Dihybrid crossing problems.

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

Genetics, its tasks

Advances in natural science and cell biology in the 18th-19th centuries allowed a number of scientists to make assumptions about the existence of certain hereditary factors that determine, for example, the development of hereditary diseases, but these assumptions were not supported by relevant evidence. Even the theory of intracellular pangenesis formulated by H. de Vries in 1889, which assumed the existence in the cell nucleus of certain “pangenes” that determine the hereditary inclinations of the organism, and the release into protoplasm of only those of them that determine the type of cell, could not change the situation, as well as the theory of “germ plasm” by A. Weissman, according to which characteristics acquired during the process of ontogenesis are not inherited.

Only the works of the Czech researcher G. Mendel (1822-1884) became the foundation stone of modern genetics. However, despite the fact that his works were cited in scientific publications, his contemporaries did not pay attention to them. And only the rediscovery of the patterns of independent inheritance by three scientists at once - E. Chermak, K. Correns and H. de Vries - forced the scientific community to turn to the origins of genetics.

Genetics is a science that studies the patterns of heredity and variability and methods of controlling them.

The tasks of genetics at the present stage are the study of qualitative and quantitative characteristics of hereditary material, analysis of the structure and functioning of the genotype, deciphering the fine structure of the gene and methods for regulating gene activity, searching for genes that cause the development of hereditary human diseases and methods for “correcting” them, creating a new generation of drugs according to the type DNA vaccines, the construction, using genetic and cellular engineering, of organisms with new properties that could produce the medicines and food products needed by humans, as well as the complete deciphering of the human genome.

Heredity and variability - properties of organisms

Heredity is the ability of organisms to transmit their characteristics and properties over a series of generations.

Variability- the property of organisms to acquire new characteristics during life.

Signs- these are any morphological, physiological, biochemical and other characteristics of organisms by which some of them differ from others, for example, eye color. Properties also called any functional characteristics of organisms, which are based on a certain structural characteristic or group of elementary characteristics.

The characteristics of organisms can be divided into quality And quantitative. Qualitative signs have two or three contrasting manifestations, which are called alternative signs, for example, blue and brown eye colors, while quantitative ones (milk yield of cows, wheat yield) do not have clearly defined differences.

The material carrier of heredity is DNA. In eukaryotes, there are two types of heredity: genotypic And cytoplasmic. The carriers of genotypic heredity are localized in the nucleus and will be discussed further, while the carriers of cytoplasmic heredity are the circular DNA molecules located in mitochondria and plastids. Cytoplasmic heredity is transmitted mainly with the egg, therefore it is also called maternal.

A small number of genes are localized in the mitochondria of human cells, but their changes can have a significant impact on the development of the organism, for example, leading to the development of blindness or a gradual decrease in mobility. Plastids play an equally important role in plant life. Thus, in some areas of the leaf, chlorophyll-free cells may be present, which leads, on the one hand, to a decrease in plant productivity, and on the other hand, such variegated organisms are valued in decorative landscaping. Such specimens reproduce mainly asexually, since sexual reproduction often produces ordinary green plants.

Genetics methods

1. The hybridological method, or the method of crossings, consists of selecting parental individuals and analyzing the offspring. In this case, the genotype of an organism is judged by the phenotypic manifestations of genes in descendants obtained through a certain crossing scheme. This is the oldest informative method of genetics, which was most fully first used by G. Mendel in combination with the statistical method. This method is not applicable in human genetics for ethical reasons.

2. The cytogenetic method is based on the study of the karyotype: the number, shape and size of the organism’s chromosomes. The study of these features allows us to identify various developmental pathologies.

3. The biochemical method allows you to determine the content of various substances in the body, especially their excess or deficiency, as well as the activity of a number of enzymes.

4. Molecular genetic methods are aimed at identifying variations in the structure and deciphering the primary nucleotide sequence of the DNA sections under study. They make it possible to identify genes for hereditary diseases even in embryos, establish paternity, etc.

5. The population statistical method makes it possible to determine the genetic composition of a population, the frequency of certain genes and genotypes, genetic load, and also outline the prospects for the development of a population.

6. The method of hybridization of somatic cells in culture makes it possible to determine the localization of certain genes in chromosomes during the fusion of cells of different organisms, for example, a mouse and a hamster, a mouse and a human, etc.

Basic genetic concepts and symbolism

Gene is a section of a DNA molecule, or chromosome, that carries information about a specific trait or property of an organism.

Some genes can influence the manifestation of several traits at once. This phenomenon is called pleiotropy. For example, the gene that causes the development of the hereditary disease arachnodactyly (spider fingers) also causes curvature of the lens and pathologies of many internal organs.

Each gene occupies a strictly defined place in the chromosome - locus. Since in the somatic cells of most eukaryotic organisms the chromosomes are paired (homologous), each of the paired chromosomes contains one copy of the gene responsible for a certain trait. Such genes are called allelic.

Allelic genes most often exist in two variants - dominant and recessive. Dominant called an allele that manifests itself regardless of which gene is located on the other chromosome and suppresses the development of the trait encoded by the recessive gene. Dominant alleles are usually designated in capital letters of the Latin alphabet (A, B, C, etc.), and recessive alleles are designated in lowercase letters (a, b, c, etc.). Recessive alleles can only be expressed if they occupy loci on both paired chromosomes.

An organism that has the same alleles on both homologous chromosomes is called homozygous for this gene, or homozygous(AA, aa, AABB, aabb, etc.), and an organism that has different gene variants on both homologous chromosomes - dominant and recessive - is called heterozygous for this gene, or heterozygous(Aa, AaBb, etc.).

