Colour perception in twins: individual variation beyond common genetic inheritance

The twin method was used to examine the genotype/phenotype relationship in colour vision, by determining concordance in colour perception within pairs of monozygotic (MZ) twins and dizygotic (DZ) twins. For MZ twins, whose photopigments are genetically identical, higher concordance in colour perception was expected; conversely, differences within each MZ pair would indicate a non-genetic contribution.

Ratings of dissimilarity between successively presented colours were elicited from four MZ and three DZ twin pairs. A non-twin sibling pair and three unrelated normal trichromats were enrolled, for comparison. Concordance for each twin (sibling) pair was estimated by Spearman correlations (rs) between data matrices and by Procrustes distances (gl) between colour spaces, reconstructed from individual data using multidimensional scaling (MDS).

For MZ twins, rs) values (0.94-0.97) were comparable to intra-individual variability and significantly higher than those for DZ twins and siblings (0.72-0.82). Further, colour spaces for MZ co-twins were less discordant, with gl values (0.008-0.029) lower than for DZ co-twins (0.073-0.079) and siblings (0.052). Finally, concordances among all pairs of subjects were summarised by a geometrical 'subject space': the mean distance between MZ co-twins was 29 per cent of that between DZ and sibling pairs.

Lower concordance rates in DZ twins and siblings can be attributed to differences in the inherited arrays of photopigment genes. The high concordance for MZ twins is in line with their shared photopigment genotype, placing an upper limit on contributions to discordance from possible individual variations in non-genetic factors. Potential photoreceptor, ocular and cognitive sources of inter-twin variation are discussed.