Chromosome X

The so-called sex chromosomes differentiate the sexes: females are XX and males XY, which is the basis for the development of a fetus into a girl or a boy (Figure 1). All other chromosomes (called autosomes) are present in two copies in both males and females. It is the presence of a Y chromosome that determines the male sex of a baby, because the Y carries a gene that induces undifferentiated gonads to turn into testes in the fetus. The number of X chromosomes does not change the sex of a baby. Indeed, people with a single X chromosome and no Y chromosome are females with Turner syndrome, a rare genetic disorder characterized by short stature and infertility. Conversely, people who have two X chromosomes and a Y chromosome are males with Klinefelter's syndrome, which includes tall stature and infertility.