Test of the hypothesis that embryonic face shape is a causal factor in genetic predisposition to cleft lip in mice.

It was proposed that embryonic face shape is one of the quantitative variables underlying the threshold trait, cleft lip. To test the hypothesis several facial dimensions were measured in photographs of embryos at an early stage of formation of the face in 3 lines of mice. The lines had been selected from one original population using frequency of cleft lip induced by 6-aminonicotinamide as the selection criterion. Line L was susceptible to spontaneous cleft lip; lines M and C were not. Line L had a significantly smaller distance between the nasal pits than the other 2 lines, but did not differ in overall head size. A trend towards reduced angle between the medial nasal processes in the L line was noted, though this was not statistically significant. These results were predicted by the face-shape hypothesis.