An international collaboration including scientists from the WTCHG has discovered that the same genes that determine asymmetries in the internal body plan of mammals may also play a role in establishing human hand preferences.
William Brandler, the lead author and a PhD student in the Centre, had a personal interest in performing this study: ‘As a left-hander I’ve always been interested in why humans are biased to the right’, he says. ‘Our hands are symmetrical after all, so why isn’t the ratio closer to 50:50 like it is in chimpanzees? Understanding the genetic basis of handedness may begin to offer clues to solving this mystery.’
The team, led by Dr Silvia Paracchini, formerly at the Centre but now at the University of St Andrews, scanned the genomes of thousands of individuals, searching for common variations in the genetic code that are associated with handedness. As they report in PLOS Genetics, they uncovered a number of associations in a network of genes that were previously known to be involved in the development of left/right asymmetry in the early embryo.
In rare cases where these genes are switched off or non-functional, individuals develop a complete mirror-reversal of body asymmetry in which the heart and stomach are located on the right side of the body, with the liver on the left, known as situs inversus. The variants identified in this study do not switch off the genes: they probably act more like dimmer switches, increasing or decreasing the activity of a gene.
What was surprising about this finding is that individuals with situs inversus are no more likely to be left-handed. Paracchini and her team suggest that compensatory mechanisms allow handedness to develop normally when these genes are individually switched off, but the collective action of gene variants does influence handedness.
Brandler cautions that these results do not completely explain the variation in handedness. ‘As with all aspects of human behaviour, nature and nurture go hand-in-hand’, he says. ‘The development of handedness derives from a mixture of genes, environment, and cultural pressure to conform to right-handedness.’
Brandler, W.M. et al., Common variants in left/right asymmetry genes and pathways are associated with relative hand skill. 2013, PLoS Genet 9(9): e1003751. doi:10.1371/journal.pgen.1003751