-handed man with a brown-eyed, left-handed woman, a blue-eyed, left-handed child was born. How many types of gametes does the mother produce? How many types of gametes does the father produce? How many different genotypes can there be among children? How many different phenotypes can there be among children? What is the probability of having a blue-eyed, left-handed child in this family (%)?

Task 4 Crestedness in chickens dominates over the absence of a crest, and black plumage color dominates over brown. From crossing a heterozygous black hen without a crest with a heterozygous brown crested rooster, 48 chickens were obtained. How many types of gametes does a chicken produce? How many types of gametes does a rooster produce? How many different genotypes will there be among the chickens? How many tufted black chickens will there be? How many black chickens will there be without a crest?

Task 5 In cats, the short hair of the Siamese breed is dominant over the long hair of the Persian breed, and the black coat color of the Persian breed is dominant over the fawn color of the Siamese breed. Siamese cats crossed with Persian cats. When crossing hybrids with each other in the second generation, 24 kittens were obtained. How many types of gametes are produced in a Siamese cat? How many different genotypes were produced in the second generation? How many different phenotypes were produced in the second generation? How many second generation kittens look like Siamese cats? How many second generation kittens look like Persians?

Solving problems at home Option 1 1) A blue-eyed right-hander married a brown-eyed right-hander. They had two children - a brown-eyed left-hander and a blue-eyed right-hander. From this man’s second marriage to another brown-eyed, right-handed woman, 8 brown-eyed children were born, all right-handed. What are the genotypes of all three parents? 2) In humans, the gene for protruding ears dominates over the gene for normal flat ears, and the gene for non-red hair dominates over the gene for red hair. What kind of offspring can be expected from the marriage of a floppy-eared red-haired man, heterozygous for the first sign, with a heterozygous red-haired woman with normal flat-back ears. Option 2 1) In humans, clubfoot (R) dominates over the normal structure of the foot (R) and normal carbohydrate metabolism (O) over diabetes. A woman with a normal foot structure and normal metabolism married a club-footed man. From this marriage two children were born, one of whom developed clubfoot and the other diabetes mellitus. Determine the genotype of parents by the phenotype of their children. What phenotypes and genotypes of children are possible in this family? 2) In humans, the gene for brown eyes dominates over the gene for blue eyes, and the ability to use the right hand dominates left-handedness. Both pairs of genes are located on different chromosomes. What kind of children can they be if: the father is left-handed, but heterozygous for eye color, and the mother is blue-eyed, but heterozygous for the ability to use her hands.

Let's solve problems 1. In humans, normal carbohydrate metabolism dominates over the recessive gene responsible for the development of diabetes mellitus. The daughter of healthy parents is sick. Determine whether a healthy child can be born in this family and what is the probability of this event? 2. In people, brown eye color is dominant over blue. The ability to better use the right hand dominates over left-handedness; the genes for both traits are located on different chromosomes. A brown-eyed right-hander marries a blue-eyed left-hander. What kind of offspring should be expected in this pair?

This article briefly and clearly describes Mendel's three laws. These laws are the basis of all genetics; by creating them, Mendel actually created this science.

Here you will find a definition of each law and learn a little something new about genetics and biology in general.

Before you start reading the article, you should understand that the genotype is the totality of the genes of an organism, and the phenotype is its external characteristics.

Who is Mendel and what did he do?

Gregor Johann Mendel is a famous Austrian biologist, born in 1822 in the village of Gincice. He studied well, but his family had financial difficulties. To deal with them, Johann Mendel in 1943 decided to become a monk at a Czech monastery in the city of Brno and received the name Gregor there.

Gregor Johann Mendel (1822 - 1884)

Later he studied biology at the University of Vienna, and then decided to teach physics and natural history in Brno. At the same time, the scientist became interested in botany. He conducted experiments on crossing peas. Based on the results of these experiments, the scientist derived three laws of heredity, which are the subject of this article.

Published in the work “Experiments with Plant Hybrids” in 1866, these laws did not receive wide publicity, and the work was soon forgotten. It was remembered only after Mendel’s death in 1884. You already know how many laws he derived. Now it's time to move on to looking at each.

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

Consider the experiment conducted by Mendel. He took two types of peas. These species were distinguished by the color of their flowers. One had them purple, and the other had them white.

Having crossed them, the scientist saw that all the offspring had purple flowers. And yellow and green peas produced completely yellow offspring. The biologist repeated the experiment many more times, checking the inheritance of different traits, but the result was always the same.

Based on these experiments, the scientist derived his first law, here is its formulation: all hybrids in the first generation always inherit only one trait from their parents.

