Sex, Hermaphroditism

The organisms of the vast majority of species differ from each other by belonging to the male or female sex. This division is associated with the specific participation of organisms of different sexes in the process of reproduction.

Male and female organisms produce different, specialized male and female reproductive cells (sperm and eggs), the fusion of which during the process of fertilization gives rise to a new organism. External manifestations of sex characteristics, such as body shape and size, color (in animals), the nature of hair growth, structural features of the external genitalia and vocal apparatus, psychophysiological characteristics, are called secondary sexual characteristics, and the very manifestation of external sex characteristics is called sexual dimorphism.

Cytogenetic studies (cytology - the science of cell structure, genetics - the science of heredity and variability) established the genetic basis for the division of the sexes. It was shown that the division of sexes is of a chromosomal nature and that the set of chromosomes in the cells of individuals of different sexes is different.

In addition to the chromosomes that are the same for both sexes (they are called autosomes), there are chromosomes that are characteristic only of the cells of the female or male body. Such chromosomes (X- and Y-chromosomes) are called sex chromosomes.

All of a woman's eggs carry one X chromosome, while in men, half of the sperm contain an X chromosome and half have a Y chromosome. If during the process of fertilization the egg meets a sperm carrying an X chromosome, the new organism will be female, but if it meets a sperm carrying a Y chromosome, then it will be male. The presence of a Y chromosome in the cells of the embryo reliably indicates that the new organism will be male.

This does not exclude the possibility that genes located on autosomes play a significant role in the formation of sex. Consequently, the formation of sex is associated with a certain balance of genes. The inheritance of a number of characteristics is associated with sex chromosomes and their “behavior” during the maturation of germ cells.

Non-sex (somatic) cells of male and female organisms can be distinguished not only by the set of chromosomes, but also by the presence or absence in their nuclei of a special substance called sex chromatin. In the nuclei of cells of the male body, in 90-95% of cases, sex chromatin is not detected. Women have one sex chromatin body. This difference serves as a reliable indicator of whether an individual is male or female and is widely used in medical and forensic practice.

At the beginning of embryonic development, the sex glands (gonads) are neutral, that is, they do not show any signs by which one could decide whether a given gonad will develop into a testis or an ovary. The early gonad consists of two layers: cortical and medulla. During development, in embryos with a Y chromosome (a male set of XY sex chromosomes), the cortical layer degenerates, and the testes develop from the medulla; in females (XX chromosomes), the medulla layer of the embryonic gonad degenerates, and the ovaries develop from the cortical layer.

As soon as the testes or ovaries are formed, they, with the help of the hormones they produce, begin to control the sexual differentiation of the developing organism. Any change in external conditions that affects the production of sex hormones or the response of developing tissues to these hormones can cause a change in the expression of sex in the mature organism. Therefore, such organisms belonging to one sex can sometimes have characteristics of the opposite sex, that is, they are intersex. This phenomenon is called hermaphroditism.

In the vast majority of animals and humans, the ratio of individuals of different sexes at birth is approximately equal. This ratio (1:1) is typical only for newborns. Subsequently, the sex ratio changes significantly.