Names

Genetics biology exam. Methodical development in biology to help students passing the exam. Problems in genetics. Tasks for independent solution

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

Patterns of heredity, their cytological basis. Mono- and dihybrid crossing. Patterns of inheritance established by G. Mendel. Linked inheritance of traits, violation of the linkage of genes. Laws of T. Morgan. Sex genetics. Inheritance of sex-linked traits. Interaction of genes. Solution of genetic problems. Drawing up cross-breeding schemes.

Variability of traits in organisms: modification, mutation, combinative. Types of mutations and their causes. The value of variability in the life of organisms and in evolution. reaction rate. The harmful effects of mutagens, alcohol, drugs, nicotine on the genetic apparatus of the cell. Protection of the environment from pollution by mutagens. Identification of sources of mutagens in the environment (indirectly) and assessment of the possible consequences of their influence on one's own body. Human hereditary diseases, their causes, prevention.

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

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

Basic terms of genetics

Gene- This is a section of the 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 the same gene encoding an alternative manifestation of the same trait. Alternative signs - signs that cannot be in the body at the same time.
Homozygous organism- an organism that does not give splitting for one reason or another. Its allelic genes equally affect the development of this trait.
heterozygous organism- an organism that gives splitting according to one or another feature. Its allelic genes affect the development of this trait in different ways.
dominant gene is responsible for the development of a trait that manifests itself in a heterozygous organism.
recessive gene is responsible for the trait, the development of which is suppressed by the dominant gene. A recessive trait appears 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 a genome.
Phenotype- the totality of all the characteristics of an organism.

When solving problems in genetics, it is necessary:

Determine the types of crossing and interactions of allelic and non-alelic genes (determine the nature of crossing).
Determine the dominant and recessive trait(s) by the condition of the problem, drawing, scheme or by the results of crossing F 1 and F 2.
Enter the letter designations of the dominant (capital letter) and recessive (capital letter) traits, if they are not given in the problem statement.
Write down the phenotypes and genotypes of the parent forms.
Write down the phenotypes and genotypes of the offspring.
Draw up a crossbreeding scheme, be sure to indicate the gametes that form the parental forms.
Write down the answer.

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

Make a brief note of the task.
Conduct an analysis of each feature separately, making an appropriate entry for each feature.
Apply monohybrid cross formulas, if none of them are suitable, then
- Add the weight of the numbers in the offspring, divide the sum by 16, find one part and express all the numbers in parts.
- Based on the fact that the splitting in F 2 of dihybrid crossing goes according to the formula 9A_B_: 3A_bb: 3 aaB_: l aabb, find the Fr genotypes
- By F 2 find F genotypes
- F to find the genotypes of the parents.

Formulas for determining the nature of crossing:

where n is the number of alleles, pairs of traits

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

Basic rules for solving genetic problems:

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

Patterns of heredity, their cytological basis. Patterns of inheritance established by G. Mendel, their cytological foundations (mono- and dihybrid crossing). Laws of T. Morgan: linked inheritance of traits, violation of the linkage of genes. Sex genetics. Inheritance of sex-linked traits. Interaction of genes. The genotype as an integral system. Human genetics. Methods for studying human genetics. Solution of genetic problems. Drawing up crossbreeding 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 of the chromosomes carries only one of these factors. If we imagine that genes are point objects on straight lines - chromosomes, then schematically homozygous individuals can be written as A||A or a||a, while heterozygous individuals - A||a. During the formation of gametes during meiosis, each of the genes of a heterozygote pair will be in one of the germ cells.

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

In cases where genes are located on different chromosomes, then during the formation of gametes, the distribution between them of alleles from a given pair of homologous chromosomes 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 the diversity of allele recombination in gametes.

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

However, the number of traits 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 a certain individual, heterozygous for two pairs of genes located in homologous chromosomes, produces gametes, then one should take into account not only the probability of formation of gametes with the original chromosomes, but also gametes that have received chromosomes changed as a result of crossing over in prophase I of meiosis. Consequently, new combinations of traits will arise in the offspring. The data obtained in experiments on Drosophila formed the basis chromosome theory of heredity.

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

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

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

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

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

For the study, G. Mendel selected only seven signs that have alternative (contrasting) manifestations. Already in the first crossings, he noticed that in the offspring of the first generation, when plants with yellow and green seeds were crossed, all the offspring had yellow seeds. Similar results were obtained in the study of other features. The signs that prevailed in the first generation, G. Mendel called dominant. Those of them that did not appear in the first generation were called recessive.

