10. In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (B

Question

10. In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (B") is recessive to the allele for an un-banded shell (B°). Genes at a different locus determine the background color of the shell; yellow (C") is recessive to brown (C). A banded, yellow snail is crossed with a homozygous brown, un-banded snail. The F, are then crossed with banded, yellow snails (a testcross). a. What will the results of the testcross be if the loci that control banding and color are linked with no crossing over?

Explanation / Answer

a. Recombination is having inverse relation with linkage. So,

If there is no crossing over or recombination, the first progeny, i.e. F1 will be having the gametes with the equal proportion of these alleles.

So, the answer will be-

50% BBCY and 50% BOCBW

c. Test cross result when-

1. loci are linked

2.15 mu apart

The distance between the two characters is 15 mu means the recombination frequency if 15%, so we can say the rest 85% will be the parental progeny.

So, the test cross result should be-

Half of the parental progeny i.e. 42.5% will be BBCBW

Half of the parental progeny i.e. 42.5% will be BOCY

Half of cross over progeny or recombinants i.e. 7.5% will be BBCY

Half of cross over progeny or recombinants i.e. 7.5% will be BOCBW

Thanks

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