Question

In the snail cepaea nemoralis, an autosomal allele causing a banded shell, u, is recessive to the allele for an unbanded shell, u. genes at a different locus determine the background color of the shell. the allele for a yellow shell, b, is recessive to the allele for a brown shell,
b. a banded, yellow snail is crossed with a homozygous brown, unbanded snail. the f1 progeny are then crossed with banded, yellow snails in a testcross. predict the percentage of progeny in each phenotype if the two genes are linked with no recombination, if the two genes assort independently, and if the genes are 20 m.u. apart. place the percentage for each phenotype for each of the scenarios in the table below. percentages can be used more than once.

Answer 1

There will be no recombinant progeny in absolute linkage. The F1 inherted banded and yellow alleles (BBCY) together on one chromosome from the banded yellow parent and unbanded and brown allels (BOCBw) together on the homologous chromosome from the unbanded parent. All the F1 gametes will contain only these two allelic combinations, in equal proportions if there is no recombination. F2 cross progeny will have 1/2 banded, that is yellow and 1/2 unbanded brown.