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

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

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

The scheme of monohybrid crossing is as follows:

Crossing 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 was the father. The same results were obtained in crosses for other traits, which gave G. Mendel reason to formulate law of uniformity of hybrids of the first generation, which is also called Mendel's first law And the 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 both in genotype and phenotype.

A - yellow seeds; but- green seeds

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

Mendel's second law:

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

However, by the phenotype of an individual, it is far from always possible to establish its genotype, since both homozygotes for the dominant gene ( AA), and heterozygotes ( Ah) will have a manifestation of the dominant gene in the phenotype. Therefore, for organisms with cross-fertilization apply analyzing cross A cross in which an organism with an unknown genotype is crossed with a homozygous recessive gene to test the genotype. At the same time, homozygous individuals for the dominant gene do not give splitting in the offspring, while in the offspring of heterozygous individuals, an equal number of individuals with both dominant and recessive traits is observed:

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 a pair gets into each of the gametes (i.e., the gametes are genetically pure), and during fertilization, the pair is restored. These assumptions are called gamete purity rules.

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 ways, 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 traits, is called dihybrid. If the manifestation of a larger number of hereditary traits is analyzed, then this is already polyhybrid cross.

Dihybrid cross scheme:

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

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

For dihybrid crossing, G. Mendel chose two traits: the color of the seeds (yellow and green) and their shape (smooth and wrinkled). In the first generation, the law of uniformity of hybrids of the first generation was observed, and in the second generation there were 315 yellow smooth seeds, 108 green smooth seeds, 101 yellow wrinkled and 32 green wrinkled. The calculation showed that the splitting approached 9:3:3:1, but the ratio of 3:1 was maintained for each of the signs (yellow - green, smooth - wrinkled). This pattern has been named law of independent feature splitting, 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, independent splitting of these traits will occur in a ratio of 3:1 (9:3:3:1 in 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 is applicable only to cases of independent inheritance, when genes are located in different pairs of homologous chromosomes. In cases where genes are located in the same pair of homologous chromosomes, patterns of linked inheritance are valid. The patterns of independent inheritance of traits established by G. Mendel are also often violated during the interaction of genes.

Laws of T. Morgan: linked inheritance of traits, violation of gene linkage

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

To test this assumption, the American geneticist T. Morgan chose a pair of genes in Drosophila that control body color (gray - black) and wing shape (long - rudimentary), which are located in one pair of homologous chromosomes. The gray body and long wings are dominant characters. 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, in fact, mainly parental phenotypes were obtained in a ratio close to 3:1, however, there was also an insignificant number of individuals with new combinations of these traits. . These individuals are called recombinant.

However, after analyzing the crossing of first-generation hybrids with homozygotes for recessive genes, T. Morgan found that 41.5% of individuals had a gray body and long wings, 41.5% had a black body and rudimentary wings, 8.5% had a gray body and rudimentary wings, and 8.5% - black body and rudimentary wings. He associated the resulting splitting with the crossing over occurring in prophase I of meiosis and proposed to consider 1% of the crossing over as a unit of distance between genes in the chromosome, subsequently named after him. morganide.

The patterns of linked inheritance, established in the course of experiments on Drosophila, are called T. Morgan's law.

Morgan's Law:

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

Genes located in the chromosome directly one after another (the probability of crossing over is extremely small) are called fully linked, and if there is at least one more gene between them, then they are not 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 build maps of chromosomes with the order of genes plotted on them. Genetic maps of chromosomes have been created for many genetically well-studied objects: Drosophila, mice, humans, corn, wheat, peas, etc. The study of genetic maps allows you to compare the structure of the genome in different types of organisms, which is important for genetics and breeding, as well as evolutionary studies .

Sex Genetics

Floor- this is a combination of morphological and physiological features of the body that ensure sexual reproduction, the essence of which is reduced to fertilization, that is, the fusion of male and female germ cells into a zygote, from which a new organism develops.

