Repair

Dihybrid crossing. Examples of solving typical problems

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

Patterns of heredity, their cytological basis

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

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

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

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

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

Other fundamental confirmation of the cytological basis of heredity was obtained from the study of various diseases. Thus, in humans, one form of cancer is caused by the loss of a small section of one of the chromosomes.

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

The basic patterns of independent inheritance of traits were discovered by G. Mendel, who achieved success by using a new hybridological method in his research at that time.

The success of G. Mendel was ensured by the following factors:

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

For the study, G. Mendel selected only seven traits that had alternative (contrasting) manifestations. Already in the first crosses, he noticed that in the offspring of the first generation, when crossing plants with yellow and green seeds, all the offspring had yellow seeds. Similar results were obtained when studying other characteristics. The traits that predominated in the first generation were called by G. Mendel dominant. Those of them that did not appear in the first generation were called recessive.

Individuals that produced cleavage in their offspring were called heterozygous, and individuals that did not split - homozygous.

Traits of peas, the inheritance of which was studied by G. Mendel

A cross in which the manifestation of only one trait is studied is called monohybrid. In this case, patterns of inheritance of only two variants of one trait can be traced, the development of which is determined by a pair of allelic genes. For example, the trait “flower corolla color” in peas has only two manifestations - red and white. All other characteristics characteristic of these organisms are not taken into account and are not taken into account in the calculations.

The monohybrid crossing scheme is as follows:

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

Mendel's first law:

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

A - yellow seeds; A- green seeds.

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

Mendel's second law:

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

However, it is not always possible to determine its genotype from the phenotype of an individual, since they are homozygous for the dominant gene ( AA), and heterozygotes ( Ahh) will have a manifestation of a dominant gene in the phenotype. Therefore, for organisms with cross-fertilization, they use test cross- a cross in which an organism with an unknown genotype is crossed with a homozygote for a recessive gene to test the genotype. At the same time, homozygous individuals for the dominant gene do not produce segregation in the offspring, while in the offspring of heterozygous individuals there is an equal number of individuals with both dominant and recessive traits:

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

Gamete purity rule:

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

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

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

Dihybrid crossing scheme:

With a greater diversity of gametes, determining the genotypes of descendants becomes difficult, so the Punnett grid is widely used for analysis, into which male gametes are entered horizontally and female gametes vertically. The genotypes of the offspring are determined by the combination of genes in the columns and rows.

$♀$/$♂$	aB	ab
AB	AaBB	AaBb
Ab	AaBb	Aabb

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

Mendel's third law:

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

$♀$/$♂$	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

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

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

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

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

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

However, after analyzing the crossing of first-generation hybrids with homozygotes for recessive genes, T. Morgan discovered that 41.5% of individuals had a gray body and long wings, 41.5% had a black body and rudimentary wings, 8.5% had a gray body and rudimentary wings, and 8.5% - black body and rudimentary wings. He associated the resulting split with crossing over occurring in prophase I of meiosis and proposed that the unit of distance between genes on a chromosome be considered 1% crossing over, which was later named in his honor Morganida.

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

Morgan's Law:

Genes localized on the same chromosome occupy a specific place called a locus and are inherited linked, with the strength of linkage inversely proportional to the distance between genes.

Genes located on the chromosome directly next to each other (the probability of crossing over is extremely low) are called fully linked, and if there is at least one more gene between them, then they are not completely linked and their linkage is broken during crossing over as a result of the exchange of sections of homologous chromosomes.

The phenomena of gene linkage and crossing over make it possible to construct maps of chromosomes with the order of gene arrangement marked on them. Genetic maps of chromosomes have been created for many genetically well-studied objects: fruit flies, mice, humans, corn, wheat, peas, etc. The study of genetic maps allows us to compare the genome structure of different species of organisms, which is important for genetics and selection, as well as evolutionary studies .

Genetics of sex

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

The characteristics by which one sex differs from the other are divided into primary and secondary. Primary sexual characteristics include the genitals, and all others are secondary.

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

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

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

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

A man's karyotype is written as $♂$ 46, XY, and a woman's karyotype is written as $♀$ 46, XX.