Let us designate the gene responsible for purple flowers as A, and for white flowers as a. The genotype of one parent is AA (purple), and the second is aa (white). The A gene will be inherited from the first parent, and a from the second. This means that the genotype of the offspring will always be Aa. A gene designated by a capital letter is called dominant, and a lowercase letter is called recessive.

If the genotype of an organism contains two dominant or two recessive genes, then it is called homozygous, and an organism containing different genes is called heterozygous. If the organism is heterozygous, then the recessive gene, designated by a capital letter, is suppressed by a stronger dominant one, resulting in the manifestation of the trait for which the dominant one is responsible. This means that peas with genotype Aa will have purple flowers.

Crossing two heterozygous organisms with different characteristics is a monohybrid cross.

Codominance and incomplete dominance

It happens that a dominant gene cannot suppress a recessive one. And then both parental characteristics appear in the body.

This phenomenon can be observed in the example of camellia. If in the genotype of this plant one gene is responsible for red petals and the other for white, then half of the camellia petals will become red and the rest white.

This phenomenon is called codominance.

Incomplete dominance is a similar phenomenon, in which a third characteristic appears, something between what the parents had. For example, a night beauty flower with a genotype containing both white and red petals turns pink.

Mendel's second law - the law of segregation

So, we remember that when crossing two homozygous organisms, all offspring will take on only one trait. But what if we take two heterozygous organisms from this offspring and cross them? Will the offspring be uniform?

Let's get back to peas. Each parent is equally likely to pass on either gene A or gene a. Then the offspring will be divided as follows:

• AA - purple flowers (25%);
• aa - white flowers (25%);
• Aa - purple flowers (50%).

It can be seen that there are three times more organisms with purple flowers. This is a splitting phenomenon. This is the second law of Gregor Mendel: when heterozygous organisms are crossed, the offspring are split in a ratio of 3:1 in phenotype and 1:2:1 in genotype.

However, there are so-called lethal genes. If they are present, a deviation from the second law occurs. For example, the offspring of yellow mice are split in a 2:1 ratio.

The same thing happens with platinum-colored foxes. The fact is that if in the genotype of these (and some other) organisms both genes are dominant, then they simply die. As a result, a dominant gene can only be expressed if the organism is heterozyotic.

The law of gamete purity and its cytological basis

Let's take yellow peas and green peas, the yellow gene is dominant, and the green gene is recessive. The hybrid will contain both of these genes (although we will only see the manifestation of the dominant one).

It is known that genes are transferred from parent to offspring using gametes. A gamete is a sex cell. There are two genes in the hybrid genotype; it turns out that each gamete - and there are two of them - contained one gene. Having merged, they formed a hybrid genotype.

If in the second generation a recessive trait characteristic of one of the parent organisms appeared, then the following conditions were met:

• the hereditary factors of the hybrids did not change;
• each gamete contained one gene.

The second point is the law of gamete purity. Of course, there are not two genes, there are more of them. There is a concept of allelic genes. They are responsible for the same sign. Knowing this concept, we can formulate the law as follows: one randomly selected gene from an allele penetrates into the gamete.

The cytological basis of this rule: cells in which there are chromosomes containing pairs of alleles with all the genetic information, divide and form cells in which there is only one allele - haploid cells. In this case, these are gametes.

Mendel's third law - the law of independent inheritance

The fulfillment of the third law is possible with dihybrid crossing, when not one trait is studied, but several. In the case of peas, this is, for example, the color and smoothness of the seeds.

We denote the genes responsible for seed color as A (yellow) and a (green); for smoothness - B (smooth) and b (wrinkled). Let's try to carry out dihybrid crossing of organisms with different characteristics.

The first law is not violated during such crossing, that is, the hybrids will be identical in both genotype (AaBb) and phenotype (with yellow smooth seeds).

What will the split be like in the second generation? To find out, you need to find out what gametes the parent organisms can secrete. Obviously these are AB, Ab, aB and ab. After this, a circuit called a Pinnett lattice is constructed.

All the gametes that can be released by one organism are listed horizontally, and all the gametes that can be released by another are listed vertically. Inside the grid, the genotype of the organism that would appear with the given gametes is recorded.

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

If you study the table, you can come to the conclusion that the splitting of second-generation hybrids by phenotype occurs in the ratio 9:3:3:1. Mendel also realized this after conducting several experiments.

In addition, he also came to the conclusion that which of the genes of one allele (Aa) gets into the gamete does not depend on the other allele (Bb), that is, there is only independent inheritance of traits. This is his third law, called the law of independent inheritance.

Conclusion

Mendel's three laws are the basic genetic laws. Thanks to the fact that one person decided to experiment with peas, biology received a new section - genetics.

