Genetic and behavioral tests of the McManus hypothesis relating response to selection for lateralization of handedness in mice to degree of heterozygosity

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Abstract
McManus advanced a genetic hypothesis to explain differences of lateralization between HI and LO lines of mice selectively bred for degree of handedness. It states that lateralization is a function of heterozygosity. Specifically it predicts that (a) the HI line will be more heterozygous than the LO line and (b) populations with a greater average heterozygosity (AH) will be more strongly lateralized. Both genetic and behavioral preheterozygosity (AH) will be more strongly lateralized. Both genetic and behavioral predictions were tested here. Results using coat color and biochemical variants show that AH in the HI line is somewhat less (not greater) than that in the LO line. The handedness of HET control mice and HI by LO reciprocal hybrids, where AH is greater than that of the HI line, exhibits lessened (not greater) lateralization. Results reject the heterozygosity hypothesis. A model for the inheritance of human handedness that accounts, for difficulty in detecting heritable differences in degree of asymmetry is presented.