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

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

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

Inheritance of sex-linked traits

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

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

X H - normal blood clotting

X h - blood incoagulability (hemophilia)

Traits encoded in the genes of the Y chromosome are transmitted purely through the male line and are called holandric(presence of membranes between the toes, increased hair growth on the edge of the auricle).

Gene interaction

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

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

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

Co-dominance call this type of interaction of allelic genes in which both alleles appear without weakening the effects of each other. A typical example of codominance is the inheritance of blood groups according to the ABO system.

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

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

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

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

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

Polymeria is a phenomenon in which non-allelic genes have the same effect on the development of a trait. This is how quantitative characteristics are most often encoded. For example, human skin color is determined by at least four pairs of non-allelic genes - the more dominant alleles in the genotype, the darker the skin.

Genotype as an integral system

The genotype is not a mechanical sum of genes, since the possibility of a gene’s manifestation and the form of its manifestation depend on environmental conditions. In this case, the environment refers not only to the environment, but also to the genotypic environment—other genes.

The manifestation of qualitative traits rarely depends on environmental conditions, although if you shave an area of the body with white hair on an ermine rabbit and apply an ice pack to it, then over time black hair will grow in this place.

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

Thus, only the “abilities” of the organism are recorded in the genotype, but they manifest themselves only in interaction with environmental conditions.

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

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

Human genetics

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

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

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

Methods for studying human genetics

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

Twin method helps determine the contribution of heredity and the influence of environmental conditions on the manifestation of a trait based on an analysis of the coincidence of these traits in identical and fraternal twins. Thus, most identical twins have the same blood type, eye and hair color, as well as a number of other characteristics, while both types of twins suffer from measles at the same time.

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

Genealogical method is a method of compiling pedigrees, with the help of which the nature of inheritance of the characteristics being studied, including hereditary diseases, is determined, and the birth of descendants with the corresponding characteristics is predicted. It made it possible to identify the hereditary nature of diseases such as hemophilia, color blindness, Huntington's chorea, etc. even before the discovery of the basic laws of heredity. When compiling pedigrees, records are kept about each family member and the degree of relationship between them is taken into account. Next, based on the data obtained, a family tree is built using special symbols.

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

Solving genetic problems. Drawing up crossing schemes

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

Calculation tasks.
Problems to determine the genotype.
Tasks to establish the type of inheritance of a trait.

Feature calculation problems is the availability of information about the inheritance of the trait and the phenotypes of the parents, from which it is easy to determine the genotypes of the parents. They require establishing the genotypes and phenotypes of the offspring.

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

Solution.

1. We designate genes:

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

2. We draw up a crossing scheme:

a) first we write down the genotypes of the parents, who, according to the conditions of the problem, are homozygous:

$P (♀AA)↙(\text"dark seeds")×(♂aa)↙(\text"light seeds")$

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

Gametes A a

c) we merge the gametes in pairs and record the genotypes of the descendants:

F 1 A A

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

Phenotype dark seeds

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

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

Task 2. In rats, black coat color dominates over brown, and normal tail length dominates over short tail. How many second-generation offspring from crossing homozygous rats with black fur and a normal tail with homozygous rats with brown fur and a short tail had black fur and a short tail, if a total of 80 rat pups were born?

Solution.

1. Write down the condition of the problem:

A - black wool, A- brown wool;

B - normal tail length, b- shortened tail.

F 2 A_ bb ?

2. Write down the crossing scheme:

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

From the Punnett grid it follows that the proportion of rat pups with black fur and a short tail was 3/16.

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

80 × 3/16 × 15.

Answer: 15 rat pups had black fur and a short tail.

IN genotype determination tasks The nature of inheritance of the trait is also given and the task is set to determine the genotypes of the offspring from the genotypes of the parents or vice versa.

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

Solution.

1. Let us recall the nature of inheritance of blood groups:

Inheritance of blood groups according to the AB0 system

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

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

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

Thus, our assumptions were confirmed.

Answer: the parents are heterozygous for blood group genes: the mother’s genotype is I A i, the father’s genotype is I B i.

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

Solution.

1. We designate genes:

X D - normal color vision;

X d - color blindness.