With its help, scientists from all over the world have learned many things, from disease prevention to genetic engineering. Genetics is one of the most interesting and promising branches of biology.

Genetics- the science of the laws of heredity and variability. The date of the “birth” of genetics can be considered 1900, when G. De Vries in Holland, K. Correns in Germany and E. Cermak in Austria independently “rediscovered” the laws of inheritance of traits established by G. Mendel back in 1865.

Heredity- the ability of organisms to transmit their characteristics from one generation to another.

Variability- the property of organisms to acquire new characteristics compared to their parents. In a broad sense, variability refers to differences between individuals of the same species.

Sign- any structural feature, any property of the body. The development of a trait depends both on the presence of other genes and on environmental conditions; the formation of traits occurs during the individual development of individuals. Therefore, each individual individual has a set of characteristics characteristic only of it.

Phenotype- the totality of all external and internal signs of the body.

Gene- a functionally indivisible unit of genetic material, a section of a DNA molecule encoding the primary structure of a polypeptide, transfer or ribosomal RNA molecule. In a broad sense, a gene is a section of DNA that determines the possibility of developing a separate elementary trait.

Genotype- a set of genes of an organism.

Locus- location of the gene on the chromosome.

Allelic genes- genes located in identical loci of homologous chromosomes.

Homozygote- an organism that has allelic genes of one molecular form.

Heterozygote- an organism that has allelic genes of different molecular forms; in this case, one of the genes is dominant, the other is recessive.

Recessive gene- an allele that determines the development of a trait only in the homozygous state; such a trait will be called recessive.

Dominant gene- an allele that determines the development of a trait not only in a homozygous, but also in a heterozygous state; such a trait will be called dominant.

Genetics methods

The main one is hybridological method- a system of crossings that allows one to trace the patterns of inheritance of traits over a series of generations. First developed and used by G. Mendel. Distinctive features of the method: 1) targeted selection of parents who differ in one, two, three, etc. pairs of contrasting (alternative) stable characteristics; 2) strict quantitative accounting of the inheritance of traits in hybrids; 3) individual assessment of the offspring from each parent in a series of generations.

Crossing in which the inheritance of one pair of alternative characters is analyzed is called monohybrid, two pairs - dihybrid, several pairs - polyhybrid. Alternative features are understood as different meanings of a feature, for example, the feature is the color of peas, alternative features are the color yellow, the green color of peas.

In addition to the hybridological method, the following are used in genetics: genealogical— compilation and analysis of pedigrees; cytogenetic— study of chromosomes; twin— study of twins; population-statistical method - studying the genetic structure of populations.

Genetic symbolism

Proposed by G. Mendel, used to record the results of crossings: P - parents; F - offspring, the number below or immediately after the letter indicates the serial number of the generation (F 1 - first generation hybrids - direct descendants of parents, F 2 - second generation hybrids - arise as a result of crossing F 1 hybrids with each other); × — crossing icon; G—male; E—female; A is a dominant gene, a is a recessive gene; AA is a homozygote for a dominant, aa is a homozygote for a recessive, Aa is a heterozygote.

The law of uniformity of first generation hybrids, or Mendel's first law

The success of Mendel's work was facilitated by the successful choice of the object for crossing - different varieties of peas. Features of peas: 1) it is relatively easy to grow and has a short development period; 2) has numerous offspring; 3) has a large number of clearly visible alternative characteristics (corolla color - white or red; cotyledon color - green or yellow; seed shape - wrinkled or smooth; pod color - yellow or green; pod shape - round or constricted; arrangement of flowers or fruits - along the entire length of the stem or at its top; stem height - long or short); 4) is a self-pollinator, as a result of which it has a large number of pure lines that stably retain their characteristics from generation to generation.

Mendel conducted experiments on crossing different varieties of peas for eight years, starting in 1854. On February 8, 1865, G. Mendel spoke at a meeting of the Brunn Society of Naturalists with a report “Experiments on plant hybrids,” where the results of his work were summarized.

Mendel's experiments were carefully thought out. If his predecessors tried to study the patterns of inheritance of many traits at once, Mendel began his research by studying the inheritance of just one pair of alternative traits.

Mendel took pea varieties with yellow and green seeds and artificially cross-pollinated them: he removed the stamens from one variety and pollinated them with pollen from another variety. The first generation hybrids had yellow seeds. A similar picture was observed in crosses in which the inheritance of other traits was studied: when crossing plants with smooth and wrinkled seed shapes, all the seeds of the resulting hybrids were smooth; when crossing red-flowered plants with white-flowered plants, all the resulting ones were red-flowered. Mendel came to the conclusion that in first-generation hybrids, of each pair of alternative characters, only one appears, and the second seems to disappear. Mendel called the trait manifested in first-generation hybrids dominant, and the suppressed trait recessive.

