Inherited red-green colour vision defects are quite common, affecting nearly 1 in 10 males, but are less common in women, affecting about 1 in 250. However because red-green defects are X-linked, nearly 15% of females are heterozygous carriers of red-green colour deficiency. In addition, about 1 in 150 females are “double carriers”, where both of their X chromosomes have L/M gene arrays encoding a red-green defect. If a woman carries the same type of colour vision defect on each X-chromosome, she herself will be red-green colour deficient, whereas if she carries opposing defects (protan vs. deutan) on each X chromosome she will be trichromatic, owing to the process of X-inactivation. These women are referred to as compound heterozygotes, though very few have been reported. Moreover, questions remain as to whether the colour vision capacity of these women is comparable to that of “normal” trichromats.
We examined a compound heterozygote carrier of both protanopia and deuteranomaly. We also examined male members of her family representing both forms of red-green defect carried by the female proband. Complete colour vision testing was done, including Rayleigh matches, pseudoichromatic plates, unique hue measurements, and 100-Hue tests. Flicker-photometric ERG estimates of L:M cone ratio were obtained, as were Medmont C100 settings.
Genetic analyses provided direct confirmation of compound heterozygosity. The compound heterozygote showed Schmidt’s sign, consistent with an extreme skew in her L:M cone ratio, and usually associated with protan carrier status.
Apart from Schmidt’s sign, we found the colour vision of the compound heterozygote to be indistinguishable from that of a normal trichromat.