(Problem 8c part 1) In the snail Cepaea nemoralis, an autosomal allele causing a

Question

(Problem 8c part 1) In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for an unbanded shell (BO). Genes at a different locus determine the background color of the shell; here, yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The F1 are then crossed with banded, yellow snails (a testcross). What proportion of banded yellow snails will result in the testcross if the loci are linked and 20 m.u. apart?

Please be detailed and clear. Thanks in advance!

A. 0% B. 25% C. 40% D. 50% E. 60% F. 100%

Explanation / Answer

Brown (CBw) and Unbanded shell (BO) are dominant characters.

F1 genotype is : BO BB CBw CY

Upon test cross

BO BB CBw CY X BBBB CYCY

BO CBw X BBCY

BO CY X BBCY

BBCBw X BBCY

BBCY X BBCY = 25% BANDED YELLOW

AS IT IS HOMOZYGOUS RECESSIVE, NO EFFECT OF RECOMBINATION ON THESE PROGENY

SO ANSWER IS B

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