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

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

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

IN tasks to determine the nature of inheritance of a trait Only the phenotypes of parents and offspring are given. The issue of such tasks is precisely to clarify the nature of inheritance of a trait.

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

Solution.

1. Determine splitting in the offspring:

161: 79 $≈$ 2: 1.

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

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

C - short legs, s - long legs.

3. We write down the crossing scheme:

Our assumptions were confirmed.

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

Dihybrid crossing. Examples of solving typical problems

Task 1. In humans, complex forms of myopia dominate over normal vision, and brown eye color dominates over blue. A brown-eyed, myopic man, whose mother had blue eyes and normal vision, married a blue-eyed woman with normal vision. What is the percentage chance of having a child with the mother's characteristics?

Solution

Gene Trait

A development of myopia

a normal vision

B brown eyes

b Blue eyes

P ♀ aabb x ♂ AaBb

G ab, AB, Ab aB, ab

F 1 AaBb; Aabb; aaBb; aabb

Answer: a child with the aabb genotype has blue eyes and normal vision. The probability of having a child with these signs is 25%.

Problem 2. In humans, red hair color dominates over light brown hair, and freckles dominate over their absence. A heterozygous red-haired man without freckles married a fair-haired woman with freckles. Determine the % probability of having a red-haired child with freckles.

Solution

Gene Trait

A red hair

a brown hair

B presence of freckles

b absence of freckles

P ♀ Aabb x ♂ aaBB

F 1 AaBb; aaBb

A red-haired child with freckles has the genotype AaBb. The probability of having such a child is 50%.

Answer: There is a 50% chance of having a red-haired baby with freckles.

Problem 3. A heterozygous woman with a normal hand and freckles marries a six-fingered heterozygous man who does not have freckles. What is the probability of having a child with a normal hand and no freckles?

Solution

Gene Trait

A six-fingered (polydactyly),

a normal hand

B presence of freckles

b no freckles

P ♀ aaBb x ♂ Aabb

G aB, ab, Ab, ab

F 1 AaBb; Aabb; aaBb; aabb

Answer: the probability of having a child with the aabb genotype (with a normal hand, without freckles) is 25%.

Problem 4. The genes that determine a predisposition to cataracts and red hair are located on different pairs of chromosomes. A red-haired woman with normal vision married a fair-haired man with cataracts. What phenotypes can they have children with if the man’s mother has the same phenotype as his wife?

Solution

Gene Trait

A blonde hair,

a red hair

B development of cataracts

b normal vision

P ♀ aabb x ♂ AaBb

G ab, AB, Ab, aB, ab

F 1 AaBb; Aabb; aaBb; aabb

Answer: phenotypes of children – fair-haired with cataracts (AaBb); fair-haired without cataracts (Aabb); red-haired with cataracts (aaBb); red-haired without cataracts (aabb).

Task 5. What is the percentage probability of having a child with diabetes mellitus if both parents are carriers of the recessive gene for diabetes mellitus. In this case, the mother’s blood Rh factor is positive, and the father’s is negative. Both parents are homozygous for the gene that determines the development of the Rh factor. What Rh factor will the children of this couple have?

Solution

Gene Trait

A normal carbohydrate metabolism

a development of diabetes mellitus

Rh+ Rh positive blood

rh- Rh negative blood.

P♀ AaRh + Rh + x ♂ Aarh - rh -

G ARh + , aRh + , Arh - ,arh -

F 1 AARh + rh - ; AaRh + rh - ; AaRh + rh - ; aaRh + rh-

Answer: the probability of having a child with diabetes is 25%; all children in this family will have a positive Rh factor.

Problem 6. Normal growth in oats dominates over gigantism, early ripening over late ripening. The genes for both traits are located on different pairs of chromosomes. What percentage of late-ripening plants of normal growth can be expected from crossing plants heterozygous for both traits?

Solution

P♀AaBb x ♂AaBb

G AB, Ab, AB, Ab,

In humans, dark hair color (A) dominates light color (a), brown eye color (B) dominates blue color (b). Write down the genotypes of the parents, possible phenotypes and genotypes of the children born from the marriage of a fair-haired, blue-eyed man and a heterozygous brown-eyed, fair-haired woman.

