In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (U) is
ID: 62902 • Letter: I
Question
In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (U) is dominant to one for unbanded shell (u). A gene at a different locus determines the background color of the shell: green (B) or brown (b), with green being the dominant phenotype. A banded brown snail is crossed to an unbanded green snail; assume each parent is homozygous for these genes in the initial cross. The F1 progeny are then testcrossed. A) What will the results of the testcross look like if these loci assort independently? B) What will the results of the testcross look like if these loci are 38 M.U. apart? C) What will the results of the testcross look like if these loci are 7 M.U. apart?
Explanation / Answer
Cross between a banded brown snail (UUbb) is crossed to an unbanded green snail (uuBB) will have the offspring with the following genotypes.
UUbb* uuBB = 100% UuBb -----------> F1
Cross between F1 will have the offspring with the following genotypes.
A). UuBb* UuBb = UUBB (1/16), UUbB (2/16), UUbb (1/16), UuBB (2/16), UuBb (4/16), uuBb (2/16), uuBB (1/16), Uubb (2/16), uubb (1/16).
Means, only 50% of the offspring are having parental phenotypes (U_B_), while remaining offpsring are recombinant ones.
b). If these loci are 38 m.u. apart, the resulting offspring will be 38% recombinant and 72% parental. The genotypes are,
UUbb ---------> 14%, uuBB -------> 14%, UuBb -----------> 72%
c). If these loci are 7 m.u. apart, the resulting offspring will be 7% recombinant and 93% parental. The genotypes are,
UUbb ---------> 3.5%, uuBB -------> 3.5%, UuBb -----------> 93%