Heritable aspects of anomalous myelinated fibre tracts in the forebrain of the laboratory mouse.

Serial coronal or sagittal sections were stained for myelin and examined in 6 inbred, 4 hybrid, and 2 outbred mouse strains. Absent corpus callosum was seen only in BALB/cJ as reported by Wimer, but a wide range of the size of corpus callosum was also noted. The action of a major gene was not evident in backcross or F2 generations; polygenic and perhaps epistatic inheritance was indicated. In A/J, and to a lesser extent A/HeJ and BALB/cJ, the columns of fornix frequently collided with the anterior commissure and either passed around it to make a normal termination or deflected dorsally to make an abnormal termination in lateral septum. In some BALB/cJ brains the anterior commissure instead was displaced and passed behind or through the columns of fornix. Backcross and F2 data suggested inheritance was polygenic and that genetic variation affected the spatio-temporal coordination of ontogeny of the two tracts. Finally, unusual longitudinal bundles were detected in the septal region of BALB/cJ. Results of crosses were consistent with the hypothesis that a single, incompletely dominant gene was acting, but further study of both the anatomy and heredity of the defect was deemed necessary.