Lateralization of paw usage in the laboratory mouse may be a useful model system in which to assess the genetic and developmental cause of asymmetry of hand usage. With a set number of paw reaches from a centrally placed food tube, individual mice from an inbred strain will exhibit a reliable number of left and right paw reaches. For a single inbred strain, there are approximately equal numbers of left-pawed and right-pawed mice, but strain differences have been reported in the degree of lateralization of paw preference. We reported a preliminary strain survey in which the strains appeared to fall into two groups of highly lateralized and weakly lateralized paw preference (Biddle et al., 1993). We review here our expanded survey of genetically different strains and stocks of the laboratory mouse, including different species and subspecies. The major genetic trait is the degree of lateralization of paw preference and the strain differences appear to fall into three major classes of highly lateralized, weakly lateralized, and ambilateral preference. The trait exhibits both additivity and dominance in preliminary reciprocal crosses, depending on which strain pairs are used. The wide difference between strains that have highly lateralized and ambilateral paw preference suggests specific genetic tools that could be used to begin a genetic dissection of the causes of this trait. Preliminary assessment of the size of the corpus callosum in three strains with significantly different degrees of lateralization suggests that genetically determined deficiencies and absence of this structure are not the direct cause of the strain differences in the trait of degree of lateralization. In the expanded survey, some strains appear to exhibit a directional deviation from equal numbers of mice with left and right paw usage. Therefore, direction of paw usage may not be a genetically neutral trait, but replicate assessments and genetic tests are needed to confirm this.

Lateralization of paw preference in laboratory mice in a single-paw reaching task has been used as a model system for left- and right-hand usage. Given a set number of paw reaches for food from a centrally placed food tube, an individual mouse will exhibit a reliable number of left and right paw reaches. Within any single inbred strain, there are approximately equal numbers of left-pawed and right-pawed mice. Nevertheless, significant strain differences have been reported for the degree of lateralization of paw preference. We report here a systematic survey of paw preference in 12 inbred strains of the mouse in which the degree of lateralization falls into two groups of weakly lateralized and highly lateralized paw preference. The genetic inference is that a single major gene may control some function, and alternate alleles at this locus are expressed as weakly and highly lateralized paw preference. Reciprocal crosses indicate the trait is additive with no maternal or X-linked effects. The direction of paw preference has previously appeared to be genetically neutral, but in some strains there is evidence of significant deviation of the numbers of mice to the left and right of equal paw usage, independent of degree of lateralization, and this suggests that direction of left-right paw usage may be a separate genetic trait in the mouse model.

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