The human genome is made up of forty-six chromosomes, the rod-like structures that reside in the nucleus of every cell. These chromosomes carry all of our genes, which, in turn, are made of DNA. Two of these chromosomes, called the “X” and the “Y,” are different from the rest: they are “sex chromosomes.“ Men carry one X and one Y chromosome, while women carry two X chromosomes. All the obvious physical differences between the sexes ultimately spring from this humble difference in chromosomal constitution.