Like most other male mammals, a man's genome usually inherits an Xchromosome from the mother and a Ychromosome from the father. The male fetus produces larger amounts of androgens and smaller amounts of estrogens than a female fetus. This difference in the relative amounts of these sex steroids is responsible for the physiological differences that distinguish men from women. During puberty, hormones which stimulate androgen production result in the development of secondary sexual characteristics, thus exhibiting greater differences between the sexes. These include greater muscle mass, the growth of facial hair and a lower body fat composition.