The Genetic Basis of Lateralization
chapterposted on 13.10.2009, 13:05 by Marian Annett
Modern theories of genetic influence on lateralization suggest that random or fluctuating asymmetry has an important role. Theories of directional asymmetry or chance can be distinguished from the right shift (RS) theory that accidental asymmetries are universal for bilaterally symmetrical organisms, but that in most humans left hemisphere advantage gives an incidental bias of the random distribution toward the right hand. Whereas many assume there is a true incidence of left-handedness, the RS theory suggests that degrees of hand preference map onto a continuous baseline of asymmetry that can be cut at any point to represent observed incidences of left-handedness. The parameters of the RS genetic model were derived from findings for speech lateralization in aphasics, supporting the argument that the relevant genetic locus is ‘for’ cerebral dominance, not handedness. Genetic predictions are given for two levels of parental left-handedness (10% and 20%). Studies of family handedness distinguishing for sex in both generations gave generally good fits where handedness was assessed by self-report, but more variable fits for indirect report of relatives’ handedness. The tendency to find a higher proportion of left-handed children born to left-handed mothers than left-handed fathers is not likely due to X-linked inheritance, but rather to slightly stronger expression of the RS + gene in females than males, and also under-reporting of left-handedness in mothers by right-handed children. Monozygotic (MZ) twins discordant for handedness have led to doubts about genetic influence but these doubts are misplaced if random asymmetry affects every individual, twin and singleborn. The similarity of handedness distributions in MZ and dizygotic (DZ) twin pairs suggests that similar mechanisms are at work in both types of pair. Twins are more likely to be left-handed than the singleborn, by some 3-4%. There is greater concordance in MZ than DZ pairs, but this is difficult to detect because variability due to accidental asymmetries is large, in comparison with the small genetic variability (due to the high prevalence of dextral genetic bias). The inheritance of brain asymmetries is difficult to research but findings are consistent with models developed for handedness. There is a continuous normal distribution of asymmetry, biased in a typical direction with negative skew. A study of strength of language lateralization in families suggests genetic influence. Studies of asymmetries of cerebral anatomy and function in twins support theories of bias in a typical direction, which is reduced in the presence of left-handers. The theory that asymmetries occur at random in the absence of the typical pattern is consistent with findings for language and visuospatial functions. The suggestion that the RS + gene might lose directional coding and cause random impairment of the cerebral hemispheres offers a possible explanation for disorders of language and loss of cerebral asymmetry in psychosis.