Chromosomal sex determination

The theory of sex inheritance was developed at the beginning of the 20th century by T. Morgan.

Sex is a set of signs and properties of an organism that ensure the reproduction of their own kind and the transfer of hereditary information to the next generation.
Sex is hereditarily programmed and is usually determined by one pair of chromosomes called sex chromosomes. Chromosomes that are the same in males and females are called autosomes.

Sex chromosomes are chromosomes that differ between males and females.

Usually sex chromosomes are designated in genetic records by the letters X and Y. These chromosomes contain genes that determine the synthesis of proteins that regulate the work of the sex glands (female or male) and determine the sex characteristics of the body.

When gametes are formed, one sex chromosome gets into them.

Sex that forms the same gametes is called homogametic.

The sex that forms different gametes is called heterogametic.
The sex of the future organism is usually determined at the time of fertilization.

Several types of sex determination are known.

I. XY type
There are many animal species in which the female sex is homogametic and the male is heterogametic. The female forms one type of gametes, and the male forms two. The sex of the future organism depends on which of the male gametes is involved in fertilization.

This is how the sex is determined in Drosophila, humans, mammals.

Example: Drosophila has 6 autosomes and two sex chromosomes in the chromosome set. The female chromosome set is 6A + XX, the male chromosome set is 6A + XY.

The female forms one type of gamete with the chromosome set 3A + X, and the male forms two types of gametes: 3A + X and 3A + Y.

Example: there are 44 autosomes and two sex chromosomes in a person’s chromosome set: a woman has 44A + XX, a man has 44A + XY.

All female gametes (eggs) have the same set of chromosomes. They contain 22 autosomes and one X chromosome (22A + X). The male body forms two types of gametes (sperm): 22A + X and 22A + Y.

Another option is heterogametic female, homogametic male. This type of sex determination is observed in birds and butterflies. In this case, the sex of the future organism is determined by the female gametes.

II. Type X0
The homogametic sex has two X chromosomes and a diploid set of chromosomes in the cell, while the heterogametic sex has only one X chromosome and an unpaired chromosome set.

Example: females of some species of bedbugs have 14 chromosomes in their cells (12A + XX), and males have 13 chromosomes (12A + X0). The female forms gametes 6A + X, and the male forms 6A + X and 6A + 0.

III. Diploid female, haploid male
Bees and ants do not have sex chromosomes. Females have a diploid set of chromosomes, while males have a haploid set.

IV. The sex of the body depends on environmental conditions
In some animals, the sex is determined by environmental conditions. So, in some species of crocodiles and turtles, the sex depends on the temperature at which the embryo develops in the egg: at high temperatures, more females appear, at low temperatures, more males.