Sex determination is the sex of men and women determined by different combinations of sex chromosomes. In human cells, there are 46 chromosomes, or 23 pairs. Among them, 22 pairs are male and female, called autosomes, and one pair is different between males and females, called sex chromosomes. In 1905, American cytoologist Wilson and others discovered that the sex chromosome of women is xx, and the sex chromosome of men is xY. After female oocytes undergo meiosis, there are only sex chromosome eggs of type x; while male spermatocytes undergo meiosis, they produce two types of sperm: one type contains sperm of x sex chromosome and another type contains sperm of the Y sex chromosome. During fertilization, if the sperm containing the x sex chromosome is combined with the egg, it will become a fertilized egg of the xx karyotype, which will develop into a female in the future; It will develop into a male in the future. It can be seen that human sex is determined by different combinations of sex chromosomes at the moment of fertilization.
Figure 1. Some chromosomal sex determination systems.
Sex determination type
Most biological cells have a pair of homologous chromosomes that are often different in shape from each other. This pair of chromosomes is directly related to gender determination and is called sex chromosome. Chromosomes other than sex chromosome are collectively called autosome.
1. XY sex determination
All male individuals have two heterotypic sex chromosomes, and female individuals have two types of identical sex chromosomes, called XY type. In this type of organism, the females are of the same sex, that is, the somatic cells contain two identical sex chromosomes, denoted XX; the male somatic cells contain two atypical sex chromosomes, one of which is the same as the female X chromosome, Let's call it X, and the other chromosome is called Y, so the somatic cells contain two sex chromosomes, XY. XY-type sex determinations account for the vast majority of animals. All mammals, most reptiles, amphibians, and dioecious plants belong to the XY type sex determination. Among the plants are female louisiana, spinach, and hemp. X and Y chromosomes play different roles in mammalian sex determination. On the short arm of the Y chromosome, there is a "testis-determining" gene, which has a strong role in determining "men"; the X chromosome has almost no effect. As long as there is Y in the zygote, it develops into a male; only the X chromosome (XO) develops into a female.
Figure 2. Drosophila sex-chromosomes.
ZW sex determination
Where female individuals have two heterotypic sex chromosomes, male individuals have two types of identical sex chromosomes, known as the ZW type. In this species, males are of the same sex. That is, the female sex chromosome composition is ZW, and the male sex chromosome composition is ZZ. The sex determination of birds, lepidopteran insects, certain amphibians and reptiles belongs to this type. For example, domestic chicken and silkworm.
XO sex determination
The sex determination of orthoptera insects such as locusts and tadpoles and a few animals such as cockroaches belongs to the XO type. Females are of the same sex, somatic cells contain two X chromosomes; males of the same sex, but only contain one X chromosome. For example, female locusts have 24 chromosomes (22+XX); male locusts have 23 chromosomes (22 + X). During meiosis, females produce only one type of X egg; males can produce two types of sperm with X and no X chromosomes, with a sex ratio of 1: 1.
Figure 3. Heredity of sex chromosomes in XO sex determination.
ZO type sex determination
A few individuals in Lepidoptera, ZZ males and ZO females are called ZO type sex determination. In this type, females produce type 2 gametes, and males produce single type gametes with a sex ratio of 1: 1.
Haploids are found in insects belonging to the order Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, ie males. Diploid individuals are usually female, but may be infertile males. Men cannot have sons or fathers. If a queen bee mate with a drone, her daughter shares ¾ of the gene with each other, not ½ in the XY and ZW systems. This may be important for the development of good deeds, as it increases the importance of kin selection, but is still controversial. Most females in the Hymenoptera can determine the sex of their offspring by placing sperm retained in the sperm into the fallopian tubes. This allowed them to create more workers based on the conditions of the colonies.
The sex of mammals depends mainly on the composition of the sex chromosomes in the body, and the environment has little effect on sex determination. However, in lower animals, such as amphibians and reptiles, the sex determination is related to the changes in the environment, in addition to the sex chromosome composition. For lower vertebrates such as frogs, even if the sex chromosome composition is XY, they will develop into female frogs in a higher temperature environment, and in lower temperature environments, they will develop into male frogs even if the sex chromosome composition is XX. In other words, the sex determination of lower vertebrate chromosomes is not very strong.
1. Bull, J. J. Sex Determination in Reptiles. Cellular and Molecular Life Sciences., 1980, 55 (1): 3-21.