Chromosomal gender. The genotype as an integral system. Interaction of genes, multiple action of genes. Types of chromosomal sex determination

Lecture

Sex genetics.

Chromosomal sex determination mechanism

One of the important problems in biology has always been the mystery of the birth of organisms of different sexes. Hundreds of hypotheses about the nature of this phenomenon were published in the works of past centuries and especially in the 19th century. However, only the chromosome theory made it possible to understand the internal mechanism of sex determination and the reason why in nature, in most cases, half of the males and half of the females are born. The chromosomal mechanism of sex inheritance was discovered in the laboratory of T. Morgan by E. Wilson in 1914 while studying the karyotype of the Drosophila fly. He proved that out of 4 pairs of male and female chromosomes, 3 pairs were identical in structure. The fourth pair was different. In the female, both chromosome pairs were the same - submetacentric. The male had different chromosomes: one homologous to a chromosome females are submetacentric, the other small is acrocentric. The submetacentric chromosome was designated as X, and the acrocentric - Y. Thus, the karyotypes of the female and male are different, and this difference is in one pair of chromosomes, which are called sexual. The chromosomes in which the male and female sexes do not differ are called autosomes.

Thus, in the Drosophila genotype there are only 8 chromosomes: 6 autosomes and 2 sex chromosomes. In the female, the chromosome set is 6A + XX, in the male - 6A + XY. In a female, one type of gametes is formed - all germ cells contain - 3A + X. AT this case such a sex is called homogametic. The male sex forms two types of gametes 3A + X - 50% and 3A + Y - 50%. This sex is called heterogametic.

Types chromosomal definition gender

Females have two XX chromosomes (homogametic sex), and males have one X chromosome and an unpaired Y chromosome (heterogametic sex). This type of sex determination in mammals, Diptera, beetles.

The male sex is heterogametic - 50% of the gametes carry the X gene, 50% do not have a sex chromosome. The female karyotype is 2A+XX, the male karyotype is 2A+XO. Described in most orthopteran insects, centipedes, beetles, spiders, nematodes.

The female sex is heterogametic - 50% of the gametes carry the X gene, 50% of the gametes carry the Y gene. In this case, other letters are used to designate the sex chromosomes: female - ZW, male - ZZ. This type of sex determination is typical for birds, butterflies, and tailed amphibians.

In moths, the female sex is heterogametic, 50% of the gametes carry the X gene, and 50% do not have a sex chromosome.

A special type of sex determination is characteristic of bees. Here, the difference between the sexes affects not one pair of chromosomes, but the entire set. Female bees are diploid, males are haploid. Females develop from fertilized eggs, males as a result of parthenogenesis.

Sex determination different organisms may take place on different stages life cycle.

The sex of the body can be determined even during the period of maturation of female germ cells - eggs. This definition of sex is called software, i.e. it occurs before fertilization. Progamous sex determination has been found in rotifers, annelids. Oocytes in these organisms differ in size as a result of the uneven distribution of the cytoplasm during oogenesis. After fertilization, males develop from small cells, and only females develop from large cells.

The most common type of sex determination is sex determination at the time of fertilization. it syngamous sex determination. Found in mammals, birds, fish, etc.

Gender can be determined by early stages individual development individuals. it epigamous type of sex determination. For example, in the marine worm Bonelia viridis. The free-swimming larvae of this worm develop into females. If the larva remains attached to the mother, then a male develops from it. The larva that has begun to develop into a male is separated from the female, then the direction of sex differentiation into a female changes, and intersex develops from it - it has signs of a male and a female.

One example of a complete redefinition of sex was described in 1953 by the Japanese scientist T. Yamamoto. The experiment was carried out on white and red doctors, in which the dominant gene for red color is located on the Y chromosome. In this case, males will always be red, females will be white. Phenotypically red males were fed supplemented with female sex hormone. As a result, it turned out that all red fish with the male genotype are females with normal ovaries and female secondary sexual characteristics.

