Like most other male mammals, a man's genome usually inherits an X chromosome from the mother and a Y chromosome from the father. Sex differentiation of the male fetus is governed by the SRY gene on the Y chromosome. During puberty, hormones which stimulate androgen production result in the development of secondary sexual characteristics that result in even more differences between the sexes. These include greater muscle mass, greater height, the growth of facial hair and a lower body fat composition. Male anatomy is distinguished from female anatomy by the male reproductive system, which includes the testicles, sperm ducts, prostate gland and epididymides, and penis. Secondary sex characteristics include a narrower pelvis and hips, and smaller breasts and nipples.