SUMMARYThe semi-dominantX-linked gene tabby (Ta) in the mouse and its recessive autosomal mimic crinkled (cr) produce the same mutant syndrome involving abnormalities of the hair, teeth and certain exocrine glands. Previous work has provided some indication of interaction between these loci in terms of vibrissa number. The results of the present study demonstrate that, for tooth size, mice doubly heterozygous for tabby and crinkled show a more extreme phenotype than either heterozygotes for tabby or crinkled alone.