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

In humans, secondary sexual characteristics are body type, voice timbre, the predominance of muscle or adipose tissue, the presence of facial hair, Adam's apple, and mammary glands. So, in women, the pelvis is usually wider than the shoulders, adipose tissue predominates, the mammary glands are expressed, and the voice is high. Men, on the other hand, differ from them in their broader shoulders, the predominance of muscle tissue, the presence of hair on the face and Adam's apple, and also in a low voice. Mankind 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 males. In turn, males produce gametes that differ in the number of chromosomes, thereby determining the sex of the offspring in advance. 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 paired chromosomes.

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. The spermatozoa of a man can carry one of these chromosomes and determine the sex of the unborn child. In this regard, human chromosomes and many other organisms are divided into two groups: autosomes and heterochromosomes, or sex chromosomes.

TO autosomes carry chromosomes that are the same for both sexes, while sex chromosomes- these are chromosomes that differ in different sexes and carry information about sexual characteristics. In cases where the sex carries the same sex chromosomes, for example XX, they say that he 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, Drosophila flies and other organisms, the female is homogametic (XX), and the male is heterogametic (XY), while in birds the male is homogametic (ZZ, or XX), and the female is heterogametic (ZW, or XY) .

The X chromosome is a large unequal chromosome that carries over 1500 genes, and many of their mutant alleles cause the development of severe hereditary diseases in humans, such as hemophilia and color blindness. The Y chromosome, in contrast, is very small, containing only about a dozen genes, including specific genes responsible for male development.

The male karyotype is written as $♂$ 46,XY, and the female karyotype 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 the actually observed.

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

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

Inheritance of sex-linked traits

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

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

X H - normal blood clotting

X h - blood incoagulability (hemophilia)

The traits encoded in the genes of the Y chromosome are transmitted purely through the male line and are called hollandic(the presence of a membrane between the toes, increased hairiness of the edge of the auricle).

Gene Interaction

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

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

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

codominance called 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 AB0 system.

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

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

A special case of the interaction of allelic genes can be considered the so-called lethal genes, which in the homozygous state lead to the death of the organism most often in the embryonic period. The reason for the death of the offspring is the pleiotropic effect of genes for gray coat color in astrakhan sheep, platinum color in foxes, and the absence of scales in mirror carps. When crossing two individuals heterozygous for these genes, the splitting for the trait under study 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 a pumpkin, when plants with spherical (AAbb) and long (aaBB) fruits are crossed, plants with disc-shaped fruits (AaBb) appear in the first generation.

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

Polymeria called the phenomenon in which non-allelic genes have the same effect on the development of a trait. Thus, most often quantitative signs are 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 gene manifestation and the form of its manifestation depend on environmental conditions. In this case, the environment is understood not only as the environment, but also as the genotypic environment - other genes.

The manifestation of qualitative signs rarely depends on environmental conditions, although if a white-haired area of the body of an ermine rabbit is shaved and an ice pack is applied to it, then black hair will grow in this place over time.

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, being in the same genotype, can strongly influence the manifestation of the action of neighboring genes. Thus, for each individual gene, there is a genotypic environment. It is possible that the development of any trait is associated with the action of many genes. In addition, the dependence of several traits on one gene was revealed. For example, in oats, the color of flower scales and the length of their awn are determined by one gene. In Drosophila, the gene for the white color of the eye simultaneously affects the color of the body and internal organs, the length of the wings, a decrease in fertility, and a decrease in life expectancy. It is possible that each gene is simultaneously the gene of the main action for "its own" trait and a modifier for other traits. Thus, the phenotype is the result of the interaction of the genes of the entire genotype with the environment in the ontogeny of the 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, while only those organisms survived in which the interaction of genes gave the most favorable reaction in ontogenesis.

human genetics

For man as a biological species, the genetic patterns 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 sections of genetics.

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

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

Methods for studying human genetics

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

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

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

genealogical method- this is a method of compiling pedigrees, with the help of which the nature of the inheritance of the studied traits, including hereditary diseases, is determined, and the birth of offspring with the corresponding traits is predicted. He made it possible to reveal the hereditary nature of such diseases as hemophilia, color blindness, Huntington's chorea, and others even before the discovery of the main patterns of heredity. When compiling pedigrees, records are kept about each of the family members and take into account the degree of relationship between them. Further, 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 they collect information about several families in which this feature is manifested. The genealogical method allows you to establish not only the heritability of the trait, but also the nature of inheritance: dominant or recessive, autosomal or sex-linked, etc. Thus, according to the portraits of the Austrian monarchs of the Habsburgs, the inheritance of prognathia (a strongly protruding lower lip) and "royal hemophilia" descendants of the British Queen Victoria.

