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In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (b) is recessive to the allele for the unbanded shell (B). Genes at a different locus determine the background color of the shell; yellow (y) is recessive to brown (Y). A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The heterozygous F1 are then crossed with banded, yellow snails (a testcross).

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