A number of genes may have three or more structural variants, for example, blood groups according to the AB0 system are encoded by three alleles - I A, I B, i. This phenomenon is called multiple allelism. However, even in this case, each chromosome of a pair carries only one allele, that is, all three gene variants cannot be represented in one organism.

Genome- a set of genes characteristic of a haploid set of chromosomes.

Genotype- a set of genes characteristic of a diploid set of chromosomes.

Phenotype- a set of characteristics and properties of an organism, which is the result of the interaction of the genotype and the environment.

Since organisms differ from each other in many traits, the patterns of their inheritance can only be established by analyzing two or more traits in the offspring. Crossing, in which inheritance is considered and an accurate quantitative count of the offspring is carried out according to one pair of alternative characteristics, is called monohybrid m, in two pairs - dihybrid, according to a larger number of signs - polyhybrid.

Based on the phenotype of an individual, it is not always possible to determine its genotype, since both an organism homozygous for the dominant gene (AA) and heterozygous (Aa) will have a manifestation of the dominant allele in the phenotype. Therefore, to check the genotype of an organism with cross-fertilization, they use test cross- a crossing in which an organism with a dominant trait is crossed with one homozygous for a recessive gene. In this case, an organism homozygous for the dominant gene will not produce splitting in the offspring, whereas in the offspring of heterozygous individuals there is an equal number of individuals with dominant and recessive traits.

The following conventions are most often used to record crossing schemes:

R (from lat. parenta- parents) - parent organisms;

$♀$ (alchemical sign of Venus - mirror with handle) - maternal specimen;

$♂$ (alchemical sign of Mars - shield and spear) - paternal individual;

$×$ — crossing sign;

F 1, F 2, F 3, etc. - hybrids of the first, second, third and subsequent generations;

F a - offspring from an analyzing cross.

Chromosomal theory of heredity

The founder of genetics, G. Mendel, as well as his closest followers, did not have the slightest idea about the material basis of hereditary inclinations, or genes. However, already in 1902-1903, the German biologist T. Boveri and the American student W. Satton independently suggested that the behavior of chromosomes during cell maturation and fertilization makes it possible to explain the splitting of hereditary factors according to Mendel, i.e., in their opinion, genes must be located on chromosomes. These assumptions became the cornerstone of the chromosomal theory of heredity.

In 1906, English geneticists W. Bateson and R. Punnett discovered a violation of Mendelian segregation when crossing sweet peas, and their compatriot L. Doncaster, in experiments with the gooseberry moth butterfly, discovered sex-linked inheritance. The results of these experiments clearly contradicted Mendelian ones, but if we consider that by that time it was already known that the number of known characteristics for experimental objects far exceeded the number of chromosomes, and this suggested that each chromosome carries more than one gene, and the genes of one chromosomes are inherited together.

In 1910, experiments by T. Morgan's group began on a new experimental object - the Drosophila fruit fly. The results of these experiments made it possible by the mid-20s of the 20th century to formulate the basic principles of the chromosomal theory of heredity, to determine the order of genes in chromosomes and the distances between them, i.e., to draw up the first maps of chromosomes.

Basic provisions of the chromosomal theory of heredity:

Genes are located on chromosomes. Genes on the same chromosome are inherited together, or linked, and are called clutch group. The number of linkage groups is numerically equal to the haploid set of chromosomes.
Each gene occupies a strictly defined place on the chromosome - a locus.
Genes on chromosomes are arranged linearly.
Disruption of gene linkage occurs only as a result of crossing over.
The distance between genes on a chromosome is proportional to the percentage of crossing over between them.
Independent inheritance is typical only for genes on non-homologous chromosomes.

Modern ideas about the gene and genome

In the early 40s of the twentieth century, J. Beadle and E. Tatum, analyzing the results of genetic studies conducted on the neurospora fungus, came to the conclusion that each gene controls the synthesis of an enzyme, and formulated the principle of “one gene - one enzyme” .

However, already in 1961, F. Jacob, J. L. Monod and A. Lvov managed to decipher the structure of the E. coli gene and study the regulation of its activity. For this discovery they were awarded the Nobel Prize in Physiology or Medicine in 1965.

In the process of research, in addition to structural genes that control the development of certain traits, they were able to identify regulatory ones, the main function of which is the manifestation of traits encoded by other genes.

Structure of a prokaryotic gene. The structural gene of prokaryotes has a complex structure, since it includes regulatory regions and coding sequences. The regulatory regions include the promoter, operator, and terminator. Promoter called the region of the gene to which the enzyme RNA polymerase is attached, which ensures the synthesis of mRNA during transcription. WITH operator, located between the promoter and the structural sequence, can bind repressor protein, which does not allow RNA polymerase to begin reading the hereditary information from the coding sequence, and only its removal allows transcription to begin. The structure of the repressor is usually encoded in a regulatory gene located in another part of the chromosome. Reading of information ends at a section of the gene called terminator.

Coding sequence A structural gene contains information about the amino acid sequence of the corresponding protein. The coding sequence in prokaryotes is called cistronome, and the totality of coding and regulatory regions of a prokaryotic gene is operon. In general, prokaryotes, which include E. coli, have a relatively small number of genes located on a single circular chromosome.

The cytoplasm of prokaryotes may also contain additional small circular or open DNA molecules called plasmids. Plasmids are able to integrate into chromosomes and be transmitted from one cell to another. They may carry information about sex characteristics, pathogenicity and antibiotic resistance.