Solution of genetic problems. Drawing up crossbreeding schemes

All variety of genetic problems can be reduced to three types:

Calculation tasks.
Tasks for determining 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, by which it is easy to establish the genotypes of the parents. They need to establish 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, if it is known that dark color dominates 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, but- Light colored seeds.

2. We draw up a crossing scheme:

a) first we write down the genotypes of the parents, which, according to the condition 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 purity of gametes:

Gametes BUT a

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

F 1 A but

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

Solution.

1. Write down the condition of the problem:

A - black wool but- brown wool;

B - normal tail length, b- shortened tail

F 2 A_ bb ?

2. We write down the crossing scheme:

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

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

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

80×3/16×15.

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

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

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

Solution.

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

Inheritance of blood groups according to the AB0 system

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

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

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

Thus, our assumptions were confirmed.

Answer: parents are heterozygous for the genes of blood groups: 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 a woman who normally distinguish colors, 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 a man and a woman whose fathers were color blind.

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

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

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

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

Solution.

1. Determine the splitting in the offspring:

161: 79 $≈$ 2: 1.

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

2. Since there were twice as many hens with short legs than with long ones, let's assume that this is a dominant trait, and this allele is characterized by a lethal effect. Then the original chickens were heterozygous. Let's name the genes:

C - short legs, c - long legs.

3. We write down the crossing scheme:

Our assumptions were confirmed.

Answer: short-legged dominates over long-legged, this allele is characterized by a lethal effect.

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

Tasks 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 exam.

The sixth type of tasks 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 sets out theoretical foundations of genetics necessary for successful preparation for task C6, as well as solutions to problems of all types are considered and examples for independent work are given.

Basic terms of genetics

Gene- This is a section of the 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 the same gene encoding an alternative manifestation of the same trait. Alternative signs - signs that cannot be in the body at the same time.

Homozygous organism- an organism that does not give splitting for one reason or another. Its allelic genes equally affect the development of this trait.

heterozygous organism- an organism that gives splitting according to one or another feature. Its allelic genes affect the development of this trait in different ways.

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

recessive gene is responsible for the trait, the development of which is suppressed by the dominant gene. A recessive trait appears 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 uniformity of hybrids

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

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

Yellow seed color
- green seed color

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

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

Mendel's second law - the law of splitting

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



	(plants have a dominant trait, - recessive)

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

Mendel's third law - the law of independent inheritance

This law was derived on the basis of data obtained during dihybrid crossing. Mendel considered the inheritance of two pairs of traits in peas: seed color and shape.

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

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



	(yellow smooth).

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

The Punnett grid is used to record and determine genotypes.

Gametes

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

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

The wording of the law: when crossing organisms that differ from each other by two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and combined in all possible combinations.

Mendel's third law holds only if the genes are on 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. In both genes are manifested, which is possible only if the hybrids form two types of gametes: one carries a dominant gene, the other 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 proved after studying the processes occurring in meiosis.

The hypothesis of "purity" of gametes is the cytological basis of Mendel's first and second laws. With its help, splitting by phenotype and genotype can be explained.

Analyzing 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 it could be concluded that the original organism is homozygous for the trait under study.

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

Inheritance of blood groups (AB0 system)

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

Inheritance of sex-linked traits

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

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

The USE includes tasks only for traits linked to the -chromosome. Basically, they relate to two signs of a person: blood clotting (- normal; - hemophilia), color vision (- normal, - color blindness). Tasks for the inheritance of sex-linked traits in birds are much less common.

In humans, the female sex may be homozygous or heterozygous for these genes. Consider the possible genetic sets in a woman on the example of hemophilia (a similar picture is observed with color blindness): - healthy; - healthy, but is a carrier; - sick. The male sex for these genes is homozygous, tk. - chromosome does not have alleles of these genes: - healthy; - is ill. Therefore, men are most often affected by these diseases, and women are their carriers.

Typical USE tasks in genetics

Determination of the number of types of gametes

The number of gamete types is determined by the formula: , where is the number of gene pairs in the heterozygous state. For example, an organism with a genotype has no genes in a heterozygous state; , therefore, and it forms one type of gamete. 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.

