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In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for 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).a. What will the results of the testcross be if the loci that control banding and color are linked with no crossing over?
b. What will the results of the testcross be if the loci assort independently?
c. What will the results of the testcross be if the loci are linked and 20 m.u. apart?

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Answer:

a) As a result of the testcross when the loci that control banding and color are linked with no crossing over, then the F2 testcross progeny will be half banded, yellow and half unbanded, brown.

b) As a result of the testcross if the loci are assorted independently, the progeny formed would be:

  • One fourth banded, yellow
  • one fourth banded, brown
  • one fourth unbanded, yellow
  • one fourth unbanded, brown

c) In case of the conclusionof the testcross, if the loci are linked and 20 m.u. apart then,

  • 40 percent banded, yellow
  • 40 percent unbanded, brown
  • 10 percent banded, brown
  • 10 percent unbanded, yellow.

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