Gender redefinition may be a consequence mutations certain genes involved in sex differentiation. So, in Drosophila, a recessive gene was found in one of the autosomes tra , the presence of which in the homozygous state causes the development of female zygotes (XX) into phenotypic males that turn out to be sterile. XY males homozygous for this gene are fertile. Similar genes have been found in plants. For example, in corn, the recessive mutation silkless in the homozygous state causes sterility of the ovules, in connection with which the bisexual plant functions as a male. Sorghum has two dominant genes, the complementary interaction of which also causes female sterility.

Inheritance of sex-linked traits

genetic research showed that sex chromosomes are responsible not only for determining the sex of an organism, they, like autosomes, contain genes that control the development of certain traits.

Inheritance of traits whose genes are localized in X - or Y chromosomes is called sex-linked inheritance.

T. Morgan was engaged in the study of the inheritance of genes localized in the sex chromosomes. In Drosophila, red eyes dominate over white. 1) When red-eyed females were crossed with white-eyed males in the first generation, all offspring turned out to be red-eyed.

R: well. red-eyed X m. white-eyed

2) If hybrids of the first generation are crossed among themselves, then in the second generation all females turn out to be red-eyed, and in males splitting occurs - 50% white-eyed and 50% red-eyed.

R: well. red-eyed X m. red-eyed

F : f. red-eyed, 50% m. red-eyed, 50% m. white-eyed

3) If you cross white-eyed females and red-eyed males, then in the first generation all females turn out to be red-eyed, and males are white-eyed. In the second generation, half of the females and males are red-eyed, half are white-eyed.

R: well. white-eyed X m. red-eyed

F : f. red-eyed, m. white-eyed

What conclusions can be drawn?

The gene responsible for eye color in Drosophila is located on the X chromosome, while the Y chromosome does not contain such genes. Females receive one X chromosome from their father and one from their mother, while males receive an X chromosome only from their mother and a Y chromosome from their father. X and Y chromosomes are not homologous. Genes found on the X chromosome are absent on the Y chromosome. So in humans, there are 200 genes on the X chromosome that are not related to the development of sex: hemophilia, color blindness, muscular dystrophy, etc. If the genes responsible for the development of these traits are found in a male representative, then they will manifest themselves phenotypically, as they are presented in the genotype in a single variant. Such genes are called hemizygous. If the genes are localized on the Y chromosome and do not have alleles on the X chromosome, then the traits caused by them are transmitted from father to son. Such inheritance is hollandic. The following signs are attributed to hollandic: hypertrichosis, membranes between the toes.

Gender-limited traits

Traits that manifest differently in different sexes, or appear only in one sex, are gender-limited traits. These traits can be determined by genes located both on autosomes and on sex chromosomes. The possibility of developing a trait depends on the sex of the organism. For example, the timbre of a baritone or bass voice is typical only for men. The manifestation of sex-limited genes is associated with the realization of the genotype in the environment of the whole organism. In addition to the genes responsible for the development of secondary sexual characteristics, which normally work only in one of the sexes, they are present in the other, but are silent. The functional activity of other genes is determined by the hormonal activity of the organism. For example, bulls have genes that control milk production and its qualitative characteristics (fat content, protein content), but in bulls they are silent, and function only in cows. The potential ability of the bull to produce high-milk offspring makes it a valuable producer of a dairy herd.

Gender related traits

There are signs that are gender dependent. Genes whose degree of manifestation is determined by the level of sex hormones are called sex-dependent genes. These genes can be found not only on the sex chromosomes, but also on any autosomes. For example, the gene that determines baldness, typical for men, is localized in the autosome and its manifestation depends on male sex hormones. This gene is dominant in males and recessive in females. If women have this gene in a heterozygous state, then the trait does not appear. Even in the homozygous state, this trait is less pronounced in women than in men.