At monohybrid crossing of homozygous individuals having different values ​​of alternative characteristics, hybrids are uniform in genotype and phenotype.

Genetic diagram of Mendel's law of uniformity

(A is the yellow color of peas, and is the green color of peas)

Law of segregation, or Mendel's second law

G. Mendel gave the first generation hybrids the opportunity to self-pollinate. The second generation hybrids obtained in this way showed not only a dominant, but also a recessive trait. The experimental results are shown in the table.

Signs	Dominant		Recessive		Total
	Number	%	Number	%
Seed shape	5474	74,74	1850	25,26	7324
Coloring of cotyledons	6022	75,06	2001	24,94	8023
Seed coat color	705	75,90	224	24,10	929
Bob shape	882	74,68	299	25,32	1181
Bob coloring	428	73,79	152	26,21	580
Flower arrangement	651	75,87	207	24,13	858
Stem height	787	73,96	277	26,04	1064
Total:	14949	74,90	5010	25,10	19959

Analysis of the table data allowed us to draw the following conclusions:

1. There is no uniformity of hybrids in the second generation: some hybrids carry one (dominant), some - another (recessive) trait from an alternative pair;
2. the number of hybrids carrying a dominant trait is approximately three times greater than the number of hybrids carrying a recessive trait;
3. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation.

The phenomenon in which part of the second generation hybrids carries a dominant trait, and part - a recessive one, is called splitting. Moreover, the splitting observed in hybrids is not random, but is subject to certain quantitative patterns. Based on this, Mendel made another conclusion: when crossing hybrids of the first generation, the characteristics in the offspring are split in a certain numerical ratio.

At monohybrid crossing of heterozygous individuals in hybrids there is a cleavage according to phenotype in a ratio of 3:1, according to genotype 1:2:1.

Genetic diagram of Mendel's law of segregation

(A is the yellow color of peas, and is the green color of peas):

Law of gamete purity

From 1854, for eight years, Mendel conducted experiments on crossing pea plants. He discovered that as a result of crossing different varieties of peas with each other, the first generation hybrids have the same phenotype, and in the second generation hybrids, the characteristics are split in certain proportions. To explain this phenomenon, Mendel made a number of assumptions, which were called the “gamete purity hypothesis”, or the “gamete purity law”. Mendel suggested that:

1. some discrete hereditary factors are responsible for the formation of traits;
2. organisms contain two factors that determine the development of a trait;
3. during the formation of gametes, only one of a pair of factors enters each of them;
4. when male and female gametes merge, these hereditary factors do not mix (remain pure).

In 1909, V. Johansen called these hereditary factors genes, and in 1912, T. Morgan showed that they are located in chromosomes.

To prove his assumptions, G. Mendel used crossing, which is now called analyzing ( test cross- crossing an organism of an unknown genotype with an organism homozygous for a recessive). Mendel probably reasoned as follows: “If my assumptions are correct, then as a result of crossing F 1 with a variety that has a recessive trait (green peas), among the hybrids there will be half green peas and half yellow peas.” As can be seen from the genetic diagram below, he actually received a 1:1 split and was convinced of the correctness of his assumptions and conclusions, but he was not understood by his contemporaries. His report “Experiments on plant hybrids,” made at a meeting of the Brunn Society of Naturalists, was met with complete silence.

Cytological basis of Mendel's first and second laws

At the time of Mendel, the structure and development of germ cells had not been studied, so his hypothesis of the purity of gametes is an example of brilliant foresight, which later found scientific confirmation.

The phenomena of dominance and segregation of characters observed by Mendel are currently explained by the pairing of chromosomes, the divergence of chromosomes during meiosis and their unification during fertilization. Let us denote the gene that determines the yellow color by the letter A, and the green color by a. Since Mendel worked with pure lines, both organisms crossed are homozygous, that is, they carry two identical alleles of the seed color gene (AA and aa, respectively). During meiosis, the number of chromosomes is halved, and only one chromosome from a pair ends up in each gamete. Since homologous chromosomes carry the same alleles, all gametes of one organism will contain a chromosome with gene A, and of the other - with gene a.

During fertilization, the male and female gametes fuse and their chromosomes combine to form a single zygote. The resulting hybrid becomes heterozygous, since its cells will have the Aa genotype; one variant of the genotype will give one variant of the phenotype - the yellow color of peas.

