Edinburgh Napier University

  Synaesthesia is a neurodevelopmental condition characterised by anomalous sensory perceptions and associated alterations in cognitive function. This chapter summarises what is known about the familial transmission of synaesthesia and its genetic underpinnings. Early familiality studies showed evidence for a strong genetic predisposition, a highly skewed female: male ratio, and an absence of male-to-male transmission. These patterns supported an early hypothesis of a single-gene X-linked dominant mode of inheritance with male lethality. Subsequent analyses in larger samples indicated that the mode of inheritance was likely to be more complex, with both the strong female skew and absence of male-to-male transmission brought into question. We review in detail the first whole genome linkage scan for auditory-visual synaesthesia (Asher et al. 2009) and a subsequent family linkage study on coloured sequence synaesthesia (Tomson et al. 2011). Together these results suggest linkage to five chromosomes (2, 5, 6, 12 and 16) but give no support for linkage to the X-chromosome. We discuss candidate genes within these regions and potential implications for the aetiology of synaesthesia. We also discuss the implications of these pioneering genome scans for our understanding of synaesthesia and of how different synaesthetic phenotypes may arise from different genetic predispositions interacting with other genes and with the environment.