Answer

Blonde-haired blue-eyed man aabb.
Heterozygous brown-eyed blonde woman aaBb.

Congenital myopia is inherited as an autosomal dominant trait, while the absence of freckles is inherited as an autosomal recessive trait. Traits are found on different pairs of chromosomes. The father has congenital myopia and no freckles, the mother has normal vision and freckles. There are three children in the family, two are nearsighted without freckles, one with normal vision and with freckles. Make a diagram for solving the problem. Determine the genotypes of parents and born children. Calculate the probability of having children who are nearsighted and have freckles. Explain what law applies in this case.

Answer

A - congenital myopia, and - normal vision.
B - freckles, b - absence of freckles.

Father A_bb, mother aaB_.
Children A_bb, aaB_.

If the father is bb, then all his children have b, which means the second child is aaBb.
If the mother is aa, then all her children have a, so the first child is Aabb.
If the first child has bb, then he took one b from his mother and one from his father, so the mother is aaBb.
If the second child has aa, then he took one a from the mother and one from the father, then the father is Aabb.

The probability of having nearsighted children with freckles is 25%, the law of independent inheritance works.

Parents with a loose earlobe and a triangular dimple on the chin gave birth to a child with a fused earlobe and a smooth chin. Determine the genotypes of the parents, the first child, phenotypes and genotypes of other possible offspring. Make a diagram for solving the problem. Traits are inherited independently.

Answer

The offspring showed recessive traits that were latent in the parents.

A - free earlobe, a - fused earlobe.
B - triangular fossa on the chin, b - smooth chin.

Child aabb, parents A_B_.
Child aa received one a from his father, the other from his mother; one b from the father, the other from the mother, therefore, the parents are AaBb.

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

9 A_B_ free earlobe, triangular dimple on the chin
3 A_bb loose earlobe, smooth chin
3 aaB_ fused earlobe, triangular dimple on the chin
1 aabb fused earlobe, smooth chin

A black crested rooster is crossed with the same hen. From them 20 chickens were obtained: 10 black crested, 5 brown crested, 3 black without crest and 2 brown without crest. Determine the genotypes of parents, offspring and the pattern of inheritance of traits. The genes of the two traits are not linked, the dominant traits are black plumage (A), crestedness (B).

Answer

A - black plumage, and - brown plumage.
B - crested, b - without crested.

Rooster A_B_, hen A_B_.
Chickens A_B_ 10 pcs., aaB_ 5 pcs., A_bb 3 pcs., aabb 2 pcs.

If a child has aa, then he took one a from his mother and one from his father, which means the parents are AaB_.
If a child has bb, then he took one b from his mother and one from his father, which means the parents are AaBb.

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

9 A_B_ black crested
3 A_bb black without crest
3 aaB_ brown crested
1 aabb brown without crest

The pattern of inheritance of characteristics is the law of independent inheritance.

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

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

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

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

Basic terms of genetics

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

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

Homozygous organism- an organism that does not split according to one or another characteristic. Its allelic genes equally influence the development of this trait.

Heterozygous organism- an organism that produces cleavage according to certain characteristics. Its allelic genes have different effects on the development of this trait.

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

Recessive gene is responsible for a trait whose development is suppressed by a dominant gene. A recessive trait occurs in a homozygous organism containing two recessive genes.

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

Phenotype- the totality of all the characteristics of an organism.

G. Mendel's laws

Mendel's first law - the law of hybrid uniformity

This law was derived based on the results of monohybrid crosses. For the experiments, two varieties of peas were taken, differing from each other in one pair of characteristics - the color of the seeds: one variety was yellow in color, the second was green. The crossed plants were homozygous.

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

Yellow color of seeds
- green color of seeds

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

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

Mendel's second law - the law of segregation

Plants were grown from seeds obtained by crossing a homozygous plant with yellow colored seeds with a plant with green colored seeds and obtained by self-pollination.



	(plants have a dominant trait - recessive)

Statement of the law: in the offspring obtained from crossing first-generation hybrids, there is a split in phenotype in the ratio , and in genotype -.