In a hybrid organism that has the Aa genotype during meiosis, the chromosomes separate into different cells and two types of gametes are formed - half of the gametes will carry gene A, the other half will carry gene a. Fertilization is a random and equally probable process, that is, any sperm can fertilize any egg. Since two types of sperm and two types of eggs were formed, four types of zygotes are possible. Half of them are heterozygotes (carry the A and a genes), 1/4 are homozygous for a dominant trait (carry two A genes) and 1/4 are homozygous for a recessive trait (carry two a genes). Homozygotes for the dominant and heterozygotes will produce yellow peas (3/4), homozygotes for the recessive - green (1/4).

The law of independent combination (inheritance) of characteristics, or Mendel's third law

Organisms differ from each other in many ways. Therefore, having established the patterns of inheritance of one pair of traits, G. Mendel moved on to studying the inheritance of two (or more) pairs of alternative traits. For dihybrid crosses, Mendel took homozygous pea plants that differed in seed color (yellow and green) and seed shape (smooth and wrinkled). The yellow color (A) and smooth shape (B) of the seeds are dominant traits, the green color (a) and wrinkled shape (b) are recessive traits.

By crossing a plant with yellow and smooth seeds with a plant with green and wrinkled seeds, Mendel obtained a uniform hybrid generation F 1 with yellow and smooth seeds. From self-pollination of 15 first-generation hybrids, 556 seeds were obtained, of which 315 were yellow smooth, 101 yellow wrinkled, 108 green smooth and 32 green wrinkled (splitting 9:3:3:1).

Analyzing the resulting offspring, Mendel drew attention to the fact that: 1) along with combinations of characteristics of the original varieties (yellow smooth and green wrinkled seeds), during dihybrid crossing new combinations of characteristics appear (yellow wrinkled and green smooth seeds); 2) splitting for each individual trait corresponds to splitting during monohybrid crossing. Of the 556 seeds, 423 were smooth and 133 wrinkled (ratio 3:1), 416 seeds were yellow in color, and 140 were green (ratio 3:1). Mendel came to the conclusion that splitting in one pair of traits is not associated with splitting in the other pair. Hybrid seeds are characterized not only by combinations of characteristics of parent plants (yellow smooth seeds and green wrinkled seeds), but also by the emergence of new combinations of characteristics (yellow wrinkled seeds and green smooth seeds).

When dihybrid crossing diheterozygotes in hybrids, there is a cleavage according to the phenotype in the ratio 9:3:3:1, according to the genotype in the ratio 4:2:2:2:2:1:1:1:1, the characters are inherited independently of each other and are combined in all possible combinations.

R	♀AABB yellow, smooth	×	♂aаbb green, wrinkled
Types of gametes	AB		ab
F 1	AaBb yellow, smooth, 100%
P	♀AaBb yellow, smooth	×	♂AаBb yellow, smooth
Types of gametes	AB Ab aB ab		AB Ab aB ab

Genetic scheme of the law of independent combination of traits:

Gametes:	♂	AB	Ab	aB	ab
♀
AB		AABB yellow smooth	AABb yellow smooth	AaBB yellow smooth	AaBb yellow smooth
Ab		AABb yellow smooth	AAbb yellow wrinkled	AaBb yellow smooth	Aabb yellow wrinkled
aB		AaBB yellow smooth	AaBb yellow smooth	aaBB green smooth	aaBb green smooth
ab		AaBb yellow smooth	Aabb yellow wrinkled	aaBb green smooth	aabb green wrinkled

Analysis of crossbreeding results by phenotype: yellow, smooth - 9/16, yellow, wrinkled - 3/16, green, smooth - 3/16, green, wrinkled - 1/16. Phenotype splitting is 9:3:3:1.

Analysis of crossbreeding results by genotype: AaBb - 4/16, AABb - 2/16, AaBB - 2/16, Aabb - 2/16, aaBb - 2/16, AABB - 1/16, Aabb - 1/16, aaBB - 1/16, aabb - 1/16. Genotype splitting is 4:2:2:2:2:1:1:1:1.

If in a monohybrid crossing the parent organisms differ in one pair of characters (yellow and green seeds) and give in the second generation two phenotypes (2 1) in the ratio (3 + 1) 1, then in a dihybrid they differ in two pairs of characters and give in the second generation four phenotypes (2 2) in the ratio (3 + 1) 2. It is easy to calculate how many phenotypes and in what ratio will be formed in the second generation during a trihybrid cross: eight phenotypes (2 3) in the ratio (3 + 1) 3.

If the splitting by genotype in F 2 with a monohybrid generation was 1: 2: 1, that is, there were three different genotypes (3 1), then with a dihybrid crossing 9 different genotypes are formed - 3 2, with a trihybrid crossing 3 3 - 27 different genotypes are formed.

Mendel's third law is valid only for those cases when the genes for the analyzed traits are located in different pairs of homologous chromosomes.

