Shell Coiling in Limnaea

Limnaea peregra is a freshwater snail whose shell coils spirally, either clockwise (dextral) or anticlockwise (sinistral); dextral coiling is the normal form and sinistral coiling is a mutant character. Coiling direction is set by a single pair of nuclear genes — dominant D (dextral) and recessive d (sinistral)) — but the twist is that it is the mother's genotype, not the offspring's own genotype, that determines which way an individual snail's shell actually coils.

When a dextral snail (providing the egg) is crossed with a sinistral snail (providing the sperm), every F1 offspring is dextral in shell shape even though its genotype is heterozygous Dd — it matches its dextral mother's phenotype. Self-crossing these Dd F1 individuals gives an F2 generation that is still entirely dextral in shell shape (even though its genotypes now split 1 DD : 2 Dd : 1 dd), because every F2 individual's phenotype is fixed by its Dd mother's genotype, not by its own. Only in the next generation, F3, does true 3 dextral : 1 sinistral segregation finally become visible in the coiling direction itself — a full generation behind where the genotype ratio actually appeared. The reciprocal cross (sinistral female x dextral male) reproduces the same lag in mirror image: the F1 are genotypically identical (Dd) but phenotypically sinistral, matching their sinistral mother, and only the F2 (from a Dd sinistral mother) turns out phenotypically all-dextral. …