Chromosomal sex determination mechanism

Phenotypic differences between individuals of different sexes are due to the genotype. Genes are located on chromosomes. There are rules of individuality, constancy, pairing of chromosomes. The diploid set of chromosomes is called karyotype. There are 23 pairs (46) of chromosomes in the female and male karyotype (Fig. 78).

22 pairs of chromosomes are the same. They are called autosomes. 23rd pair of chromosomes - sex chromosomes. In the female karyotype, the same

Rice. 78. Karyotypes of various organisms. 1 - a person; 2 - mosquito; 3 - skerda plants.

sex chromosomes XX. In the male karyotype, the sex chromosomes are XY. The Y chromosome is very small and contains few genes. The combination of sex chromosomes in the zygote determines the sex of the future organism.

During the maturation of germ cells, as a result of meiosis, gametes receive a haploid set of chromosomes. Each egg has 22 autosomes + an X chromosome. The sex that produces gametes that are the same on the sex chromosome is called the homogametic sex. Half of the spermatozoa contain - 22 autosomes + X-chromosome, and half of 22 autosomes + Y. The sex that forms gametes that are different on the sex chromosome is called heterogametic. The sex of the unborn child is determined at the time of fertilization. If the egg is fertilized by a sperm with an X chromosome, a female organism develops, if the Y chromosome is male (Fig. 79).

Rice. 79. Chromosomal mechanism of sex formation.

The probability of having a boy or a girl is 1:1 or 50%:50%. This definition of sex is typical for humans and mammals. Some insects (grasshoppers and cockroaches) do not have a Y chromosome. Males have one X chromosome (X0), and females have two (XX). In bees, females have 2n sets of chromosomes (32 chromosomes), while males have n (16 chromosomes). Women have two sex X chromosomes in their somatic cells. One of them forms a lump of chromatin, which can be seen in interphase nuclei when treated with a reagent. This lump is the Barr body. Males do not have a Barr body because they only have one X chromosome. If during meiosis two XX chromosomes enter the egg at once and such an egg is fertilized by a sperm, then the zygote will have more chromosomes.

For example, an organism with a set of chromosomes XXX (trisomy on the X chromosome) phenotype is a girl. She has underdeveloped gonads. There are two Barr bodies in the nuclei of somatic cells.

An organism with a set of chromosomes XXY (Klinefelter syndrome) phenotype is a boy. His testicles are underdeveloped, physical and mental retardation. There is a Barr body.

Chromosomes XO (monosomy on the X chromosome)- determine Shereshevsky-Turner syndrome. An organism with such a set is a girl. She has underdeveloped gonads, small stature. No Barr body. An organism that does not have an X chromosome, but contains only a Y chromosome, is not viable.

The inheritance of traits whose genes are located on the X or Y chromosomes is called sex-linked inheritance. If the genes are on the sex chromosomes, they are inherited sex-linked.

A person has a gene on the X chromosome that determines the sign of blood clotting. The recessive gene causes the development of hemophilia. The X chromosome has a gene (recessive) that is responsible for the manifestation of color blindness. Women have two X chromosomes. recessive trait(hemophilia, color blindness) manifests itself only if the genes responsible for it are located on two X chromosomes: X h X h; X d X d . If one X chromosome has a dominant H or D gene, and the other has a recessive h or d gene, then there will be no hemophilia or color blindness. Men have one X chromosome. If it has the H or h gene, then these genes will definitely show their effect, because the Y chromosome does not carry these genes.

A woman can be homozygous or heterozygous for genes located on the X chromosome, but recessive genes appear only in the homozygous state.

If the mother is a carrier of the gene

If the genes are on the Y chromosome ( hollandic inheritance), then the signs conditioned by them are transmitted from father to son. For example, ear hairiness is inherited through the Y chromosome. Men have one X chromosome. All genes in it, including recessive ones, appear in the phenotype. In the heterogametic sex (male), most of the genes located on the X chromosome are located on hemizygous state, i.e., do not have an allelic pair.

