Deficiency of the Corpus Callosum: Incomplete Penetrance and Substrain Differentiation in BALB/c Mice

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Abstract
In a foundation population of BALB/c mice used to establish a colony at Waterloo in 1977, about 11% of adults showed either total absence or gross deficiency of the corpus callosum. Comparisons between parents and offspring, between progenies of different males, and between 13 separate lines established by full-sib inbreeding showed that variation in the adult corpus callosum reflected a genuine incomplete penetrance in a genetically uniform population. However, after seven generations of inbreeding, a spontaneous change occurred in one line (BALB/cWah 1), resulting in more than 50% of adults with deficient corpus callosum, among which about 20% have complete absence of callosal axons traversing the hemispheres.