title: Synaesthesia: discordant male monozygotic twins.
abstract: Grapheme-color synaesthesia, a condition in which achromatic graphemes elicit vivid experiences of color is believed to be a genetically determined trait. We describe a study of 10-year-old twin brothers who are physically identical in appearance but who have considerably different conscious experiences. A phenotypic analysis that measured the consistency of grapheme-color pairings over test-retest confirmed that one twin has grapheme-color synaesthesia and the other twin does not. A genotypic analysis using sixteen microsatellite loci confirmed that the twins are monozygotic. These findings are problematic for previous suggestions that synaesthesia is an X-linked dominant trait. At the very least, the findings show that the penetrance of the genotype for synaesthesia is incomplete and that any view suggesting that synaesthesia is simply an X-linked dominant trait is therefore also incomplete and possibly even incorrect. The findings also negate a previous suggestion, based on a study of female monozygotic twins, that discordance of synaesthesia in identical twins is due to X-inactivation. In general, the findings raise serious questions regarding whether it is possible at this time to establish the genetic contribution to synaesthesia.
authors.affiliation: Department of Psychology, University of Waterloo, Waterloo, Ontario, Canada. dsmilek@uwaterloo.ca
authors.forename: Daniel
authors.initials: D
authors.lastname: Smilek
authors.forename: Mike J
authors.initials: MJ
authors.lastname: Dixon
authors.forename: Philip M
authors.initials: PM
authors.lastname: Merikle
mesh_terms: D002648:Child; D003118:Color Perception / Q000235:genetics*; D004200:Diseases in Twins; D005838:Genotype; D006212:Hallucinations / Q000235:genetics*; D006801:Humans; D008297:Male; D018895:Microsatellite Repeats / Q000235:genetics*; D010468:Perceptual Disorders / Q000235:genetics* / Q000503:physiopathology; D010641:Phenotype; D010775:Photic Stimulation / Q000379:methods; D014430:Twins, Monozygotic