The Y chromosome contains some genes that are homologous to the genes of the X chromosome, for example, genes for hemorrhagic diathesis, general color blindness, etc. These genes are inherited both through the X and through the Y chromosome.

Questions for self-control

1. What are the rules of chromosomes?
2. What is a karyotype?
3. How many autosomes does a person have?
4. Which chromosomes in humans are responsible for the development of sex?
5. What is the probability of having a boy or a girl?
6. How is sex determined in grasshoppers and cockroaches?
7. How is sex determined in bees?
8. How is sex determined in butterflies and birds?
9. What is a Barr body?
10. How can you determine the presence of a Barr body?
11. How can one explain the appearance of a larger or smaller number of chromosomes in the karyotype?
12. What is sex-linked inheritance?
13. What genes are sex-linked in humans?
14. How and why do sex-linked recessive genes show their effect in women?
15. How and why do X-linked recessive genes act in men?

Keywords of the topic "Chromosomal sex determination"

• autosomes
• butterflies
• probability
• ear hairiness
• gametes
• genotype
• heterogametic sex
• lump of chromatin
• homogametic sex
• color blindness
• girl
• action
• woman
• zygote
• individuality
• karyotype
• grasshoppers
• boy
• meiosis
• mammal
• moment
• monosomy
• the male
• kit
• insects
• inheritance
• carrier
• reagent treatment
• fertilization

• organism
• individual
• pairing
• sex cells
• offspring
• regulations
• sign
• birds
• bees
• development
• differences
• birth
• blood clotting
• testicles
• down syndrome
• Klinefelter syndrome
• Shershevsky-Turner syndrome
• blindness
• maturation
• condition
• combination
• spermatozoa
• cockroaches
• Barr body
• trisomy
• Y chromosome
• phenotype
• chromosome
• X chromosome
• human
• egg

Most animals are dioecious organisms. Sex can be considered as a set of features and structures that provide a way for the reproduction of offspring and the transmission of hereditary information. Sex is most often determined at the time of fertilization, that is, in sex determination leading role plays the karyotype of the zygote. The karyotype of each organism contains chromosomes that are the same for both sexes - autosomes, and chromosomes in which the female and male sexes differ from each other - sex chromosomes. In humans, the "female" sex chromosomes are two X chromosomes. During the formation of gametes, each egg receives one of the X chromosomes. The sex in which gametes of the same type are formed, carrying the X chromosome, is called homogametic. In humans, the female sex is homogametic. The "male" sex chromosomes in humans are the X chromosome and the Y chromosome. During the formation of gametes, half of the spermatozoa receive the X chromosome, the other half - the Y chromosome. The sex that produces gametes different type is called heterogametic. In humans, the male sex is heterogametic. If a zygote is formed that carries two X chromosomes, then a female organism will be formed from it, if an X chromosome and a Y chromosome - a male.

Animals have the following four types of chromosomal sex determination.

1. The female sex is homogametic (XX), the male is heterogametic (XY) (mammals, in particular, humans, Drosophila).

   Genetic scheme of chromosomal sex determination in humans:

   R	♀46,XX	×	♂46,XY
   Types of gametes	23 X		23, X 23, Y
   F	46,XX females, 50%		46 XY males, 50%

   Genetic scheme of chromosomal sex determination in Drosophila:

   R	♀8,XX	×	♂8,XY
   Types of gametes	4, X		4, X 4, Y
   F	8,XX females, 50%		8,XY males, 50%

2. The female sex is homogametic (XX), the male is heterogametic (X0) (Orthoptera).

   Genetic scheme of chromosomal sex determination in the Desert Locust:

   R	♀24,XX	×	♂23,X0
   Types of gametes	12 X		12, X 11, 0
   F	24,XX females, 50%		23,X0 males, 50%

3. The female sex is heterogametic (XY), the male is homogametic (XX) (birds, reptiles).

   Genetic scheme of chromosomal sex determination in a pigeon:

   R	♀80,XY	×	♂80,XX
   Types of gametes	40, X 40, Y		40X
   F	80 XY females, 50%		80,XX males, 50%

4. The female sex is heterogametic (X0), the male is homogametic (XX) (some insect species).

   Genetic scheme of chromosomal sex determination in moths:

   R	♀61,X0	×	♂62,XX
   Types of gametes	31, X 30, Y		31 X
   F	61,X0 females, 50%		62,XX males, 50%

Inheritance of sex-linked traits

It has been established that the sex chromosomes contain genes responsible not only for the development of sexual, but also for the formation of non-sexual characteristics (blood clotting, tooth enamel color, sensitivity to red and green, etc.). The inheritance of non-sexual traits whose genes are located on the X or Y chromosomes is called sex-linked inheritance.

T. Morgan was engaged in the study of the inheritance of genes localized in the sex chromosomes.

In Drosophila, red eyes dominate over white. Reciprocal crossing- two crossings, which are characterized by a mutually opposite combination of the analyzed trait and sex in the forms participating in this crossing. For example, if in the first crossing the female had dominant trait, and the male is recessive, then in the second crossing, the female should have a recessive trait, and the male should be dominant. Carrying out reciprocal crossing, T. Morgan received following results. When red-eyed females were crossed with white-eyed males in the first generation, all offspring turned out to be red-eyed. If F 1 hybrids are crossed among themselves, then in the second generation all females turn out to be red-eyed, and among males - half are white-eyed and half are red-eyed. If, however, white-eyed females and red-eyed males are crossed, then in the first generation all females turn out to be red-eyed, and males are white-eyed. In F 2, half of the females and males are red-eyed, half are white-eyed.

T. Morgan was able to explain the results of the observed splitting in eye color only by assuming that the gene responsible for eye color is localized on the X chromosome (X A - red eyes, X a - White color eye), and the Y chromosome does not contain such genes.

Scheme of human sex chromosomes and genes linked to them:
1 - X chromosome; 2 - Y-chromosome.

In humans, a male receives an X chromosome from his mother and a Y chromosome from his father. A woman receives one X chromosome from her mother and the other X chromosome from her father. X-chromosome - medium submetacentric, Y-chromosome - small acrocentric; X-chromosome and Y-chromosome have not only different sizes, structure, but also for the most part carry different sets of genes. Depending on the gene composition in the human sex chromosomes, the following sections can be distinguished: 1) a non-homologous section of the X chromosome (with genes found only on the X chromosome); 2) a homologous region of the X-chromosome and Y-chromosome (with genes present both in the X-chromosome and in the Y-chromosome); 3) a non-homologous section of the Y chromosome (with genes found only on the Y chromosome). Depending on the localization of the gene, in turn, the following types of inheritance are distinguished.

Most X-linked genes are not present on the Y chromosome, so these genes (even recessive ones) will show up phenotypically because they are present in the genotype in singular. Such genes are called hemizygous. The human X chromosome contains a number of genes whose recessive alleles determine the development of severe anomalies (hemophilia, color blindness, etc.). These anomalies are more common in men (because they are hemizygous), although the carrier of the genes that cause these anomalies is more often a woman. For example, if X A is normal blood clotting, X a is hemophilia, and if the woman is a carrier of the hemophilia gene, then phenotypically healthy parents may have a hemophilic son:

(for differences in sex determination between Drosophila and mammals, see "Balance theory of sex determination in Drosophila: difference from humans"). Sex is determined in the same way in humans: women have only X chromosomes, and men have X and Y chromosomes. As far as plants are concerned, back in 1919 Allen discovered X chromosomes in female plants and a set of XY chromosomes in male plants in liver moss. In 1921, the same chromosomes were found in elodea, then they were found in hemp, spinach and other plants. The sex of most dioecious plants is also determined.