Tasks for mono- and dihybrid crossing

For a monohybrid cross

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

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

	(the black)	(White)

	(black) : (white)

For a dihybrid cross

Dominant genes are known

A task: Crossed tomatoes of normal growth 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 the red color of the fruit dominates over yellow, and normal growth over dwarfism.

Solution: Denote dominant and recessive genes: - normal growth, - dwarfism; - red fruits, - yellow fruits.

Let us analyze the inheritance of each trait separately. All offspring have normal growth, i.e. splitting on this basis is not observed, so the original forms are homozygous. Splitting 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

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

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

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

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

Solution:



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

Solving problems on the inheritance of sex-linked traits

Such tasks may well occur both in part A and in part C of the USE.

A 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

A task: A man with brown eyes and blood type marries a woman with brown eyes and 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 may have the genotype or, the first - only. Since the child has the first blood type, therefore, he received the gene from both his father and mother, therefore his father has a genotype.

	(father)	(mother)

	(was born)

A task: The 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, the best possession of the right hand dominates over left-handedness, therefore - right-handed, - lefty. Male genotype (because he received the gene from a left-handed mother), and women -.

A color-blind man has the genotype, and his wife -, 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 ()

Tasks for independent solution

Determine the number of types of gametes in an organism with a genotype.
Determine the number of types of gametes in an organism with a genotype.
They crossed tall plants with short plants. B - all plants are medium in size. What will be?
They crossed a white rabbit with a black rabbit. All rabbits are black. What will be?
They crossed two rabbits with gray wool. B with black wool, - with gray and white. Determine the genotypes and explain this splitting.
They crossed a black hornless bull with a white horned cow. They received black hornless, black horned, white horned and white hornless. Explain this split if black and the absence of horns are dominant traits.
Drosophila 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 sign?
The mother and father have a blood type (both parents are heterozygous). What blood group is possible in children?
The mother has a blood group, the child has a blood group. What blood type is impossible for a father?
The father has the first blood type, the mother has the second. What is the probability of having a child with the first blood type?
A blue-eyed woman with a blood type (her parents had a third blood type) married a brown-eyed man with a blood type (his father had blue eyes and a first blood type). What kind of children can be born?
A right-handed hemophilic man (his mother was left-handed) married a left-handed woman with normal blood (her father and mother were healthy). What kind of children can be born from this marriage?
Strawberry plants with red fruits and long-leaved leaves were crossed with strawberry plants with white fruits and short-leaved leaves. What offspring can there be if red color and short-leaved leaves dominate, while both parental plants are heterozygous?
A man with brown eyes and blood type marries a woman with brown eyes and blood type. They had a blue-eyed child with a blood type. Determine the genotypes of all individuals indicated in the problem.
They crossed melons with white oval fruits 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 the white color of the melon dominates over the yellow, the oval shape of the fruit is over the spherical.

Answers

gamete type.
gamete types.
gamete type.
high, medium and low (incomplete dominance).
black and white.
- black, - white, - grey. 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:
but)
- Brown eyes, - blue; - dark hair, - light. Father mother - .

- brown eyes, dark hair
- brown eyes, blonde hair
- blue eyes, dark hair
- blue eyes, blonde hair
- right-handed, - left-handed; Rh positive, Rh negative. Father mother - . Children: (right-handed, Rh positive) and (right-handed, Rh negative).
Father and mother - . In children, a third blood type (probability of birth -) or a first blood group (probability of birth -) is possible.
Mother, child; He received the gene from his mother, and from his father -. The following blood types 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, - lefty. Man Woman . Children (healthy boy, right-handed), (healthy girl, carrier, right-handed), (healthy boy, left-handed), (healthy girl, carrier, left-hander).
- red fruit - white; - short-stalked, - long-stalked.
Parents: and Offspring: (red fruit, short stem), (red fruit, long stem), (white fruit, short stem), (white fruit, long stem).
Strawberry plants with red fruits and long-leaved leaves were crossed with strawberry plants with white fruits and short-leaved leaves. What offspring can there be if red color and short-leaved leaves dominate, while both parental 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 who are just starting out in biology or exam preparation in particular, they are terrifying. Very in vain. One has only to figure out how 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. Methods of genetics. Basic genetic concepts and symbolism. Chromosomal theory of heredity. Modern ideas about the gene and genome

"I will solve the exam" divides tasks into two large groups: monohybrid crossing and dihybrid crossing.

