The genetic basis of synasthesia

Almost every study that has investigated the topic has suggested that synesthesia clusters within families, consistent with a genetic origin for the condition. The earliest references to the familial component of synesthesia date to the 1880s, when Francis Galton first described the condition in the pages of Nature. Since then, other studies have supported this conclusion. However, early studies (Baron-Cohen et al., 1993; 1996) which claimed a much higher prevalence in women than in men (up to 6:1) most likely suffered from a sampling bias due to the fact that women are more likely to self-disclose than men. More recent studies, using random samples find a sex ratio of approximately 1:1 (Simner et al., in press).

The observed patterns of inheritance have suggested an x-linked mode of inheritance, although research into the genetics of synesthesia is still very preliminary. For example, father-to-son transmission is rare, while mother-to-son and mother-to-daughter transmission are quite common. Pairs of identical twins have been identified where only one member of the pair experiences synesthesia (Smilek et al., 2002; 2005) and it has been noted that synesthesia can skip generations within a family (Hubbard and Ramachandran, 2003), consistent with models of incomplete penetrance. Additionally, Simner et al. (in press) note that it is quite common for synesthetes within a family to experience different types of synesthesia, suggesting that the gene or genes involved in synesthesia do not lead to specific types of synesthesia. Rather developmental factors such as gene-expresssion and environment must also play a role in determining which types of synesthesia an indivudal synesthete will experience.

Papers
Bailey, Mark E.S., and Keith J. Johnson (1997). "Synaesthesia: Is a Genetic Analysis Feasible?" In Baron-Cohen and Harrison, eds.; Synaesthesia, Cambridge,Massachusetts: Blackwell.

Papers

 * Google Scholar