Structure of a eukaryotic gene. Unlike prokaryotes, eukaryotic genes do not have an operon structure, since they do not contain an operator, and each structural gene is accompanied only by a promoter and terminator. In addition, in eukaryotic genes significant regions ( exons) alternate with insignificant ones ( introns), which are completely transcribed into mRNA and then excised during their maturation. The biological role of introns is to reduce the likelihood of mutations in significant regions. The regulation of genes in eukaryotes is much more complex than that described for prokaryotes.

Human genome. In each human cell, the 46 chromosomes contain about 2 m of DNA, tightly packed into a double helix, which consists of approximately 3.2 $×$ 10 9 nucleotide pairs, which provides about 10 1900000000 possible unique combinations. By the end of the 80s of the twentieth century, the location of approximately 1,500 human genes was known, but their total number was estimated at approximately 100 thousand, since humans have approximately 10 thousand hereditary diseases alone, not to mention the number of various proteins contained in cells .

In 1988, the international Human Genome project was launched, which by the beginning of the 21st century ended with a complete decoding of the nucleotide sequence. He made it possible to understand that two different people have 99.9% similar nucleotide sequences, and only the remaining 0.1% determine our individuality. In total, approximately 30-40 thousand structural genes were discovered, but then their number was reduced to 25-30 thousand. Among these genes there are not only unique ones, but also repeated hundreds and thousands of times. However, these genes encode a much larger number of proteins, for example tens of thousands of protective proteins - immunoglobulins.

97% of our genome is genetic “junk” that exists only because it can reproduce well (RNA that is transcribed in these regions never leaves the nucleus). For example, among our genes there are not only “human” genes, but also 60% of genes similar to the genes of the Drosophila fly, and up to 99% of our genes are similar to chimpanzees.

In parallel with the decoding of the genome, chromosome mapping also took place, as a result of which it was possible not only to discover, but also to determine the location of some genes responsible for the development of hereditary diseases, as well as drug target genes.

Decoding the human genome has not yet given a direct effect, since we have received a kind of instruction for assembling such a complex organism as a person, but have not learned how to manufacture it or at least correct errors in it. Nevertheless, the era of molecular medicine is already on the threshold; all over the world, so-called gene preparations are being developed that can block, delete or even replace pathological genes in living people, and not just in a fertilized egg.

We should not forget that in eukaryotic cells DNA is contained not only in the nucleus, but also in mitochondria and plastids. Unlike the nuclear genome, the organization of genes in mitochondria and plastids has much in common with the organization of the prokaryotic genome. Despite the fact that these organelles carry less than 1% of the cell's hereditary information and do not even encode the full set of proteins necessary for their own functioning, they are capable of significantly influencing some of the body's characteristics. Thus, variegation in plants of chlorophytum, ivy and others is inherited by a small number of descendants even when crossing two variegated plants. This is due to the fact that plastids and mitochondria are transmitted mostly with the cytoplasm of the egg, therefore such heredity is called maternal, or cytoplasmic, in contrast to genotypic, which is localized in the nucleus.

Problem 1
When crossing two varieties of tomato with red spherical and yellow pear-shaped fruits in the first generation, all the fruits are spherical and red. Determine the genotypes of parents, first-generation hybrids, and the ratio of second-generation phenotypes.
Solution:
Since when crossing peas, all offspring individuals have the trait of one of the parents, which means that the genes for red color (A) and the genes for the spherical shape of fruits (B) are dominant in relation to the genes for yellow color (a) and pear-shaped fruits (b). genotypes of parents: red spherical fruits - AABB, yellow pear-shaped fruits - aabb.
To determine the genotypes of the first generation, the ratio of phenotypes of the second generation, it is necessary to draw up crossing schemes:

First crossing scheme:

Uniformity of the first generation is observed, the genotypes of individuals are AaBb (Mendel's 1st law).

Second crossing scheme:

The ratio of phenotypes of the second generation: 9 – red spherical; 3 – red pear-shaped; 3 - yellow spherical; 1 – yellow pear-shaped.
Answer:
1) genotypes of the parents: red spherical fruits - AABB, yellow pear-shaped fruits - aabb.
2) genotypes F 1: red spherical AaBb.
3) ratio of phenotypes F 2:
9 – red spherical;
3 – red pear-shaped;
3 - yellow spherical;
1 – yellow pear-shaped.

Problem 2
The absence of small molars in humans is inherited as a dominant autosomal trait. Determine the possible genotypes and phenotypes of parents and offspring if one of the spouses has small molars, while the other does not have them and is heterozygous for this trait. What is the likelihood of having children with this anomaly?
Solution:
Analysis of the problem conditions shows that the crossed individuals are analyzed according to one characteristic - molars, which is represented by two alternative manifestations: the presence of molars and the absence of molars. Moreover, it is said that the absence of molars is a dominant trait, and the presence of molars is a recessive trait. This task is on, and to designate alleles it will be enough to take one letter of the alphabet. We denote the dominant allele by the capital letter A, and the recessive allele by the lowercase letter a.
A - absence of molars;
a - the presence of molars.
Let's write down the genotypes of the parents. We remember that the genotype of an organism includes two alleles of the studied gene “A”. The absence of small molars is a dominant trait, therefore a parent who lacks small molars and is heterozygous means his genotype is Aa. The presence of small molars is a recessive trait, therefore a parent who lacks small molars is homozygous for the recessive gene, which means its genotype is aa.
When a heterozygous organism is crossed with a homozygous recessive organism, two types of offspring are formed, both genotype and phenotype. Crossbreeding analysis confirms this statement.

