A team of scientists have located a gene mutation that appears to be the link to what causes intersex conditions. The researchers who discovered this have reported their finding in the American Journal of Human Genetics this month. The genetic mutation seems to act as an on/off switch.
About one in 1,000 people are born with an intersex condition. Most of us realize that what usually determines gender has to do with the presence or absence of X or Y chromosomes. If a fetus has one X chromosome, and one Y chromosome, it will develop male sexual organs and secondary sexual characteristics. If the fetus has two X chromosomes, it will develop female sexual organs and secondary sexual characteristics.
The current research focuses on various mutations that appear on a gene called MAP3K1. It appears that this gene acts as an on/off switch, which either encourages or discourages an intersex condition from occurring. Researchers found that a person who was born with one X chromosome and one Y chromosome, whose MAP3K1 has been switched on will develop as a female, despite having male chromosomes. This person wouldn't have the ability to menstruate, and may have ovaries that are underdeveloped. The person might also have an overdeveloped clitoris, excessive hair, and the potential to develop ovarian tumors. This person may also be infertile.
Conversely, if a person was born with two X chromosomes, and had a MAP3K1 gene that was switched on, this person would develop as a male, despite having the chromosomes that typically would cause the person to develop as a female. This person would have some physical traits that usually would be found on a female, such as a urethral opening on the underside of the penis, or an underdeveloped or very small penis.
In short, the discovery of the function of the mutated MAP3K1 gene may provide an answer as to why many individuals have an intersex condition, instead of developing in a more typical way. It also goes against the idea that gender is entirely determined by the presence of two X chromosomes, or by the presence of one X chromosome and one Y chromosome.