In other organisms, sex determination is carried out differently. For example, females of most butterflies and grasshoppers have two X chromosomes, while males have only one. The genotype of females on sex chromosomes is XX, and males are X0 (zero means that the second sex chromosome is absent). Thus, in butterflies, males also have a heterogametic sex, while they have one less chromosome than females (Fig. 109). This same type of sex determination is found in some centipedes, spiders, and nematodes.

The original way to determine sex is in bees, ants and some other Hymenoptera - they lack sex chromosomes. The queen bee mates only once in her life during her nuptial flight. Sperm is stored in her special seminal receptacles. When the next ripe egg passes by the seed receptacle, its lumen opens slightly and the egg is fertilized. However, the uterus may not open the opening of the seed receptacle. Then the egg will remain unfertilized. Male bees develop from such unfertilized eggs - drones, therefore, drones do not have fathers, they receive all their chromosomes from their mother. Thus, some zygotes contain diploid sets of chromosomes (in bees - 32), females develop from them, and others - haploid sets of chromosomes (in bees - 16), drones develop from them (Fig. 109). During the formation of spermatozoa, meiosis does not occur in drones. True, diploidy is restored in the somatic cells of drones, but at the same time such cells are homozygous for all genes. (Because of this, harmful recessive alleles appear in drones. Drones with such genes die or do not leave offspring, due to which the harmful recessive alleles are removed from the population).

Exercise 1. Let the genotype of the female bee for some gene aa, and the genotype of the drone A. Determine the genotypes in F1, given that they are different for different sexes.

Exercise 2. Consider the same problem with other genotypes of the parental generation.

In some animals, such as crocodiles, neither sex chromosomes nor sex-linked traits have been found at all. The sex of crocodiles that hatch from eggs depends on the temperature at which the eggs develop: the higher this temperature, the more females are hatched.

Phenotypic differences between individuals of different sexes are due to the genotype. Genes are located on chromosomes. A diploid set of chromosomes is called a karyotype. There are 23 pairs (46) of chromosomes in the female and male karyotype. 22 pairs of chromosomes are the same, they are called autosomes. The 23rd pair of chromosomes are the sex chromosomes. In the female karyotype, the same sex chromosomes - XX. AT male body sex chromosomes - XY. The Y chromosome is small and contains few genes. Sex is inherited as a Mendelian trait. The combination of sex chromosomes in the zygote determines the sex of the future organism. During the maturation of germ cells, as a result of meiosis, gametes receive a haploid set of chromosomes. Each egg has 22 autosomes + an X chromosome. An organism that produces gametes that are the same on the sex chromosome is called homogametic.

Spermatozoa produce two types of gametes: half contains 22 autosomes + X sex chromosome, and half contains 22 autosomes + Y sex chromosome. An organism that produces different gametes is called heterogametic. The sex of the unborn child is determined at the time of fertilization and depends on which sperm will fertilize this egg. If the egg is fertilized by a sperm having the X chromosome, a female organism develops, if the Y chromosome - male. Theoretically, the probability of having a boy and a girl is 1:1 or 50%:50%. However, more boys are born, but because the male body has only one X chromosome, and all genes (dominant and recessive) show their effect, then the male body is less viable.

This definition of sex is typical for humans and mammals.

Some insects (grasshoppers, cockroaches) do not have a Y chromosome. Males have one X chromosome and females have two XXs. In bees, females have a 2n set of chromosomes (32 chromosomes), and males have n (16) chromosomes. Females develop from fertilized eggs, and males from unfertilized ones. In birds and butterflies, females are heterogametic and have ZW sex chromosomes, while males are homogametic and have ZZ sex chromosomes.

In some organisms, sex depends on environmental factors. For example, in the marine worm bonnelia, the larvae are asexual. If the larva falls on the female's oral lobe, microscopic males develop from it, and vice versa, females form from the larva if it has not been in contact with the female.

