Abstract

This paper focuses on the inheritance of human handedness and cerebral lateralization within the more general context of structural biological asymmetries. The morphogenesis of asymmetrical structures, such as the heart in vertebrates, depends upon a complex interaction between information coded in the cytoplasm and in the genes, but the polarity of asymmetry seems to depend on the cytoplasmic rather than the genetic code. Indeed it is extremely difficult to find clear-cut examples in which thedirectionof an asymmetry is under genetic control. As one possible case, there is some evidence that the direction, clockwise or counterclockwise, of rotation of the abdomen in certain mutant strains ofDrosophilais controlled by a particular gene locus, although there appears to be some degree of confusion on this point. By contrast, it is much easier to find examples in which thedegreebut not the direction of asymmetry is under genetic control. For instance, there is a mutant strain of mice in which half of the animals displaysitus inversusof the viscera. The proportion has remained at one half despite many years of inbreeding, suggesting that the mutant allele effectively cancels the normalsitusand allows the asymmetry to be specified in random fashion.Although this account does not deny that the right hemisphere of humans may be the more specialized for certain functions, it does attribute a leading or dominant role to the left hemisphere (at least in most individuals). We suggest that so-called “right-hemisphere” functions are essentially acquired by default, due to the left hemisphere's prior involvement with speech and skilled motor acts; we note, for instance, that these right-hemisphere functions include rather elementary perceptual processes. But perhaps the more critical prediction from our account is that the phenomenon of equipotentiality should be unidirectional: the right (lagging) hemisphere should be more disposed to take over left-hemisphere functions following early lesions than is the left (leading) hemisphere to take over right-hemisphere functions. We note preliminary evidence that this may be so.