Sex-specific manifestations of disease are most often attributed to differences in the
reproductive apparatus or in life experiences. However, a good deal of sex differences in
health issues have their origins in the genes on the sex chromosomes themselves and in X
inactivation—the developmental program that equalizes their expression in males and
females. Most females are mosaics, having a mixture of cells expressing either their mother's
or father's X-linked genes. Often, cell mosaicism is advantageous, ameliorating the …