Mendel's third law - the law of independent inheritance

This law was derived from data obtained from dihybrid crosses. Mendel considered the inheritance of two pairs of characteristics in peas: color and seed shape.

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

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



	(yellow smooth).

Mendel then grew plants from seeds and obtained second-generation hybrids through self-pollination.

The Punnett grid is used to record and determine genotypes

Gametes

There was a split into phenotypic classes in the ratio. All seeds had both dominant traits (yellow and smooth), - the first dominant and second recessive (yellow and wrinkled), - the first recessive and second dominant (green and smooth), - both recessive traits (green and wrinkled).

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

Statement of the law: when crossing organisms 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 combined in all possible combinations.

Mendel's third law is true only if the genes are located in different pairs of homologous chromosomes.

Law (hypothesis) of “purity” of gametes

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

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

Analysis cross

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

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

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

Inheritance of blood groups (AB0 system)

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

Inheritance of sex-linked traits

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

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

The Unified State Exam includes tasks only for traits linked to the - chromosome. They mainly concern two human characteristics: blood clotting (- normal; - hemophilia), color vision (- normal, - color blindness). Tasks on the inheritance of sex-linked traits in birds are much less common.

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

Typical USE tasks in genetics

Determination of the number of gamete types

The number of gamete types is determined using the formula: , where is the number of gene pairs in the heterozygous state. For example, an organism with a genotype does not have genes in a heterozygous state, i.e. , therefore, and it forms one type of gametes. An organism with a genotype has one pair of genes in a heterozygous state, i.e. , therefore, and it forms two types of gametes. An organism with a genotype has three pairs of genes in a heterozygous state, i.e. , therefore, and it forms eight types of gametes.

Mono- and dihybrid crossing problems

For monohybrid crossing

Task: Crossed white rabbits with black rabbits (black color is the dominant trait). In white and black. Determine the genotypes of parents and offspring.

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

	(black)	(white)

	(black) : (white)

For dihybrid crossing

Dominant genes are known

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

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

Let's analyze the inheritance of each trait separately. All descendants have normal growth, i.e. no segregation for this trait is observed, therefore the initial forms are homozygous. Segregation is observed in fruit color, so the original forms are heterozygous.

		(dwarfs, red fruits)

	(normal growth, red fruits) (normal growth, red fruits) (normal growth, red fruits) (normal growth, yellow fruits)

Dominant genes unknown

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

Solution: Let's analyze the splitting for each characteristic separately. Among the descendants of plants with red flowers are, with white flowers -, i.e. . That's why it's red, - white color, and the parental forms are heterozygous for this trait (since there is cleavage in the offspring).

There is also a split in flower shape: half of the offspring have saucer-shaped flowers, the other half have funnel-shaped flowers. Based on these data, it is not possible to unambiguously determine the dominant trait. Therefore, we accept that - saucer-shaped flowers, - funnel-shaped flowers.

(red flowers, saucer-shaped)

(red flowers, funnel-shaped)

Gametes

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

Solving problems on blood groups (AB0 system)

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

Solution:



	(the probability of having a child with the second blood group is , with the third - , with the fourth - ).

Solving problems on the inheritance of sex-linked traits

Such tasks may well appear in both Part A and Part C of the Unified State Examination.

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

Solution:



	girl, healthy () girl, healthy, carrier () boy, healthy () boy with hemophilia ()

Solving problems of mixed type

Task: A man with brown eyes and a blood type married a woman with brown eyes and a blood type. They had a blue-eyed child with a blood type. Determine the genotypes of all individuals indicated in the problem.

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

	(father)	(mother)

	(was born)

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

Solution: In a person, better control of the right hand dominates over left-handedness, therefore - right-handed, - left-handed. The genotype of the man (since he received the gene from a left-handed mother), and women - .

A colorblind man has the genotype, and his wife has the genotype, because. her parents were completely healthy.

