Dactylaplasia in mice a two-locus model for development anomalies

Dactylaplasia, characterized by the absence of phalangeal bones in the middle digits of each foot, resulted from a mutation that occurred in the SM7B/SM inbred strain of mice. Breeding tests revealed the mutant gene is an autosomal dominant that is homozygous lethal. Further investigation by outcrossing with a number of inbred strains showed that the manifestation of the mutant gene is controlled by another locus. At this locus are found two alleles: one, a dominant inhibiting dactylaplasia gene expression; the other, a recessive allowing the expression of the mutant gene. In each of the tested inbred strains, one or the other allele is present at this locus. We propose Dac as a symbol for the mutant gene, and mdac for the locus controlling the Dac expression. Mouse dactylaphasia closely resembles split hand/foot in man and in monkeys in gross morphology and mode of inheritance. The significance of the present findings explainable by a two-locus model is discussed relative to irregular mode of inheritance of certain other congenital defects, and also relative to the maintenance of genetic loads in populations.