Crossing scheme

Answer:
1) genotypes and phenotypes P: aa – with small molars, Aa – without small molars;
2) genotypes and phenotypes of the offspring: Aa – without small molars, aa – with small molars; the probability of having children without small molars is 50%.

Problem 3
In humans, the gene for brown eyes (A) dominates over blue eyes, and the gene for color blindness is recessive (color blindness - d) and linked to the X chromosome. A brown-eyed woman with normal vision, whose father had blue eyes and suffered from color blindness, marries a blue-eyed man with normal vision. Make a diagram for solving the problem. Determine the genotypes of the parents and possible offspring, the likelihood of having color-blind children with brown eyes and their gender in this family.
Solution:

Since the woman is brown-eyed, and her father suffered from color blindness and was blue-eyed, she received the recessive blue-eyed gene and the color blindness gene from her father. Consequently, a woman is heterozygous for the eye color gene and is a carrier of the color blindness gene, since she received one X chromosome from a color blind father, her genotype is AaX D X d. Since the man is blue-eyed with normal vision, his genotype will be homozygous for the recessive gene a and the X chromosome will contain a dominant gene for normal vision, his genotype is aaX D Y.
Let's determine the genotypes of possible offspring, the probability of birth in this family of color-blind children with brown eyes and their gender, drawing up a crossing scheme:

Crossing scheme

Answer:
The scheme for solving the problem includes: 1) mother’s genotype – AaX D X d (gametes: AX D, aX D, AX d, aX D), father’s genotype – aaX D Y (gametes: aX D, aY);
2) genotypes of children: girls – AaX D X D, aaX D X D, AaX D X d, aaX D X d; boys – AaX D Y, aaXDY, AaX d Y, aaX D Y;
3) the probability of having color-blind children with brown eyes: 12.5% AaX d Y – boys.

Problem 4
When a pea plant with smooth seeds and tendrils was crossed with a plant with wrinkled seeds without tendrils, the entire generation was uniform and had smooth seeds and tendrils. When crossing another pair of plants with the same phenotypes (peas with smooth seeds and tendrils and peas with wrinkled seeds without tendrils), half of the plants with smooth seeds and tendrils and half of the plants with wrinkled seeds without tendrils were obtained. Make a diagram of each cross.
Determine the genotypes of parents and offspring. Explain your results. How are dominant traits determined in this case? What law of genetics is manifested in this case?
Solution:
This task is for dihybrid crossing, since the crossed organisms are analyzed according to two pairs of alternative characteristics. The first pair of alternative characters: seed shape - smooth seeds and wrinkled seeds; the second pair of alternative characters: the presence of antennae - the absence of antennae. Alleles of two different genes are responsible for these traits. Therefore, to designate alleles of different genes, we will use two letters of the alphabet: “A” and “B”. Genes are located on autosomes, so we will designate them only using these letters, without using the symbols of the X and Y chromosomes.
Since when crossing a pea plant with smooth seeds and tendrils with a plant with wrinkled seeds without tendrils, the entire generation was uniform and had smooth seeds and tendrils, we can conclude that the trait smooth pea seeds and the sign of the absence of tendrils are dominant traits.
And the gene that determines the smooth shape of peas; a - gene that determines the wrinkled shape of peas; B - gene that determines the presence of antennae in peas; b - gene that determines the absence of tendrils in peas. Parental genotypes: AABB, aabb.

First crossing scheme

Since during the 2nd crossing there was a split in two pairs of characters in a 1:1 ratio, we can assume that the genes determining smooth seeds and the presence of tendrils (A, B) are localized on one chromosome and are inherited linked, a plant with smooth seeds and tendrils are heterozygous, which means the genotypes of the parents of the second pair of plants are: AaBb; aabb.
Crossbreeding analysis confirms these arguments.

Second crossing scheme

Answer:
1. The genes that determine smooth seeds and the presence of tendrils are dominant, since during the 1st crossing the entire generation of plants was the same and had smooth seeds and tendrils. Genotypes of the parents: smooth seeds and tendrils - AABB (ametes AB), wrinkled seeds and without tendrils - aabb (ametes - ab). The genotype of the offspring is AaBb. The law of uniformity of the first generation appears when crossing this pair of plants
2. When crossing the second pair of plants, the genes that determine smooth seeds and the presence of tendrils (A, B) are localized on one chromosome and are inherited linked, since during the 2nd crossing, splitting occurred in two pairs of characters in a 1:1 ratio. The law of linked inheritance appears.

Problem 5
The genes for coat color in cats are located on the X chromosome. Black coloring is determined by the X B gene, red coloring is determined by the X b gene, heterozygotes X B X b have a tortoiseshell coloration. From a black cat and a red cat were born: one tortoiseshell and one black kitten. Make a diagram for solving the problem. Determine the genotypes of parents and offspring, the possible sex of kittens.
Solution:
An interesting combination: the genes for black and red colors do not dominate each other, but in combination give a tortoiseshell coloration. Codominance (gene interaction) is observed here. Let's take: X B – the gene responsible for the black color, X b – the gene responsible for the red color; X B and X b genes are equivalent and allelic (X B = X b).
Since a black cat and a red cat were crossed, their gentypes will look like: cat - X B X B (gametes X B), cat - X b Y (gametes X b, Y). With this type of crossing, the birth of black and tortoiseshell kittens is possible in a 1:1 ratio. Crossbreeding analysis confirms this judgment.

Crossing scheme

Answer:
1) genotypes of the parents: cat X B X B (gametes X B), cat - X b Y (gametes X b, Y);
2) genotypes of kittens: tortoiseshell - X B X b, X B X b Y;
3) gender of kittens: female - tortoiseshell, male - black.
When solving the problem, we used the law of gamete purity and sex-linked inheritance. Gene interaction - codominance. The type of crossing is monohybrid.

