GENETIC INSTABILITY AT THE AGOUTI LOCUS OF THE MOUSE (MUS MUSCULUS) .1. INCREASED REVERSE MUTATION FREQUENCY TO THE A(W) ALLELE IN A A HETEROZYGOTES/

We have compiled the reverse mutation rate data to the white bellied a gouti (A(w)) allele in heterozygous A/a mice and shown it to be increa sed by a factor of at least 350 in comparison to the reverse mutation rate in homozygous a/a mice. Employing tightly linked flanking restric tion fragment length polymorphism DNA markers, we have shown that reve rsion to A(w) is associated with crossing over in the vicinity of the agouti locus. The non-agouti (a) allele has been recently shown to con tain an 11-kb insert within the first intron of the agouti gene. Toget her with our present results, these observations suggest possible mech anisms to explain the reversion events.