Trembler-J,TrJ, is a peripheral hypomyelinating murine mutant. In intercrosses(TrJ/+ xTrJ/ +) there are severely affected (behaviorally and pathologically), mildly affected, and normal offspring, while back-crosses(TrJ/+ x + /+) produce only mildly affected and normal offspring. We used the closely linked marker vestigial tail,vt, to test whether severely affected offspring of intercrosses(TrJvt/+ + xTrJvt/+ +) wereTrJ/ TrJ.In 5/6 intercrosses all severely affected animals were short-tailed and vice versa, while all mildly affected animals had long tails and vice versa. It is highly probable that the severely affected, mildly affected, and normal classes of intercross offspring were ofTrJ/ TrJ, TrJ/+, and + / + genotypes, respectively.