[Genetics of congenital color vision defects. II. Rare types of color blindness].

Between the rare types of colour blindness, the known best are defects of blue colour vision, which are called tritanopia or trinanomaly (tritanomalous trichromacy). Their incidence is 1 in 500 and they are inherited in autosomal dominant way with incomplete penetrance. The basis of them are mutations of the short (blue) wavelength sensitive visual pigment gene. The gene has been mapped on the chromosome 7 and has already been cloned and sequenced. However, the loci heterogeneity should not be excluded in that condition. Another rare type of colour blindness in blue cone monochromacy. It is based on the cone sensitivity to short (blue) wavelength only. The condition is inherited in X-linked recessive way and it is known, that it can be caused by 2 different mechanisms. The first one--two-step pathway--consists of green cone pigment gene deletion, and point mutation of red cone pigment gene. The second one--one-step pathway--arose by deletion of regulatory sequence of both genes of visual pigments, mapped on the X chromosome. Different types of total and partial achromatopsia are also described. The best known ones are: rod monochromacy, which is inherited in autosomal recessive way and consist of rod vision only, and cone dystrophy, usually inherited in X-linked recessive way.