R

	right-handed girl, healthy, carrier () left-handed girl, healthy, carrier () right-handed boy, healthy () left-handed boy, healthy ()

Problems to solve independently

Determine the number of gamete types in an organism with genotype.
Determine the number of gamete types in an organism with genotype.
Crossed tall plants with short plants. B - all plants are medium in size. What will it be?
Crossed a white rabbit with a black rabbit. All rabbits are black. What will it be?
Two rabbits with gray fur were crossed. In with black wool, - with gray and with white. Determine the genotypes and explain this segregation.
A black hornless bull was crossed with a white horned cow. We got black hornless, black horned, white horned and white hornless. Explain this split if black color and lack of horns are dominant characteristics.
Drosophila flies with red eyes and normal wings were crossed with fruit flies with white eyes and defective wings. The offspring are all flies with red eyes and defective wings. What will be the offspring from crossing these flies with both parents?
A blue-eyed brunette married a brown-eyed blonde. What kind of children can be born if both parents are heterozygous?
A right-handed man with a positive Rh factor married a left-handed woman with a negative Rh factor. What kind of children can be born if a man is heterozygous only for the second characteristic?
The mother and father have the same blood type (both parents are heterozygous). What blood type is possible in children?
The mother has a blood type, the child has a blood type. What blood type is impossible for the father?
The father has the first blood group, the mother has the second. What is the probability of having a child with the first blood group?
A blue-eyed woman with a blood type (her parents had a third blood group) married a brown-eyed man with a blood type (his father had blue eyes and a first blood group). What kind of children can be born?
A hemophilic man, right-handed (his mother was left-handed) married a left-handed woman with normal blood (her father and mother were healthy). What children can be born from this marriage?
Strawberry plants with red fruits and long-petioled leaves were crossed with strawberry plants with white fruits and short-petioled leaves. What kind of offspring can there be if red color and short-petioled leaves dominate, while both parent plants are heterozygous?
A man with brown eyes and a blood type married a woman with brown eyes and a blood type. They had a blue-eyed child with a blood type. Determine the genotypes of all individuals indicated in the problem.
Melons with white oval fruits were crossed with plants that had white spherical fruits. The following plants were obtained in the offspring: with white oval, with white spherical, with yellow oval and with yellow spherical fruits. Determine the genotypes of the original plants and descendants, if in a melon the white color dominates over the yellow, the oval shape of the fruit dominates over the spherical.

Answers

type of gametes.
types of gametes.
type of gametes.
high, medium and low (incomplete dominance).
black and white.
- black, - white, - gray. Incomplete dominance.
Bull: , cow - . Offspring: (black hornless), (black horned), (white horned), (white hornless).
- red eyes, - white eyes; - defective wings, - normal. Initial forms - and, offspring.
Crossing results:
A)
- brown eyes, - blue; - dark hair, - blond. Father, mother - .

- brown eyes, dark hair
- brown eyes, blond hair
- blue eyes, dark hair
- blue eyes, blond hair
- right-handed, - left-handed; - Rh positive, - Rh negative. Father, mother - . Children: (right-handed, Rh positive) and (right-handed, Rh negative).
Father and mother - . Children may have a third blood group (probability of birth - ) or first blood group (probability of birth - ).
Mother, child; he received the gene from his mother, and from his father - . The following blood groups are impossible for the father: second, third, first, fourth.
A child with the first blood group can only be born if his mother is heterozygous. In this case, the probability of birth is .
- brown eyes, - blue. Woman, man. Children: (brown eyes, fourth group), (brown eyes, third group), (blue eyes, fourth group), (blue eyes, third group).
- right-handed, - left-handed. Man, woman. Children (healthy boy, right-handed), (healthy girl, carrier, right-handed), (healthy boy, left-handed), (healthy girl, carrier, left-handed).
- red fruits, - white; - short-petioled, - long-petioled.
Parents: and. Offspring: (red fruits, short-petioled), (red fruits, long-petioled), (white fruits, short-petioled), (white fruits, long-petioled).
Strawberry plants with red fruits and long-petioled leaves were crossed with strawberry plants with white fruits and short-petioled leaves. What kind of offspring can there be if red color and short-petioled leaves dominate, while both parent plants are heterozygous?
- brown eyes, - blue. Woman, man. Child:
- white color, - yellow; - oval fruits, - round. Source plants: and. Offspring:
with white oval fruits,
with white spherical fruits,
with yellow oval fruits,
with yellow spherical fruits.

