GENETIC AND BEHAVIORAL-TESTS OF THE MCMANUS HYPOTHESIS RELATING RESPONSE TO SELECTION FOR LATERALIZATION OF HANDEDNESS IN MICE TO DEGREE OFHETEROZYGOSITY

McManus advanced a genetic hypothesis to explain differences of latera lization between HI and LO lines of mice selectively bred for degree o f handedness. It states that lateralization is a function of heterozyg osity. Specifically it predicts that (a) the HI line will be more hete rozygous than the LO line and (b) populations with a greater average h eterozygosity (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 (no t greater) than that in the LO line. The handedness of HET control mic e and HI by LO reciprocal hybrids, where AH is greater than that of th e HI line, exhibits lessened (not greater) lateralization. Results rej ect the heterozygosity hypothesis. A model for the inheritance of huma n handedness that accounts for difficulty in detecting heritable diffe rences in degree of asymmetry is presented.