In humans, it is normal for females to have two X chromosomes. For males, it is normal to have one X chromosome and one Y chromosome. A human fetus starts as female so the Y chromosome is what makes the changes necessary to switch to male. Androgen hormones control this process. These androgens are testosterone and dihydrotestosterone (DHT). The testicles in the fetus are responsible for the existence of androgens. The androgens will work together to make the sex difference of the fetus. This results in differences between males and females. Some scientists change androgen levels in mammals during fetus and early life as experiments.