The sixth building of the Unified State Examination in Biology is tasks. For people just starting out in biology, or test prep in particular, they are terrifying. Very in vain. Once you figure it out, everything will become simple and easy. 🙂

Refers to the basic level, with a correct answer you can get 1 primary point.

To successfully complete this task, you should know the following topics given in the codifier:

Topics in the codifier for task No. 6

Genetics, its tasks. Heredity and variability are properties of organisms. Genetics methods. Basic genetic concepts and symbolism. Chromosomal theory of heredity. Modern ideas about the gene and genome

“Solve the Unified State Exam” divides tasks into two large groups: monohybrid crossing and dihybrid crossing.

Before solving problems, we suggest compiling a small dictionary of terms and concepts in order to understand what is required of us.

Theory for crossing tasks

Traits are divided into two types: recessive and dominant.

« A dominant trait suppresses a recessive one" is a stable phrase. What does suppress mean? This means that in the choice between a dominant and recessive trait, the dominant one will necessarily appear. Anyway. A dominant trait is indicated by a capital letter, and a recessive trait is indicated by a small letter. Everything is logical. In order for a recessive trait to appear in the offspring, it is necessary that the gene carries the recessive trait from both the female and the male.

For clarity: let’s imagine a sign, for example, the color of a kitten’s fur. Let us have two options for the development of events:

Black wool
White wool

Black wool is dominant over white. In general, tasks always indicate what dominates what; applicants are not required to know everything, especially about genetics.

Black wool will then be indicated by a capital letter. The most commonly used are A, B, C and further alphabetically. White wool, respectively, in small letters.

A - black wool.

a- white wool.

If the fusion of gametes results in the following combinations: AA, Aa, aA, then this means that the fur of the descendants of the first generation will be black.

If the fusion of gametes results in the combination aa, then the wool will be white.

What kind of gametes the parents have will be stated in the task conditions.

Gametes, or germ cells, are reproductive cells that have a haploid (single) set of chromosomes and participate, in particular, in sexual reproduction.

Zygote- a diploid cell formed as a result of fertilization.

Heterozygote - two genes that determine one trait - identical (AA or aa)

Homozygote - two genes that determine one trait are different (Aa)

Dihybrid cross- crossing of organisms that differ in two pairs of alternative characteristics.

Monohybrid cross- crossing, in which the crossed organisms differ in only one characteristic.

Analysis cross- crossing a hybrid individual with an individual homozygous for recessive alleles.

Gregor Mendel - the “father” of genetics

So, how to distinguish these types of crossing:

When crossing a monohybrid, we are talking about one trait: color, size, shape.

In a dihybrid cross we are talking about a pair of traits.

In an analytical cross, one individual can be absolutely anything, but the other’s gametes must carry exclusively recessive traits.

Alleles- different forms of the same gene, located in the same regions of homologous chromosomes.

It doesn't sound very clear. Let's figure it out:

1 gene carries 1 trait.

1 allele carries one trait value (it can be dominant or recessive).

Genotype- the totality of genes of a given organism.

Phenotype- a set of characteristics inherent in an individual at a certain stage of development.

Problems often ask you to indicate the percentage of individuals with a certain genotype or phenotype or to indicate the breakdown by genotype or phenotype. If we simplify the definition of phenotype, then phenotype is the external manifestation of characteristics from the genotype.

In addition to all sorts of concepts, you need to know the laws of Gregor Mendel, the father of genetics.

Gregor Mendel crossed peas with fruits that differed in color and smoothness of the skin. Thanks to his observations, the three laws of genetics emerged:

I. Law of uniformity of first generation hybrids:

In a monohybrid crossing of different homozygotes, all descendants of the first generation will be identical in phenotype.

II. Law of splitting

When crossing descendants of the first generation, a splitting of 3:1 in phenotype and 1:2:1 in genotype is observed.

III. Law of independent cleavage

When a dihybrid crossing of two different homozygotes occurs in the second generation, phenotypic cleavage is observed in a ratio of 9:3:3:1.

