Because genetic determination of coat colors in calico cats is linked to the X chromosome, calicos are nearly always female, with one color linked to the maternal X chromosome and a second color linked to the paternal X chromosome. In most cases, males are only one color (for instance, black) as they have only one X chromosome. Male calicoes can happen when a male cat has two X chromosomes (Klinefelter syndrome, with XXY sex chromosomes and generally sterile); is a chimera, with two different cell types; or, rarely, when some skin cells of the developing kitten spontaneously mutate.