Inheritance of sex-linked traits.

Chromosomal sex determination.

Most animals and dioecious plants are dioecious organisms, and within a species the number of males is approximately equal to the number of females.

Sex can be considered as one of the signs of the organism. Inheritance of traits of an organism is usually determined by genes. The mechanism of sex determination is of a different nature – chromosomal.

Sex is most often determined at the time of fertilization. Drosophila has only 8 chromosomes: 6 autosomes and 2 sex chromosomes, the chromosome set of the male is 8,XY, the female is 8,XX. In this case, the female sex is homogametic, that is, all eggs carry the X chromosome. The male sex is heterogametic, that is, it forms two types of gametes – 50% of the gametes carry the X chromosome and 50% carry the Y chromosome.

A person has 44 autosomes and 2 sex chromosomes, the chromosome set of a man is 46,XY, of a woman – 46,XX. If a zygote is formed that carries two X chromosomes, then a female organism will be formed from it, if X and Y chromosomes are found in the zygote, a male one (Fig. 4). Since the female body has two identical sex chromosomes, it can be considered as homozygous, male, forming two types of gametes – as heterozygous.

There are four main types of chromosomal sex determination:

The male sex is heterogametic; 50% of gametes carry the X-chromosome, 50% – the Y-chromosome (mammals, Diptera, beetles, bugs).

The male sex is heterogametic; 50% of gametes carry X-, 50% do not have a sex chromosome (grasshoppers).

The female sex is heterogametic; 50% of the gametes carry the X-chromosome, 50% – the Y-chromosome (birds, reptiles, tailed amphibians, silkworms).

The female sex is heterogametic; 50% of gametes carry X-, 50% do not have a sex chromosome (mol).

Inheritance of sex-linked traits.

Scheme of sex chromosomes and genes linked to them.

Genetic studies have established 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.

The inheritance of traits whose genes are located on the X or Y chromosomes is called sex-linked inheritance. The study of the inheritance of genes localized in the sex chromosomes was carried out by T.H. Morgan. In Drosophila, red eyes dominate over white. When red-eyed females were crossed with white-eyed males, in the first generation, all offspring turned out to be red-eyed. If you cross F 1 hybrids with each other, then in the second generation all females turn out to be red-eyed, and males split – 50% white-eyed and 50% 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.H. Morgan could 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, while the Y chromosome does not contain such genes.

Thus, thanks to the crosses carried out, a very important conclusion was made: the eye color gene is sex-linked, that is, it is located on the X chromosome.

In humans, the male receives the X chromosome from the mother. Human sex chromosomes have small homologous regions that carry the same genes (for example, the gene for general color blindness), these are conjugation regions. But most of the genes linked to the X chromosome are absent from the Y chromosome, so these genes (even recessive ones) will appear phenotypically, since they are represented in the genotype in the 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). These anomalies are more common in men, although women are more likely to be carriers. In men, these genes are hemizygous, their recessive alleles cause diseases: X h Y – a man with hemophilia; X d Y – colorblind.

In most organisms, only the X chromosome is genetically active, while the Y chromosome is practically inert, since it does not contain genes that determine the characteristics of the organism. In humans, only some genes that are not vital are localized on the Y chromosome (for example, hypertrichosis – increased hairiness of the auricle). Genes located on the Y chromosome are inherited in a special way – only from father to son.

As an example of the recording of genes located on the X chromosome, consider the inheritance of color in cats. The black color is determined by the allele of gene B, located on the X chromosome – X B X B , red – by the allele b – X b X b . If alleles B and b – X B X b are found, then the color of the cat’s coat will be tortoiseshell. The genotype of a black cat is X B Y, a red cat is X b Y. A tricolor cat can only be with Klinefelter’s syndrome, with trisomy on the X chromosome – X B X b Y, in which case the cat will be infertile.

Written work with cards for 10 minutes:

1. Task . From a black cat, one tortoiseshell and several black kittens were born. Determine: a) the phenotype and genotype of the father; b) sex of tortoiseshell and black kittens.

2. Task . In what case can a color-blind man have a color-blind grandson? It is known that the gene responsible for color blindness is recessive and is located on the X chromosome (X d ).

3. In what case can a girl with color blindness be born? It is known that the gene responsible for color blindness is recessive and is located on the X chromosome (X d ).

4. Give definitions or reveal the essence of the terms: autosomes, sex chromosomes, male heterogamety, sex-linked inheritance, hemophilia, hemizygous genes.

Work with the card at the board:

1. How many autosomes are in the human genome?

2. How many autosomes are in the human genotype?

3. What groups of organisms have homogametic male organisms?

4. Where is the gene that causes color blindness located in a person?

5. Where is the gene that causes hemophilia located in a person?

6. The mother is a carrier of the gene for color blindness, the father distinguishes colors normally. Which of the children can be color blind?

7. To whom are the genes on the Y chromosome transmitted?

8. What is the genotype of a red cat and a black cat?

9. What is the genotype of a tricolor (tortoiseshell) cat?

10. What is the genotype of a girl with color blindness?

Test:

Test 8. Sex chromosomes: 1. Sex chromosomes X and Y are completely homologous to each other. 2. Sex chromosomes X and Y are homologous to each other in a small area. 3. Do not have homologous regions at all. Test 9. Genotypes of a red cat and a black cat: 1. The cat has X B Y, the cat has X B X B. 2. Cat X b Y, cat X B X b . 3. Cat X b Y, cat X b X b . 4. Cat X b Y, cat X B X B. Test 10. The genotype of a tricolor (tortoiseshell) cat: 1. X B X B. 2. X B X b . 3. X b X b .

Test 1 . In the human genome:

1. 22 autosomes.

2. 23 autosomes.

3. 44 autosomes.

4. 46 autosomes.

Test 2 . In the human genotype:

1. 22 autosomes.

2. 23 autosomes.

3. 44 autosomes.

4. 46 autosomes.

** Test 3. Male organisms are homogametic:

1. In birds.

2. In reptiles.

3. Diptera.

4. In mammals.

Test 4. The gene that causes color blindness:

1. In the X chromosome.

2. In the Y-chromosome.

3. In 1 pair of autosomes.

4. In the 18th pair of autosomes.

Test 5. The gene that causes hemophilia:

1. In the X chromosome.

2. In the Y-chromosome.

3. In 1 pair of autosomes.

4. In the 18th pair of autosomes.

Test 6. The mother is a carrier of the gene for color blindness, the father distinguishes colors normally. Color blindness can be:

1. All sons.

2. All daughters.

3. Half of the daughters.

4. Half of the sons.

Test 7. Genes located on the Y chromosome are transmitted:

1. From father to sons.

2. From father to daughters.

3. From the father to all children.

4. From mother to sons.

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