Chromosomes that are the same in both sexes are called autosomes.


“The genetics of sex. Sex-linked inheritance.

People have long been interested in what determines the sex of a person or animal. For example, Aristotle argued that sex depends on the wind blowing at the time of conception. If the north wind blows, then a male will be born, and if the south – a female.

Mendel argued that sex is hereditarily determined. This statement was prompted by the results of splitting by sex 1: 1. Obviously, one is homogametic and the other is heterogametic.

As you know, most animals and dioecious plants are dioecious organisms, and within the species the number of males is approximately equal to the number of females.

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.

Chromosomes that are the same in both sexes are called autosomes.

Chromosomes, in which the chromosome sets of male and female organisms differ, are called sex chromosomes. They are labeled X and Y.

In humans, the female sex is homogametic , meaning all eggs carry an X chromosome .

The male body is heterogametic , that is, it forms two types of gametes – 50% of the gametes carry the X chromosome and 50% carry the Y chromosome .

If during fertilization a zygote is formed that carries two X chromosomes, then a female organism will be formed from it, if the X chromosome and Y chromosome are male.

Since the female body has two identical sex chromosomes, it can be considered as homogametic , male, forming two types of gametes – as heterogametic .

The sex of Drosophila is also determined, which has 3 pairs of autosomes and one pair of sex chromosomes. Female gender – homogametic, sex chromosomes XX; male – heterogametic, XY sex chromosomes.

Sex, like in humans, is determined at the time of the fusion of gametes.

There are four main types

chromosomal sex determination:

  1. The male sex is heterogametic; 50% of gametes carry the X-chromosome, 50% – the Y-chromosome;
  2. The male sex is heterogametic; 50% of gametes carry X-, 50% do not have a sex chromosome;
  3. The female sex is heterogametic; 50% of gametes carry the X-chromosome, 50% – the Y-chromosome;
  4. The female sex is heterogametic; 50% of gametes carry X-, 50% do not have a sex chromosome.

So, in mammals and dipterous insects, the male sex (XY) is heterogametic and the female sex (XX) is homogametic.

In birds, reptiles and tailed amphibians, female individuals have XY sex chromosomes, while males have XX.

In butterflies, the female sex is heterogametic – the sex chromosomes XY or X0.

What are the sex chromosomes in the gametes of a rooster and a hen?

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 – one gene per zygote .

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 (because they are hemizygous for these genes), although the carrier of these anomalies is more often a woman.

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. The mother passes on only the X chromosome to both her son and daughter.

From whom can a son inherit color blindness? Hemophilia? Hypertrichosis?

What hemizygous genes are shown in the figure? What genes are called hemizygous?


What children can be from the marriage of a healthy man and a woman, phenotypically healthy, carrier of the hemophilia gene?

Recording the genetic schema.

It is known that “tricolor” cats are always females. This is due to the fact that the genes for black and red coat are allelic and are located on the X chromosome, but none of them dominates, and when red and black are combined, “tricolor” individuals are formed.

1. What offspring is expected from crossing a red cat and a black cat?

  1. What offspring is expected from a black cat and a tricolor cat?
  2. From a black cat and a red cat?
  3. Why are there no tricolor cats?


When crossing a chicken with a striped plumage color with a black rooster, all the hens are black in the offspring, and the roosters are with a striped plumage color.

What are the genotypes of parents and offspring?

What offspring is expected from crossing hybrids with each other?

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