Expression of L cone pigment gene subtypes in females.

Spectral subtypes of L pigment are produced by a serine/alanine dimorphism at amino acid position 180. X-chromosomes that carry genes for the different subtypes occur with about equal frequency in normal men. Females have two X-chromosomes: thus, about 50% of women will inherit genes for both L pigment subtypes, although on different X-chromosomes. In these women, X-inactivation is expected to produce about equal numbers of LS180 and LA180 cones in addition to middle (M) and short (S) wavelength-sensitive cones to total four spectrally distinct cone types. Consistent with this expectation we found nearly equal expression of genes for two spectrally distinct subtypes of L pigment in five of nine female retinas examined.