Tests of the right shift genetic model for two new samples of family handedness and for the data of McKeever (2000)

Two new family samples are described for handedness in parents and children. In the first sample, drawn from the general population, questionnaires were completed for each individual (a self-report sample). In the second sample students completed a handedness questionnaire and described the handedness of their parents (indirect report sample). The percentages of left-handed children in the various family types (R x R, R x L etc., father x mother) were tested for goodness of fit to the predictions of the right shift genetic model, along with tests for the findings of McKeever (2000). Fits for the self-report sample were good for all family types. For the indirect report sample fits were good for many family types but there were more left-handed daughters than expected in R x L families and fewer than expected in L x R families. In the McKeever sample there was a shortfall of sons in L x R families but the fit for sons in R x L families was excellent. It is concluded that differences between samples here, and in the literature, are variable and probably due to sampling errors when the assessment of parental handedness depends on indirect report. Findings for sex differences do not suggest a need for hypotheses about X-linked mechanisms.

Heritability and linkage analysis of hand, foot, and eye preference in Mexican Americans

Functional lateralities are of interest due to their relationship with cerebral lateralisation and language development. However, genes influencing sidedness remain elusive. We measured direction and consistency of hand, foot, and eye preference in 584 Mexican-Americans from families participating in the San Antonio Family Diabetes/Gallbladder Study. Using maximum-likelihood-based variance components methods, we estimated weak (.11 <or= h2<or=.17) but significant heritability for foot preference, eye preference, several hand preferences (writing, drawing, throwing, using scissors, using spoon, striking match), and a composite hand preference trait. Self-reported handedness was significantly heritable (h2=.57), whereas hand preference for opening a box or using a toothbrush or knife was not. Many trait pairs had significant genetic correlations, and all had significant environmental correlations. Using genome-wide multipoint linkage screens using 382 highly informative autosomal STR markers, we identified suggestive linkage signals for drawing (LOD 2.10) and writing (LOD 2.00) hand preference on chromosome 12q21-23, in the region flanked by markers D12S1300 and PAH. A suggestive signal (LOD 2.46) for eye preference occurred on chromosome 22pter, near marker D22S420. No obvious candidate genes occur in these regions. Our results indicate that genes are an important component of side preferences, and suggest chromosomal regions for further investigation.

Is the handedness gene on the X chromosome? Comment on Jones and Martin (2000)

G. V. Jones and M. Martin (2000) argued, contrary to M. C. Corballis (1997), that a gene for handedness might plausibly be located in homologous, noncombining regions of the X and Y chromosomes. The specific model they proposed is unlikely to be correct, but a case can be made for an X-linked gene that has no homologue on the Y chromosome and that is subjected to X-inactivation in females. An X-linked gene predicts no overall sex difference in the incidence of left-handedness; the slight preponderance of left-handers among males might then be attributed to a higher incidence of pathologically induced left-handedness.