In many plants and animals with distinct sexes, the genes responsible for sex determination may be located on a special chromosome pair: sex chromosomes (or allosomes). All other chromosomes are known as autosomes. A human carries 46 chromosomes; 22 pairs of autosomes plus one pair of sex chromosomes. In females, the sex chromosome pair always consists of two X chromosomes, while in males, it consists of an X chromosome and a smaller Y chromosome. When eggs are formed through a type of cell division called meiosis, they always contain one X chromosome; when sperm are formed, they could either carry one X chromosome or one Y chromosome. Therefore, if a sperm carrying the X chromosome fertilizes the egg, the result is female offspring; or, if a sperm carrying the Y chromosome fertilizes the egg, the result is male offspring. In humans, the X chromosome not only carries the...