Responses to selection for plus and minus modifiers of some effects of Strong's luxoid gene on the mouse skeleton

Selection increased the heterozygous expression of Strong's luxoid gene (hindfoot polydactyly) in a high line (SH) and lowered it in a low line (SL). The realized heritability in the SH line was 0.142 ± 0.029; in the SL line, 0.159 ± 0.010. SH homozygotes showed as indirect responses increased hindfoot polydactyly, reduction of tibia and pubis, while SL homozygotes showed opposite responses for tibia and pubis. Selection in a PF subline for forefoot polydactyly in heterozygotes induced severe radial reduction as an indirect response in homozygotes. Selection increased the expression of radial duplication in a high line (SHH) and lowered it in a low line (SHL). The realized heritability in the SHH line was 0.110 ± 0.061; in the SHL line, 0.326 ± 0.180. In both lines heterozygous sibs of homozygotes with high or low radial expression were chosen for breeding since homozygotes rarely reproduce. In the SHH line heterozygotes with high expression and in the SHL line heterozygotes with low expression of polydactyly were preferred for mating. SHL homozygotes showed indirect responses in their hindlimbs similar to those observed in SL homozygotes while SHH homozygotes showed a less marked tendency to follow the SH pattern. These results suggest that the effects of Strong's luxoid gene on distal and proximal parts of the limb have a reciprocal inverse relation.