Nonallelic noncomplementation models in mice: the first arch and lidgap-Gates mutations

We tested for complementation between two Mendelian mutations in mice, Far (first arch) and lgGa (lidgap-Gates). Each of these mutations gives greater than 70% risk of the birth defect, open eyelids, in homozygotes and gives little or no risk in heterozygotes. Far and lgGa are known to not be alleles; Far maps to Chr 2 and lgGa maps to Chr 13. However the cross between +/Far (on the BALB/cGaBc strain) and lgGa/lgGa (on the LGG/Bc strain) gave 32% (48/149) of progeny affected with open eyelids at birth: 63% (45/71) of the double heterozygote, +/Far, +/lgGa, compared with 4% (3/78) of the +/+, +/lgGa progeny. That is, the complementation test suggests that Far and lgGa are alleles, whereas the mapping data show that they are not. We interpret the result of the Far by lgGa test as an example of nonallelic noncomplementation (or "false allelism") in mammals, and suggest that this phenomenon might be expected because open eyelids at birth involves a developmental threshold. Our data also show that both the embryonic and the maternal background genotypes strongly influence the risk of open eyelids in the Far by lgGa crosses. The risk to the double heterozygote (+/Far, +/lgGa) is highest (77%) with Far from the BALB/cGaBc rather than the ICR/Bc (0%) strain and in a BALB/cGaBc (77%) rather than an LGG/Bc (50%) dam in the reciprocal cross. This effect of genetic context on risk is also predicted by the threshold model. Based on our data on open eyelids at birth, we suggest that false allelism may be common in mammalian birth defects that result from failure to meet developmental thresholds, even when the "causal" mutations are Mendelian.