Problem 6
Diheterozygous male Drosophila flies with a gray body and normal wings (dominant traits) were crossed with females with a black body and shortened wings (recessive traits). Make a diagram for solving the problem. Determine the genotypes of the parents, as well as the possible genotypes and phenotypes of the offspring F 1, if the dominant and recessive genes of these traits are pairwise linked, and crossing over does not occur during the formation of germ cells. Explain your results.
Solution:
The genotype of a diheterozygous male is AaBb, the genotype of a female homozygous for recessive traits is: aabb. Since the genes are linked, the male produces two types of gametes: AB, ab, and the female produces one type of gamete: ab, so the offspring exhibit only two phenotypes in a 1:1 ratio.
Crossbreeding analysis confirms these arguments.

Crossing scheme

Answer:
1) genotypes of the parents: female aabb (gametes: ab), male AaBb (gametes: AB, ab);
2) genotypes of the offspring: 1AaBb gray body, normal wings; 1 aabb black body, shortened wings;
3) since the genes are linked, the male gives two types of gametes: AB, ab, and the female gives one type of gametes: ab, therefore the offspring exhibit only two phenotypes in a 1:1 ratio. The law of concatenated inheritance appears.

Problem 7
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, and the genotypes and phenotypes of other possible offspring. draw up a diagram for solving the problem. Traits are inherited independently.
Solution:
Given:
Each of the parents has a free earlobe and a triangular fossa and they gave birth to a child with a fused earlobe and a smooth chin, which means that a free earlobe and a triangular chin are dominant traits, and a fused earlobe and a smooth chin are recessive traits. From these arguments we conclude: the parents are diheterozygous, and the child is dihomozygous for recessive traits. Let's create a table of features:

Therefore, the genotypes of the parents: mother AaBb (gametes AB, Ab, Ab, ab), father AaBb (gametes AB, Ab, Ab, ab), genotype of the first child: aabb - fused lobe, smooth chin.
Crossbreeding analysis confirms this judgment.

Phenotypes and genotypes of offspring:
free lobe, triangular fossa, A_B_
loose lobe, smooth chin, A_bb
fused lobe, triangular fossa, aaB_

Answer:
1) genotypes of the parents: mother AaBb (gametes AB, Ab, Ab, ab), father AaBb (gametes AB, Ab, Ab, ab);
2) genotype of the first child: aabb - fused lobe, smooth chin;
3) genotypes and phenotypes of possible descendants:
loose lobe, smooth chin, A_bb;
free lobe, triangular fossa, A_B_;
fused lobe, smooth chin, aabb.

Problem 8
In chickens, a sex-linked lethal gene (a) is found that causes the death of embryos; heterozygotes for this trait are viable. Draw up a scheme for solving the problem, determine the genotypes of the parents, sex, genotype of possible offspring and the probability of embryo death.
Solution:
According to the problem:
X A - development of a normal embryo;
X a - death of the embryo;
X A X a - viable individuals.
Determine the genotypes and phenotypes of the offspring

Crossing scheme

Answer:
1) genotypes of the parents: X A Y (gametes X A, Y), X A X A (gametes X A, X A);
2) genotypes of possible offspring: X A Y, X A X A, X A X a, X a Y;
3) 25% - X and Y are not viable.

luck 9
When a plant with long striped fruits was crossed with a plant with round green fruits, the offspring were plants with long green and round green fruits. When the same watermelon (with long striped fruits) was crossed with a plant that had round striped fruits, all the offspring had round striped fruits. Determine the dominant and recessive traits, genotypes of all parent watermelon plants.
Solution:
A - gene responsible for the formation of a round fruit
a - gene responsible for the formation of a long fruit
B - gene responsible for the formation of green color of the fruit
b - gene responsible for the formation of a striped fetus
Since when crossing a plant with long striped fruits with a plant with round green fruits, the F 1 offspring produced plants with long green and round green fruits, we can conclude that the dominant traits are round green fruits, and the recessive traits are long striped ones. The genotype of a plant with long striped fruits is aabb, and the genotype of a plant with round green fruits is AaBB, because in the offspring all individuals have green fruits, and 1/2 each have round and long fruits, which means that this plant is heterozygous for the dominant trait of shape fetus and homozygous for the dominant trait of fruit color. Genotype of offspring F 1: AaBb, aaBb. Considering that when crossing a parent watermelon with long striped fruits (digomozygous for recessive traits) with a plant having round striped fruits, all F 2 offspring had round striped fruits, the genotype of the parent plant with green striped fruits taken for the second crossing is: AAbb. The genotype of the offspring F 2 is Aabb.
Analyzes of the crosses carried out confirm our assumptions.

First crossing scheme

Second crossing scheme

Answer:
1) dominant characters - fruits are round, green, recessive characters - fruits are long, striped;
2) genotypes of parents F 1: aabb (long striped) and AaBB (round green);
3) genotypes of parents F 2: aabb (long striped) and AAbb (round striped).

