Mirror writing ability is genetic and probably transmitted as a sex-linked dominant trait: it is hypothesised that mirror writers have bilateral language centres with a callosal interconnection.
Mirror writing is the ability to write from right to left, reversing each letter so that when held to a mirror the script appears normal. There is no information on the prevalence of this trait but a suggestion was received that it may be hereditary. A newspaper survey was carried out to discover the approximate prevalence and whether a hereditary factor is involved. The results indicated a prevalence of 1 in 6500. There is strong evidence that the trait is hereditary and is associated with non-right-handedness'. It is hypothesised that mirror writers may comprise a very small group of people who not only have bilateral language centres but also have an interconnecting pathway between these centres via the corpus callosum. The surprising genetic nature of a trait with no obvious evolutionary consequences is discussed. The data can best be explained on the basis of the trait being the phenotypic expression of an X-linked dominant gene, which does have evolutionary consequences. Similarities are noted between the proposed genetics of mirror writing and those of synaesthesia and of a few pathological X-linked dominant syndromes such as Rett syndrome. Other similarities and contrasts between mirror writing and synaesthesia are noted.
Mathewson
Iain
I
mathim@matilda.net.au
D000328:Adult; D001315:Australia / Q000453:epidemiology; D003337:Corpus Callosum / Q000502:physiology; D003625:Data Collection; D004292:Dominance, Cerebral / Q000235:genetics* / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000235:genetics* / Q000502:physiology; D005799:Genes, Dominant / Q000235:genetics* / Q000502:physiology; D006801:Humans; D007802:Language; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D017678:Sex Distribution; D012737:Sex Factors