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

Theory for crossbreeding tasks

There are two types of traits: recessive and dominant.

« Dominant trait overrides recessive trait' is a fixed phrase. What does it mean to suppress? This means that in the choice between a dominant and a 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 a recessive trait from both the female and the male.

For clarity, let's imagine a sign, for example, the color of a kitten's coat. Suppose we have two options for the development of events:

Black wool
White wool

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

Black wool would then be denoted by a capital letter. The most commonly used are A, B, C, and so on alphabetically. White wool, respectively, in small letters.

A black wool.

a white wool.

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

If, when the gametes are fused, the combination aa is obtained, then the wool will be white.

About what gametes the parents have will be said in the condition of the problem.

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

Zygote A diploid cell resulting from fertilization.

Heterozygous - two genes that determine one trait are the same (AA or aa)

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

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

monohybrid cross- crossing, in which the crossed organisms differ in only one trait.

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

Gregor Mendel - "father" of genetics

So, how to distinguish between these types of crossing:

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

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

With analyzing crosses, one individual can be absolutely any, but the other 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 is the totality of the genes of an organism.

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

Problems are often asked to indicate the percentage of individuals with a particular genotype or phenotype, or to indicate the splitting by genotype or phenotype. If we simplify the definition of the phenotype, then the phenotype is the external manifestation of traits from the genotype.

In addition to any 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, three laws of genetics appeared:

I. The law of uniformity of hybrids of the first generation:

With monohybrid crossing of different homozygotes, all descendants of the first generation will be the same in phenotype.

II. splitting law

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

III. Law of independent splitting

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

When the skill of solving genetic problems is obtained, the question may arise: why should I know Mendel's laws, if I can perfectly solve the problem and find splitting in particular cases? Attention answer: in some tasks it may be necessary to indicate by what law splitting occurred, but this applies more to tasks with a detailed answer.

Having been savvy in theory, you can finally move on to tasks. 😉

Analysis of typical tasks No. 6 USE in biology

Types of gametes in an individual

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

We have two pairs of allelic chromosomes:

First couple: aa

Second pair: bb

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

Types of gametes when crossing

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

Since the plants are diheterozygous, this means that, according to both traits, they have one allele dominant, and the second recessive.

We get the genotypes AaBb and AaBb.

Gametes in tasks are denoted by the letter G, moreover, without commas, in circles, gametes of one individual are indicated first, then a semicolon (;) is put, gametes of another individual are written, also in circles.

Crossing is indicated by an "x".

Let's write out the gametes, for this we will sort 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. So, we got 4 different types of gametes:

Calculation of the proportion of diheterozygotes

When crossing individuals with AaBb genotypes 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 the gametes of one individual in a column, the gametes of the other in a row, we get a table:

Let's find the 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 should be crossed, one with a dominant trait, and the second with a recessive trait. Hence, 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 analyzing dihybrid crossing and independent inheritance of traits, splitting in the phenotype in the offspring is observed in the ratio. Write down the answer in the form of 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 the Punnett lattice.

Generations are denoted by the letter F.

F1: AaBb; abb; 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 offspring obtained from crossing individuals with AaBb x AABB genotypes?

AaBb x AABB

F1: AaBb; abb; aaBb; aabb

All 4 genotypes are different.

Inheritance of certain traits or diseases

What is the probability of the birth of 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 the trait is linked to the Y chromosome, then it does not affect the X chromosome.

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

Solving problems with sex chromosomes practically does not differ from solving problems with autosomes.

Let's make a gene and trait table, 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 are dominant and recessive, they are indicated by capital and small letters, they can refer to both the H-chromosome and the Y-chromosome, depending on the task.

♀XX x XY a

F1: XX-girl, healthy

XY a - boy, sick

Boys born to this couple will be 100% sick, so 0% healthy.

Blood groups

What blood type according to the ABO system does a person with the genotype I B I 0 have? Write down a number for your answer.

Let's use the table:

In our genotype, agglutinogens B and 0 are recorded. This pair gives the third blood group.

Working with a schema

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

So, we are learning to analyze such schemes.

We see that the sign is manifested in both men and women, which means that it is not sex-linked.

It manifests itself in every generation, which means it is dominant.

Since one of the children of the couple did not show the trait, then the parents are heterozygotes.

F1: AA- appears

Aa- shows up

aa - does not appear

3 - appears from 4



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