Cytological basis of Mendel's third law

Let A be the gene that determines the development of yellow color of seeds, a - green color, B - smooth shape of the seed, b - wrinkled. First generation hybrids with genotype AaBb are crossed. During the formation of gametes, from each pair of allelic genes, only one gets into the gamete, and as a result of random divergence of chromosomes in the first division of meiosis, gene A can end up in the same gamete with gene B or gene b, and gene a - with gene B or gene b. Thus, each organism produces four types of gametes in the same quantity (25%): AB, Ab, aB, ab. During fertilization, each of the four types of sperm can fertilize any of the four types of eggs. As a result of fertilization, nine genotypic classes may appear, which will give rise to four phenotypic classes.

Go to lectures No. 16“Ontogenesis of multicellular animals that reproduce sexually”

Go to lectures No. 18"Chained inheritance"

Hybridization - This is the crossing of individuals that differ in genotype. A crossing in which one pair of alternative traits is taken into account in the parent individuals is called monohybrid; two pairs of traits are called dihybrid, more than two pairs - polyhybrid.

Crossing of animals and plants (hybridization) has been carried out by humans since time immemorial, but it has not been possible to establish patterns of transmission of hereditary characteristics. The hybridological method of G. Mendel, with the help of which these patterns were identified, has the following features:

▪ selection of pairs for crossing ("pure lines");

▪ analysis of the inheritance of individual alternative (mutually exclusive) traits in a series of generations;

▪ accurate quantitative accounting of descendants with different combinations of characteristics (use of mathematical methods).

Mendel's first law is the law of uniformity of first generation hybrids. G. Mendel crossed pure lines of pea plants with yellow and green seeds (alternative traits). Clean lines- these are organisms that do not produce splitting when crossed with the same genotype, i.e., they are homozygous for this trait:

When analyzing the results of crossing, it turned out that all descendants (hybrids) in the first generation are identical in phenotype (all plants had yellow peas) and genotype (heterozygotes). Mendel's first law is formulated as follows: when crossing homozygous individuals analyzed for one pair of alternative traits, uniformity of the first generation hybrids is observed both in phenotype and genotype.

Mendel's second law is the law of splitting. When crossing first generation hybrids, i.e. heterozygous individuals, the following result is obtained:

Individuals containing the dominant gene A have yellow seeds, and those containing both recessive genes have green seeds. Consequently, the ratio of individuals by phenotype (seed color) is 3:1 (3 parts with a dominant trait and 1 part with a recessive trait), by genotype: 1 part of individuals - yellow homozygotes (AA), 2 parts - yellow heterozygotes (Aa) and 1 part - green homozygotes (aa). Mendel's second law is formulated as follows: When crossing first-generation hybrids (heterozygous organisms) analyzed for one pair of alternative traits, a splitting ratio of 3:1 by phenotype and 1:2:1 by genotype is observed.

During experimental and selection work, quite often the need arises to find out the genotype of an individual with a dominant trait. For this purpose they carry out test cross: the test individual is crossed with a recessive homozygote. If she was homozygous, then the first generation hybrids will be uniform - all descendants will have a dominant

Patterns of inheritance 79

sign. If the individual was heterozygous, then as a result of crossing, the characteristics of the descendants are split in a 1:1 ratio:

Sometimes (usually when obtaining clean lines) they use backcrossing- crossing offspring with one of the parents. In some cases (when studying the linkage of genes) reciprocal crossing- crossing of two parental individuals (for example, AaBb and aabb), in which first the maternal individual is heterozygous, and the paternal one is recessive, and then vice versa (crossings P: AaBb x aabb and P: aabb x AaBb).

Having studied the inheritance of one pair of alleles, Mendel decided to trace the inheritance of two traits simultaneously. For this purpose, he used homozygous pea plants, differing in two pairs of alternative characters: smooth yellow seeds and green wrinkled seeds. As a result of such crossing in the first generation, he obtained plants with yellow smooth seeds. This result showed that the law of uniformity of first-generation hybrids manifests itself not only in monohybrid, but also in polyhybrid crossings if the parental forms are homozygous:

Then Mendel crossed the first generation hybrids with each other - P(F 1): AaBb x AaBb.

To analyze the results of polyhybrid crossings, they usually use Punnett grid, in which female gametes are written horizontally, and male gametes vertically:

As a result of the free combination of gametes in zygotes, different combinations of genes are obtained. It is easy to calculate that according to the phenotype, the offspring are divided into 4 groups: 9 parts of plants with yellow smooth peas (A-B-), 3 parts with yellow wrinkled peas (A-bb), 3 parts with green smooth peas (aaB-) and 1 part is green wrinkled (aabb), i.e. splitting occurs in the ratio 9:3:3:1, or (3+1) 2. From this we can conclude that when crossing heterozygous individuals analyzed for several pairs of alternative traits, the offspring exhibit phenotypic cleavage in the ratio (3+1) n, where n is the number of analyzed traits.

