THE DEGREE OF LATERALIZATION OF PAW USAGE (HANDEDNESS) IN THE MOUSE IS DEFINED BY 3 MAJOR PHENOTYPES

Lateralization of paw usage in the laboratory mouse may be a useful mo del system in which to assess the genetic and developmental cause of a symmetry of hand usage. With a set number of paw reaches from a centra lly placed food tube, individual mice from an inbred strain will exhib it a reliable number of left and right paw reaches. For a single inbre d strain, there are approximately equal numbers of left-pawed and righ t-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., 199 3). We review here our expanded survey of genetically different strain s 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 m ajor classes of highly lateralized, weakly lateralized, and ambilatera l preference. The trait exhibits both additivity and dominance in prel iminary reciprocal crosses, depending on which strain pairs are used. The wide difference between strains that have highly lateralized and a mbilateral paw preference suggests specific genetic tools that could b e used to begin a genetic dissection of the causes of this trait. Prel iminary 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 exhi bit a directional deviation from equal numbers of mice with left and r ight paw usage. Therefore, direction of paw usage may not be a genetic ally neutral trait, but replicate assessments and genetic tests are ne eded to confirm this.