Abstract
BACKGROUND: Inherited red-green colour vision defects are quite common, affecting one in 12 males, but are less common in women, affecting about one in 250. Because red-green defects are X-linked, nearly 15 per cent of females are heterozygous carriers of red-green colour deficiency. In addition, about one 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 will be red-green colour deficient, whereas if she carries opposing defects (protan versus deutan) on each X chromosome, she will have normal colour vision, owing to the process of X-inactivation. These women are referred to as compound heterozygotes, though very few have been reported. Questions remain about whether the colour vision capacity of these women is comparable to that of 'normal' trichromats. METHODS: 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, pseudoisochromatic plates, unique hue measurements and 100-Hue tests. Flicker-photometric ERG estimates of L : M cone ratio were obtained, as were Medmont C100 settings. RESULTS: 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. CONCLUSION: Apart from Schmidt's sign, we found the colour vision of the compound heterozygote to be indistinguishable from that of a normal trichromat.