It is convenient to record the results of crossing using phenotypic radical- a brief record of the genotype made on the basis of the phenotype. For example, the notation A-B- means that if the genotype contains at least one dominant gene from a pair of allelic ones, then, regardless of the second gene, a dominant trait will appear in the phenotype.

If we analyze the splitting for each pair of characters (yellow and green color, smooth and wrinkled surface), we get 12 individuals with yellow (smooth) and 4 individuals with green (wrinkled) seeds. Their ratio is 12:4, or 3:1. Therefore, in a dihybrid cross, each pair of traits in the offspring produces segregation independently of the other pair. This is the result of random combinations of genes (and their corresponding traits), resulting in new combinations of traits that were not present in the parental forms. In our example, the initial forms of peas had yellow smooth and green wrinkled seeds, and in the second generation plants were obtained not only with a combination of parental characteristics, but also with new combinations - yellow wrinkled and green smooth seeds. this implies

Mendel's third law - the law of independent combination of characteristics . When crossing homozygous organisms analyzed for two (or more) pairs of alternative traits, in the second generation an independent combination of genes of different allelic pairs and their corresponding traits is observed.

Analyzing the results of the splitting of characters in the second generation (the appearance of recessive homozygotes), Mendel came to the conclusion that in the heterozygous state, hereditary factors do not mix and do not change each other. Subsequently, this idea received a cytological substantiation (divergence of homologous chromosomes during meiosis) and was called the "gamete purity" hypothesis(W. Bateson, 1902). It can be reduced to the following two main provisions:

▪ in a hybrid organism, genes do not hybridize (do not mix), but are in a pure allelic state;

▪ from an allelic pair, only one gene enters the gamete due to the divergence of homologous chromosomes and chromatids during meiosis.

Mendel's laws are statistical in nature (they are carried out on a large number of individuals) and are universal, i.e. they are inherent in all living organisms. For Mendel's laws to manifest, the following conditions must be met:

▪ genes of different allelic pairs must be located in different pairs of homologous chromosomes;

▪ there should be no linkage or interaction between genes, other than complete dominance;

▪ there must be an equal probability of the formation of gametes and zygotes of different types, as well as an equal probability of survival of organisms with different genotypes (there should be no lethal genes).

The independent inheritance of genes of different allelic pairs is based on the genetic level of organization of the hereditary material, which consists in the fact that the genes are relatively independent of each other.

Deviations from the expected segregation according to Mendel's laws cause lethal genes. For example, when crossing heterozygous Karakul sheep, the segregation in F) is 2:1 (instead of the expected 3:1). Lambs homozygous for the dominant gray allele (W) are not viable and die due to underdevelopment of the stomach rumen:

In a similar way, humans inherit brachydactyly And sickle cell anemia. The gene for brachydactyly (short thick fingers) is dominant. Heterozygotes exhibit brachydactyly, and homozygotes for this gene die in the early stages of embryogenesis. A person has a gene for normal hemoglobin (HbA) and a gene for sickle cell anemia (HbS). Heterozygotes for these genes are viable, but homozygotes for HbS die in early childhood (hemoglobin S is not able to bind and carry oxygen).

Difficulties in interpreting the results of crossing (deviations from Mendel’s laws) can also be caused by the phenomenon of pleiotropy, when one gene is responsible for the manifestation of several traits. Thus, in homozygous gray Karakul sheep, the W gene determines not only the gray color of the coat, but also the underdevelopment of the digestive system. Examples of pleiotropic gene action in humans are Marfan and blue sclera syndromes. In Marfan syndrome, one gene causes the development of spider toes, lens subluxation, chest deformation, aortic aneurysm, and high arches. With blue sclera syndrome, a person experiences blue discoloration of the sclera, brittle bones, and heart defects.

With pleiotropy, there is probably a deficiency of enzymes that are active in several types of tissues or in one, but widespread. Marfan syndrome appears to be based on the same defect in the development of connective tissue.

Diagram of Mendel's first and second laws. 1) A plant with white flowers (two copies of the recessive allele w) is crossed with a plant with red flowers (two copies of the dominant allele R). 2) All descendant plants have red flowers and the same genotype Rw. 3) When self-fertilization occurs, 3/4 of the plants of the second generation have red flowers (genotypes RR + 2Rw) and 1/4 have white flowers (ww).

Mendel called the manifestation of the trait of only one of the parents in hybrids as dominance.

