Which statement correctly describes the sex chromosome makeup of a human male?

• A. It is XX
• B. It is XY
• C. It is XO
• D. It is YY

Answer: (B)

In humans, sex is determined by the combination of sex chromosomes. A male has one X and one Y chromosome. In reproduction, the mother always contributes an X, and the father can contribute either X or Y. If the father contributes a Y, the zygote is XY and develops as male. The Y chromosome also carries the SRY gene, which initiates the development of male characteristics. So the male’s sex chromosome makeup is XY. By contrast, XX would be female, XO is Turner syndrome, and YY is not viable in humans.