When the skill of solving genetic problems is acquired, the question may arise: why do I need to know Mendel’s laws if I can already solve the problem perfectly well and find splitting in particular cases? Attention answer: in some tasks you may need to indicate by what law the splitting occurred, but this applies more to tasks with a detailed answer.

Having gained some grounding in theory, you can finally move on to the tasks. 😉

Analysis of typical tasks No. 6 of the Unified State Exam in biology

Types of gametes in an individual

How many types of gametes are produced in an individual with the aabb genotype?

We have two pairs of allelic chromosomes:

First pair: aa

Second pair: bb

These are all homozygotes. You can make only one combination: ab.

Types of gametes in crossing

How many types of gametes are formed in diheterozygous pea plants during dihybrid crossing (the genes do not form a linkage group)? Write down the number in response.

Since plants are diheterozygous, this means that for both traits, one allele is dominant and the other is recessive.

We obtain genotypes AaBb and AaBb.

Gametes in problems are designated by the letter G, without commas, in circles; the gametes of one individual are indicated first, then a semicolon (;) is placed, and the gametes of another individual are written, also in circles.

Crossing is indicated by an "x".

Let's write out the gametes; to do this, we'll go through all the combinations:

The gametes of the first and second individuals turned out to be the same, so their genotype was also the same. This means we have 4 different types of gametes:

Calculation of the proportion of diheterozygotes

When crossing individuals with genotypes AaBb with AaBb (genes are not linked), the proportion (%) of heterozygotes for both alleles (diheterozygotes) in the offspring will be….

Let's create a Punnett lattice. To do this, we write out the gametes of one individual in a column, the gametes of another in a row, and we get a table:

Let's find diheterozygotes in the table:

Total zygotes: 16

Diheterozygotes:4

Let's calculate the percentage: =

Application of Mendel's laws

The rule of uniformity of the first generation will appear if the genotype of one of the parents is aabb, and the other is

According to the rule of uniformity, monohybrid homozygotes must be crossed, one with a dominant trait, and the other with a recessive trait. This means that the genotype of the other individual must be AABB.

Answer: AABB.

Phenotype ratio

The genotype of one of the parents will be AaBb if, during an analyzing dihybrid crossing and independent inheritance of traits, a split in phenotype is observed in the offspring in the ratio. Write your answer as a sequence of numbers showing the ratio of the resulting phenotypes, in descending order.

Analyzing dihybrid cross, which means that the second individual has a recessive dihomozygote: aabb.

Here you can do without a Punnett grid.

Generations are designated by the letter F.

F1: AaBb; Aabb; aaBb; aabb

All four variants of phenotypes are different, so they relate to each other as 1:1:1:1.

Answer: 1111

Genotype ratio

What is the ratio of genotypes in the offspring obtained from crossing individuals with genotypes AaBb x AABB?

AaBb x AABB

F1: AaBb; Aabb; aaBb; aabb

All 4 genotypes are different.

Inheritance of certain traits or diseases

What is the probability of having healthy boys in a family where the mother is healthy and the father is sick with hypertrichosis, a disease caused by the presence of a gene linked to the Y chromosome?

If a trait is linked to the Y chromosome, it means that it is not reflected in any way on the X chromosome.

The female sex is homozygous: XX, and the male is heterozygous: XY.

Solving problems with sex chromosomes is practically no different from solving problems with autosomes.

Let's make a table of genes and traits, which should also be compiled for problems about autosomal chromosomes, if the traits are indicated and this is important.

The letter above the Y indicates that the gene is linked to that chromosome. Traits can be dominant or recessive, they are designated by capital and small letters, they can relate to both the H chromosome and the Y chromosome, depending on the task.

♀ХХ x ХY a

F1: XX-girl, healthy

XY a - boy, sick

The boys born to this couple will be 100% sick, which means 0% healthy.

Blood groups

What ABO blood group does a person with genotype I B I 0 have? Write down the number in response.

Let's use the table:

Our genotype contains agglutinogens B and 0. This pair gives the third blood group.

Working with the circuit

Using the pedigree shown in the figure, determine the probability (in percentage) of the birth of parents 1 and 2 of a child with the trait indicated in black, with complete dominance of this trait. Write your answer as a number.