Inheritance of retarded forebrain commissure development in fetal mice: results from classical crosses and recombinant inbred strains.

Deficiency of the adult corpus callosum in BALB/c mice shows incomplete penetrance and is clearly polygenic, whereas the defect in fetuses shows complete penetrance and a much less complex mode of inheritance. Retardation of the growth of the corpus callosum and the hippocampal commissure in the fetal mouse forebrain was expressed by a standard score (z) derived from body weight, such that a fetus with a score less than -2.0 was held to have commissures abnormally small for the body size. By this index, almost all C57BL/6 fetuses were normal, whereas BALB/c fetuses in the body weight range 0.5 to 1.0 g were often 5 standard deviations below the expected value of 0.0. In classical crosses between C57BL/6J and BALB/cWah, inheritance of the index of abnormality (z) was recessive, and about half of the fetuses in backcrosses to BALB/c were below -2.0. However, the distribution of scores was not bimodal. The results were consistent with a two-locus but not a single-locus difference between parent strains. Among the seven recombinant inbred strains derived from the By strains of C57BL/6 and BALB/c, there were three or possibly four distinct clusters of strains, which also suggested two-locus inheritance and excluded a single-locus difference. Although substantial retardation of commissure growth was evident in fetuses, deficiency or absence of the corpus callosum in weanling and adult By recombinant inbred mice was extremely rare in all strains except BALB/cByJ. These data confirm anatomical results showing that, in all but the most extremely retarded cases, the corpus callosum recovers from an obvious prenatal defect.