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

This law is also known as the "law of trait dominance." Its formulation is based on the concept clean line regarding the trait being studied - in modern language this means homozygosity of individuals for this trait. Mendel formulated the purity of a character as the absence of manifestations of opposite characters in all descendants in several generations of a given individual during self-pollination.

When crossing pure lines of purple-flowered peas and white-flowered peas, Mendel noticed that the descendants of the plants that emerged were all purple-flowered, with not a single white one among them. Mendel repeated the experiment more than once and used other signs. If he crossed peas with yellow and green seeds, all the offspring would have yellow seeds. If he crossed peas with smooth and wrinkled seeds, the offspring would have smooth seeds. The offspring from tall and short plants were tall. So, first-generation hybrids are always uniform in this characteristic and acquire the characteristic of one of the parents. This sign (stronger, dominant), always suppressed the other ( recessive).

Codominance and incomplete dominance

Some opposing characters are not in the relation of complete dominance (when one always suppresses the other in heterozygous individuals), but in the relation incomplete dominance. For example, when pure snapdragon lines with purple and white flowers are crossed, the first generation individuals have pink flowers. When pure lines of black and white Andalusian chickens are crossed, gray chickens are born in the first generation. With incomplete dominance, heterozygotes have characteristics intermediate between those of recessive and dominant homozygotes.

The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation.

Explanation

Law of gamete purity: each gamete contains only one allele from a pair of alleles of a given gene of the parent individual.

Normally, the gamete is always pure from the second gene of the allelic pair. This fact, which could not be firmly established in Mendel's time, is also called the gamete purity hypothesis. This hypothesis was later confirmed by cytological observations. Of all the laws of inheritance established by Mendel, this “Law” is the most general in nature (it is fulfilled under the widest range of conditions).

Law of independent inheritance of characteristics

Definition

Law of independent inheritance(Mendel’s third law) - when crossing two homozygous individuals that differ from each other in two (or more) pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations (as in monohybrid crossing). When plants differing in several characters, such as white and purple flowers and yellow or green peas, were crossed, the inheritance of each character followed the first two laws and in the offspring they were combined in such a way as if their inheritance occurred independently of each other. The first generation after crossing had a dominant phenotype for all traits. In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1, that is, 9:16 were with purple flowers and yellow peas, 3:16 were with white flowers and yellow peas, 3:16 were with purple flowers and green peas, 1 :16 with white flowers and green peas.

Explanation

Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes. During meiosis, homologous chromosomes of different pairs are randomly combined in gametes. If the paternal chromosome of the first pair gets into the gamete, then with equal probability both the paternal and maternal chromosomes of the second pair can get into this gamete. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. (It later turned out that of the seven pairs of characters studied by Mendel in the pea, which has a diploid number of chromosomes 2n=14, the genes responsible for one of the pairs of characters were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since as linkage between these genes was not observed due to the large distance between them).

Basic provisions of Mendel's theory of heredity

In modern interpretation, these provisions are as follows:

• Discrete (separate, non-mixable) hereditary factors - genes are responsible for hereditary traits (the term “gene” was proposed in 1909 by V. Johannsen)
• Each diploid organism contains a pair of alleles of a given gene responsible for a given trait; one of them is received from the father, the other from the mother.
• Hereditary factors are transmitted to descendants through germ cells. When gametes are formed, each of them contains only one allele from each pair (the gametes are “pure” in the sense that they do not contain the second allele).

Conditions for the fulfillment of Mendel's laws

According to Mendel's laws, only monogenic traits are inherited. If more than one gene is responsible for a phenotypic trait (and the absolute majority of such traits), it has a more complex pattern of inheritance.

Conditions for fulfilling the law of segregation during monohybrid crossing

Splitting 3:1 by phenotype and 1:2:1 by genotype is performed approximately and only under the following conditions:

1. A large number of crosses (large number of offspring) are studied.
2. Gametes containing alleles A and a are formed in equal numbers (have equal viability).
3. There is no selective fertilization: gametes containing any allele fuse with each other with equal probability.
4. Zygotes (embryos) with different genotypes are equally viable.

Conditions for the implementation of the law of independent inheritance

1. All conditions necessary for the fulfillment of the law of splitting.
2. The location of the genes responsible for the traits being studied is in different pairs of chromosomes (unlinked).

Conditions for fulfilling the law of gamete purity

The normal course of meiosis. As a result of chromosome nondisjunction, both homologous chromosomes from a pair can end up in one gamete. In this case, the gamete will carry a pair of alleles of all genes that are contained in a given pair of chromosomes.

Dubinin N.P. General genetics. - M.: “Science”, 1986