In women, in somatic cells, in addition to autosomes, there are two sex XX chromosomes. One of them is revealed, forming a lump of chromatin, noticeable in interphase nuclei when treated with dyes. This is X-chromatin or Barr body. This chromosome is coiled and inactive. The second chromosome remains active. In the cells of male and female organisms contains one active X chromosome.

The Barr body in the cells of men is not detected. If during meiosis there is no divergence of chromosomes, then two XX chromosomes will fall into one egg. When such an egg is fertilized by a sperm, the zygote will have a greater number of chromosomes. Cells with more than two X chromosomes have more Barr bodies because only one X chromosome is active at a time.

For example, XXX (trisomy on the X chromosome) is a girl according to the phenotype. She has two Barr bodies in the nuclei of somatic cells (symptoms in question 27).

XXY - Klinefelter's syndrome - phenotype is a boy. He has a Barr body (symptoms in question 27).

XO - monosomy on the X chromosome - Shereshevsky-Turner syndrome. This is a girl, Barr's body is missing (symptoms in question 27).

YO -- not viable.

Traits whose genes are located on the sex chromosomes are inherited in a sex-linked manner. The inheritance of traits whose genes are located on the X and Y chromosomes is called sex-linked inheritance. The distribution of genes in the offspring should correspond to the distribution of sex chromosomes in meiosis and their combination during fertilization.

The Y-chromosome contains genes that determine the development of the male sex, which are necessary for the differentiation of the testes. There are no such genes on the X chromosome, but there are many other genes. The Y chromosome is very small and does not contain many of the genes found on the X chromosome.

In the heterogametic sex (male), most of the genes located on the X chromosome are in the hemizygous state, i.e. do not have an allelic pair. In male organisms, any recessive gene located in one of the non-homologous regions of the X-chromosome manifests itself in the phenotype.

The Y-chromosome contains a number of genes homologous to the genes of the X-chromosome, for example, genes for hemorrhagic diathesis, general color blindness, etc.

In humans, recessive sex-linked traits are known, such as hemophilia, color blindness, muscular dystrophy, etc.

Women have two XX chromosomes. A recessive trait appears if the genes responsible for it are located on two X chromosomes. If the organism is heterozygous for these genes, then the trait will not appear. The male body has one X chromosome. If it contains the H or h gene, then these genes will definitely show their effect, because the Y chromosome does not carry these genes.

A woman can be homozygous or heterozygous for genes located on the X chromosome, but recessive genes appear only in the homozygous state.

If the genes are on the Y-chromosome (Holandric inheritance), then the traits they cause are passed on from father to son. For example, ear hairiness is inherited this way. The Y chromosome in humans controls testicular differentiation. Men have one X chromosome. All genes in it, including recessive ones, appear in the phenotype. This is one of the reasons increased mortality males compared to females.

The signs, the manifestation of which is different in representatives of different sexes, or these signs manifested in the same sex, are called sex-limited.

These signs can be determined by genes located both in autosomes and sex chromosomes, but the possibility of their development depends on the sex of the organism. For example, baritone and bass voice timbres are typical only for men.

The manifestation of sex-limited traits is associated with the realization of the genotype in the environment of the whole organism. The genes responsible for the development of secondary sexual characteristics normally work only in one of the sexes, while in the other they are present, but "silent". functional activity a number of genes determines the hormonal activity of the body. For example, bulls have genes that control milk production and its qualitative characteristics (fat content, protein content, etc.), but in bulls they are "silent" and function only in cows. The potential ability of the bull to produce high-milk offspring makes it a valuable producer of a dairy herd.

Genes whose degree of manifestation is determined by the level of sex hormones are called sex-dependent genes. These genes can be found not only on the sex chromosomes, but also on any autosomes.

For example, the gene that determines male pattern baldness is localized in the autosome, and its manifestation depends on male sex hormones. This gene is dominant in males and recessive in females. If women have this gene in a heterozygous state, then the trait does not appear. Even in the homozygous state, this trait is less pronounced in women than in men.