Loss of heterozygosity at the dilute short ear (Myo5a-Bmp5) region of the mouse: mitotic recombination or double non-disjunction?

The occurrence of homozygous-viable dilute-short ear (Myo5a-Bmp5) double mu tants in mouse specific locus mutation experiments has generally been assum ed to be the result of double nondisjunction such that the mutant inherits two copies of chromosome 9 carrying the recessive alleles from the test-sto ck. A homozygous viable Myo5a-Bmp5 double mutant was recovered recently in our laboratory. We were able to genetically analyse both the Myo5a-Bmp5 reg ion and proximal and distal markers in the original mutant as well as in of fspring of the original mutant. Our results indicate the mutational event t o be due to mitotic recombination and not double non-disjunction.