A number of recent genetical experiments and microbiological models are directly applicable to human ophthalmogenetics. This is particularly the case in models and experiments involving X-chromosomal markers because X-chromosomal traits are well studied in the eye. The high prevalence of colour blindness in the Western world relative to primitive people has been explained as a lessening of a selectional pressure through evolution, and the emergence of trichromatic colour vision in primates has evidently been of great selective advantage. These evolutionary new characters can be ascribed to the redundancy of colour vision loci on the X chromosome. Colour-vision abnormalities are sometimes acquired, and some linkage studies are invalidated by deficient differentiation between phenotype and genotype of colour vision. Two reports of monocular deutan colour blindness in males have, so far, remained unexplained. Chimaerism might be an acceptable explanation because mouse chimaeras have been shown to have very dissimilar proportions of the constituent genotypes in the pigment epithelium of the two eyes in each animal. The clonal number of cells at the time of X-inactivation has been calculated in women heterozygous at the G-6-PD locus, in mice, this probably occurs before the differentiation of the mesoderm from the ectoderm. In man, such estimation has not yet been done. It is suggested to use the lens of carriers of G-6-PD variants in such estimations. A theory of the development of the cornea by a centripetal spiral pattern is proposed on the basis of the morphology of affected and heterozygotes for Fabry’s disease. It is also argued that Knudsons’s theory on the origin of heritable retinoblastoma from a mean of three cells can be tried by an estimation of G-6-PD variants in tumour cells from G-6-PD heterozygotes. Compound heterozygotes are at present a much-debated subject, and a list of X-borne loci with multiple alleles, supplementing earlier listing, is provided. X-linked retinitis pigmentosa and X-linked cataract are presented as polyallelic traits. Accepting the current theory of X-inactivation, evidence can be provided that the determination of human photoreceptor cells takes place before the embryo is lateralised because in this way the wide inter- and intrafamilial variation and the irrelevant inter-ocular variation in the manifestation in carriers can be accounted for. From experiments in mice carrying an X-linked marker for albinism, the same early determination has been hypothesised for the melanoblasts in the posterior part of the eye.

Keywords:
X chromosome, Ophthalmological disorders, Polyallelism, Compound heterozygotes, X inactivation
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© 1974 S. Karger AG, Basel
1974
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