Problem 10
A Datura plant with purple flowers (A) and smooth capsules (b) was crossed with a plant with purple flowers and spiny capsules. The following phenotypes were obtained in the offspring: with purple flowers and spiny capsules, with purple flowers and smooth capsules, with white flowers and smooth capsules, with white flowers and spiny capsules. Make a diagram for solving the problem. Determine the genotypes of parents, offspring and possible relationships between phenotypes. Establish the nature of inheritance of traits.
Solution:
And the gene for purple flower color;
a - gene for white flower color;
B - gene that forms the spiny capsule;
b - gene that forms a smooth capsule.
This task is for dihybrid crossing (independent inheritance of traits during dihybrid crossing), since plants are analyzed according to two characteristics: flower color (purple and white) and capsule shape (smooth and spiny). These traits are caused by two different genes. Therefore, to designate genes, we will take two letters of the alphabet: “A” and “B”. Genes are located on autosomes, so we will designate them only using these letters, without using the symbols of the X and Y chromosomes. The genes responsible for the analyzed traits are not linked to each other, so we will use the gene record of the cross.
Purple coloring is a dominant trait (A), and white coloring that appears in the offspring is a recessive trait (a). Each parent has a purple flower, which means they both carry the dominant gene A. Since they have offspring with the aa genotype, each of them must also carry the recessive gene a. Consequently, the genotype of both parental plants for the flower color gene is Aa. The trait spiny capsule is dominant in relation to the trait smooth capsule, and since when crossing a plant with a spiny capsule and a plant with a smooth capsule, offspring with both a spiny capsule and a smooth capsule appeared, the genotype of the parent with the dominant trait for the shape of the capsule will be heterozygous ( Bb), and in recessive - (bb). Then the genotypes of the parents are: Aabb, aaBb.
Now let’s determine the genotypes of the offspring by analyzing the crossing of parental plants:

Crossing scheme

Answer:
1) genotypes of the parents: Aabb (gametes Ab, ab) * AaBb (gametes AB, Ab, aB, ab);
2) genotypes and phenotype ratios:
3/8 purple spiny (AABb and AaBb);
3/8 purple smooth (AAbb and Aabb);
1/8 white spiny (aaBb);
1/8 white smooth (aabb);

Problem 11
It is known that Huntington's chorea (A) is a disease that manifests itself after 35-40 years and is accompanied by progressive impairment of brain function, and a positive Rh factor (B) is inherited as unlinked autosomal dominant traits. The father is diheterozygous for these genes, and the mother has a negative Rh factor and is healthy. Draw up a scheme for solving the problem and determine the genotypes of the parents, possible offspring and the probability of having healthy children with a positive Rh factor.
Solution:
And the gene for Huntington's disease;
a - gene for normal brain development;
B - positive Rh factor gene;
b - negative Rh factor gene
This task is for dihybrid crossing (unlinked autosomal dominant inheritance of traits in dihydride crossing). According to the conditions of the problem, the father is diheterozygous, which means his genotype is AaBb. The mother is phenotypically recessive for both traits, which means her genotype is aabb.
Now let’s determine the genotypes of the offspring by analyzing the crossing of parents:

Crossing scheme

Answer:
1) genotypes of the parents: father - AaBb (gametes AB Ab, aB, ab), mother aabb (gametes ab);
2) genotypes of the offspring: AaBb, Aabb, aaBb, aabb;
3) 25% of offspring with the aaBb genotype are Rh positive and healthy.

A plant with red fruits produces gametes that carry dominant alleles AB, and a plant with yellow fruits produces gametes carrying recessive alleles aw. The combination of these gametes leads to the formation of a diheterozygote AaVv, since genes A And IN dominant, then all first-generation hybrids will have red and smooth fruits.

Let's cross the plants with red and smooth fruits from the generation F 1 with a plant having yellow and pubescent fruits (Fig. 2). Let's determine the genotype and phenotype of the offspring.

Rice. 2. Crossing scheme ()

One of the parents is a diheterozygote, its genotype AaVv, the second parent is homozygous for recessive alleles, its genotype is aaww. A diheterozygous organism produces the following types of gametes: AB, Av, aB, aw; homozygous organism - gametes of the same type: aw. This results in four genotypic classes: AaVv, Aaww, aaVv, aaww and four phenotypic classes: red smooth, red pubescent, yellow smooth, yellow pubescent.

Splitting according to each of the characteristics: according to the color of the fruit 1:1, according to the skin of the fruit 1:1.

This is a typical test cross, which allows you to determine the genotype of an individual with a dominant phenotype. A dihybrid cross is two independently occurring monohybrid crosses, the results of which overlap each other. The described mechanism of inheritance during dihybrid crossing refers to traits whose genes are located in different pairs of non-homologous chromosomes, that is, in one pair of chromosomes there are genes responsible for the color of tomato fruits, and in the other pair of chromosomes there are genes responsible for the smoothness or pubescence of the skin of the fruit.

From crossing two pea plants grown from yellow and smooth seeds, 264 yellow smooth, 61 yellow wrinkled, 78 green smooth, 29 green wrinkled seeds were obtained. Determine which cross the observed ratio of phenotypic classes belongs to.

In the condition, the splitting from the crossing is given, four phenotypic classes are obtained with the following splitting 9: 3: 3: 1, and this indicates that two diheterozygous plants having the following genotype were crossed: AaVv And AaVv(Fig. 3).

Rice. 3 Crossing scheme for problem 2 ()

If we construct a Punnett grid, in which we write gametes horizontally and vertically, and zygotes obtained from the fusion of gametes in squares, we will obtain four phenotypic classes with the splitting specified in the problem (Fig. 4).

Rice. 4. Punnett lattice for problem 2 ()

Incomplete dominance according to one of the characteristics. In the snapdragon plant, the red color of the flowers does not completely suppress the white color; the combination of the dominant and recessive alleles causes the pink color of the flowers. The normal flower shape dominates the elongated and pyloric flower shape (Fig. 5).

Rice. 5. Crossing snapdragons ()

Homozygous plants with normal white flowers were crossed with a homozygous plant with elongated red flowers. It is necessary to determine the genotype and phenotype of the offspring.

