When does the genetic determination of sex in humans occur and what is the reason for this?

In the process of spermatogenesis in a male Drosophila, two types of gametes will be equally likely to form, containing X and Y chromosomes, i.e., the male sex in Drosophila is heterogametic (from the Greek. Heteros – different, different). During fertilization, if the egg merges with a sperm containing the X chromosome, a pair of sex chromosomes XX is formed and a female is formed. If the sperm contained the Y chromosome, the male will develop. The sex of the future individual is determined at the time of fertilization and depends on the set of sex chromosomes. By the same mechanism, gender is determined in humans. The sex chromosomes in a woman are the same, they are called X chromosomes, in men there is one X chromosome and one Y chromosome. The remaining 22 pairs of chromosomes are the same in men and women; these are autosomes. Consequently, the sex of the baby depends on which sperm fertilizes the egg. Initially, the human embryo is bisexual, but the presence of the Y chromosome directs the development of the still undifferentiated genital organs of the fetus in the male type, turning them into testes. A special gene is located in the Y chromosome that causes this specialization at the 4-8th week of the embryonic period. In the absence of the Y chromosome, the development of the embryo proceeds according to the female type.