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

Two loci are linked with no crossing over:

Banded, yellow snail * Homozygous unbanded, brown snail

BB BB CY CY                             * B0 B0 CBW CBW

B0 CBW

BB CY

B0 BB CBW CY

F1: B0 BB CBW CY

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a.)

Two loci are linked with no crossing over:

F1 (B0 BB CBW CY)   * Testcross (BB BB CY CY)

BB CY

B0 CBW

B0 BB CBW CY

BB Cy

BB BB CY CY

B0 BB CBW CY - Unbanded, Brown.

BB BB CY CY - Banded, Yellow.

B0 CBW

BB CY

B0 BB CBW CY

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