The task:

A- red coloring is a dominant trait

A- white coloring is a recessive trait

IN- normal form - dominant trait

V- pyloric form - recessive trait

aaBB- genotype of white color and normal flower shape

AAbb- genotype of red pyloric flowers

They produce gametes of the same type, in the first case gametes carrying alleles aB, in the second case - Av. The combination of these gametes leads to the emergence of a diheterozygote with the genotype AaVv- all hybrids of the first generation will have a pink color and a normal flower shape (Fig. 6).

Rice. 6. Crossing scheme for problem 3 ()

Let's cross the first generation hybrids to determine the color and shape of the flower in the generation F 2 with incomplete dominance in color.

Genotypes of parent organisms - AaVv And AvVv,

Hybrids produce four types of gametes: AB, Av, aB, aw(Fig. 7).

Rice. 7. Scheme of crossing first generation hybrids, task 3 ()

When analyzing the resulting offspring, we can say that we did not succeed in the traditional splitting according to the 9:3 and 3:1 phenotype, since the plants have incomplete dominance in flower color (Fig. 8).

Rice. 8. Punnett table for problem 3 ()

Of the 16 plants: three red normal, six pink normal, one red pyloric, two pink pyloric, three white normal and one white pyloric.

We looked at examples of solving dihybrid crossing problems.

In humans, brown eye color dominates over blue, and the ability to better use the right hand dominates over left-handedness.

Problem 4

A brown-eyed right-hander married a blue-eyed left-hander, and they had two children - a blue-eyed right-hander and a blue-eyed left-hander. Determine the mother's genotype.

Let's write down the condition of the problem:

A- Brown eyes

A- Blue eyes

IN- right-handedness

V- left-handedness

aaww- father’s genotype, he is homozygous for recessive alleles of two genes

A - ? IN- ? - the mother’s genotype has two dominant genes and theoretically can have

genotypes: AABB, AaBB, AAVv, AaVv.

F 1 - aaww, aaB - ?

If there is a genotype AABB the mother would not experience any splitting in her offspring: all children would be brown-eyed, right-handed and would have the genotype AaVv, since the father produces gametes of the same type aw(Fig. 9).

Rice. 9. Crossing scheme for problem 4 ()

Two children have blue eyes, which means the mother is heterozygous for eye color Ahh, in addition, one of the children is left-handed - this suggests that the mother has a recessive gene V, responsible for left-handedness, that is, the mother is a typical diheterozygote. The crossing scheme and possible children from this marriage are presented in Fig. 10.

Rice. 10. Crossbreeding scheme and possible children from marriage ()

A trihybrid cross is a cross in which the parent organisms differ from each other in three pairs of alternative characteristics.

Example: Crossing a pea with smooth yellow seeds and purple flowers with green wrinkled seeds and white flowers.

Trihybrid plants will display dominant characteristics: yellow color and smooth seed shape with purple flower color (Fig. 11).

Rice. 11. Trihybrid crossing scheme ()

Trihybrid plants, as a result of independent gene splitting, produce

eight types of gametes - female and male, when combined, they will give F 2 64 combinations, 27 genotypes and 8 phenotypes.

Bibliography

Mamontov S.G., Zakharov V.B., Agafonova I.B., Sonin N.I. Biology 11th grade. General biology. Profile level. - 5th edition, stereotypical. - Bustard, 2010.
Belyaev D.K. General biology. A basic level of. - 11th edition, stereotypical. - M.: Education, 2012.
Pasechnik V.V., Kamensky A.A., Kriksunov E.A. General biology, grades 10-11. - M.: Bustard, 2005.
Agafonova I.B., Zakharova E.T., Sivoglazov V.I. Biology 10-11 grade. General biology. A basic level of. - 6th ed., add. - Bustard, 2010.

Biorepet-ufa.ru ().
Kakprosto.ru ().
Genetika.aiq.ru ().

Homework

Define dihybrid cross.
Write the possible types of gametes produced by organisms with the following genotypes: AABB, CcDD.
Define trihybrid cross.

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 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-allelic genes (determine the nature of crossing).
Determine the dominant and recessive trait(s) based on the conditions of the problem, drawing, diagram, or based on the results of crossing F 1 and F 2.
Enter the letter designations of the dominant (capital letter) and recessive (capital letter) characteristics, if they are not given in the task conditions.
Record the phenotypes and genotypes of the parental forms.
Record the phenotypes and genotypes of the offspring.
Draw up a crossing diagram, be sure to indicate the gametes that form the parent forms.
Write down the answer.

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

Write a short note of the task.
Analyze each feature separately, making a corresponding entry for each feature.
Apply monohybrid crossing formulas, if none of them are suitable, then
- Add the weight of the numerical indicators in the offspring, divide the sum by 16, find one part and express all the numerical indicators in parts.
- Based on the fact that the splitting in F 2 dihybrid crossing occurs according to the formula 9А_В_: 3A_bb: 3 ааВ_: l aabb, find the genotypes Fr
- For F 2, find F genotypes
- Using F, find the genotypes of the parents.

