After the discovery of the testis-determining gene SRYmany scientists shifted to the theory that the genetic mechanism that causes a fetus to develop into a male form was initiated by the SRY gene, which was thought to be responsible for the production of testosterone and its overall effects on body and brain development. Hormone levels in the male parent affect the sex ratio of sperm in humans. Hidden categories: Once viewed simply as an impediment to fertilizationrecent research indicates the zona pellucida may instead function as a sophisticated biological security system that chemically controls the entry of the sperm into the egg and protects the fertilized egg from additional sperm.
Hello! I am Mae. I have 24 yo. I love dirty talks. My favorit is Swapping, Facial, Deepthroat
Take me now!
Aristotle claimed that the male and was the driver behind sex determination,  such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female. The first clues to the existence the a factor that determines the development of testis in mammals came from experiments carried out by Alfred Jost who castrated embryonic chromosomes in utero and noticed that they all developed as female. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located determine sex the Y chromosome that code for maleness.
XY sex-determination system
SRY is a sex-determining gene on the Y chromosome in the therians placental mammals and marsupials. Philosophical Transactions of the Royal Society B: Not all male-specific genes are located on the Y chromosome. In these species, one or more genes are present on their Y chromosome that determine maleness. Once viewed simply as an impediment to fertilizationrecent research indicates the zona pellucida may instead function as lesbians munching pussy sophisticated biological security system that chemically controls the entry of the sperm into the egg and protects the fertilized egg from additional sperm.