Formulas for determining the nature of crossing:

where n is the number of alleles, pairs of characteristics

Segregation by genotype – (3:1) n
Phenotype splitting – (1:2:1) n
Number of gamete types – 2 n
Number of phenotypic classes - 2 n
Number of genotypic classes - 3 n
Number of possible combinations, combinations of gametes – 4 n

Basic rules for solving genetic problems:

If, when crossing two phenotypically identical individuals, a split in characteristics is observed in their offspring, then these individuals are heterozygous.
If, as a result of crossing individuals that differ feiotypically in one pair of traits, offspring are obtained that have segregation in the same pair of traits, then one of the parent individuals was heterozygous, and the other was homozygous for a recessive trait.
If, when crossing feiotypically identical (one pair of traits) individuals in the first generation of hybrids, the traits 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 crossing two feiotypically identical individuals, the characteristics in the offspring are split in the ratio 9:3:3:1, then the original individuals were diheterozygous.

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

Heredity

Basic Concepts
Gene and alleles. The unit of hereditary information is the gene.
Gene(from the point of view of genetics) - a section of a chromosome that determines the development of one or more characteristics in an organism.
Alleles- different states of the same gene, located in a certain locus (region) of homologous chromosomes and determining the development of one particular trait. Homologous chromosomes are present only in cells containing a diploid set of chromosomes. They are not found in the sex cells (gametes) of eukaryotes or prokaryotes.

Sign (hairdryer)- some quality or property by which one organism can be distinguished from another.
Domination- the phenomenon of predominance of the trait of one of the parents in a hybrid.
Dominant trait- a trait that appears in the first generation of hybrids.
Recessive trait- a trait that outwardly disappears in the first generation of hybrids.

Dominant and recessive traits in humans

Signs
dominant	recessive
Dwarfism	Normal height
Polydactyly (polydactyly)	Norm
Curly hair	Straight hair
Not red hair	Red hair
Early baldness	Norm
Long eyelashes	Short eyelashes
Large eyes	Small eyes
Brown eyes	Blue or gray eyes
Myopia	Norm
Twilight vision (night blindness)	Norm
Freckles on the face	No freckles
Normal blood clotting	Poor blood clotting (hemophilia)
Color vision	Lack of color vision (color blindness)

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

G. Mendel's laws

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

Mendel's first law - the law of uniformity of first generation hybrids (law of dominance): When pure lines are crossed, all first-generation hybrids exhibit one trait (dominant).
Mendel's second law. After this, G. Mendel crossed the first generation hybrids with each other.

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

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

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

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

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

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

Cytological basis of independent combination is the random nature of the divergence of homologous chromosomes of each pair to different poles of the cell during the process of meiosis, regardless of other pairs of homologous chromosomes. This law is only valid if the genes responsible for the development of different traits are located on different chromosomes. Exceptions are cases of linked inheritance.

Chained inheritance. Loss of adhesion

The development of genetics has shown that not all traits are inherited in accordance with Mendel's laws. Thus, the law of independent inheritance of genes is valid only for genes located on different chromosomes.
The patterns of linked inheritance of genes were studied by T. Morgan and his students in the early 20s. XX century The object of their research was the fruit fly Drosophila (its lifespan is short, and several dozen generations can be obtained in a year; its karyotype consists of only four pairs of chromosomes).
Morgan's Law: genes localized on the same chromosome are inherited predominantly together.
Linked genes - genes lying on the same chromosome.
Clutch group - all genes on one chromosome.
In a certain percentage of cases, adhesion may be broken. The reason for the disruption of cohesion is crossing over (crossing of chromosomes) - the exchange of chromosome sections in prophase I of the meiotic division. Crossing over leads to genetic recombination. The farther genes are located from each other, the more often crossing over occurs between them. The construction is based on this phenomenon genetic maps- determination of the sequence of genes on the chromosome and the approximate distance between them.

Genetics of sex

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

Types of chromosomal sex determination

Type	Examples
♀ XX, ♂ ХY	Characteristic of mammals (including humans), worms, crustaceans, most insects (including fruit flies), most amphibians, some fish
♀ XY, ♂ XX	Characteristic of birds, reptiles, some amphibians and fish, some insects (Lepidoptera)
♀ XX, ♂ X0	Occurs in some insects (orthoptera); 0 means no chromosomes
♀ X0, ♂ XX	Found in some insects (homoptera)
haplo-diploid type (♀ 2n, ♂ n)	It is found, for example, in bees and ants: males develop from unfertilized haploid eggs (parthenogenesis), females from fertilized diploid eggs.

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

Blood genetics

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

Group I	0	I 0 I 0	homozygote
Group II	A	I A I A	homozygote
Group II	A	I A I 0	heterozygote
III group	IN	I B I B	homozygote
III group	IN	I B I 0	heterozygote
IV group	AB	I A I B	heterozygote

Different peoples have different ratios of blood groups in the population.

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

In addition, the blood of different people may differ in the Rh factor. Blood can be Rh positive (Rh +) or Rh negative (Rh -). This ratio varies among different nations.

Distribution of Rh factor among different peoples,%

Nationality	Rh positive	Rh negative
Australian Aboriginals	100	0
American Indians	90–98	2–10
Arabs	72	28
Basque	64	36
Chinese	98–100	0–2
Mexicans	100	0
Norse	85	15
Russians	86	14
Eskimos	99–100	0–1
Japanese	99–100	0–1

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

Gene interaction

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

Chromosomal theory of heredity

Basic provisions of the chromosomal theory of heredity:

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

Non-chromosomal inheritance

According to the chromosomal theory of heredity, the DNA of chromosomes plays a leading role in heredity. However, DNA is also contained in mitochondria, chloroplasts and in the cytoplasm. Non-chromosomal DNA is called plasmids . Cells do not have special mechanisms for uniform distribution of plasmids during division, so one daughter cell can receive one genetic information, and the second - completely different. The inheritance of genes contained in plasmids does not obey Mendelian laws of inheritance, and their role in the formation of the genotype has not yet been studied enough.