Question

Albino rabbits lack pigment and are homozygous for the recessive allele c (C allows pigment formation). Rabbits homozygous for the recessive allele b make brown pigment, while those with at least one copy of B make black pigment. True-breeding brown rabbits were crossed to albino rabbits (that also had the genotype BB). The F1 rabbits, which were all black, were crossed to rabbits with a double recessive genotype (bb cc). The progeny obtained were 34 black, 66 brown, and 100 albino.

a.What phenotypic proportions would be expected if the b and c loci were unlinked?

b.How far apart are the b and c loci?

Answer 1

Solution:

In the First (F1) Generation:- CCbb (brown rabbits) x ccBB (albinos) CcBb (Black)

	Cb	Cb
cB	CcBb	CcBb
cB	CcBb	CcBb

According to the question:-

CcBb (Black) x ccbb (recessive) 34 black: 66 brown: 100 albino rabbits.

a. in the case when the genes b and c were unlinked then it will result in the following:

	CB	Cb	cB	cb
cb	CcBb (Black)	Ccbb (Brown)	ccBb (Albino)	ccbb (Albino)
cb	CcBb (Black)	Ccbb (Brown)	ccBb (Albino)	ccbb (Albino)

Therefore, The phenotypic proportions which would be expected if the b and c loci were unlinked are :

2 Black : 4 Albino: 2 Brown.

i.e., 1/4 Black : 1/2 Albino: 1/4 Brown.

b.

If c and b are linked, then the genotype of the F1 class is C b / c B and you would expect the parental type Cb and cB gametes to predominate; cb and CB are the recombinant gametes and are therefore present at lower levels.The parental gametes are represented in the F2 by the Cc bb(brown) and cc Bb(albino) classes.

In case of c and b linking together then the genotype for the first (F1) generation would be Cb/cB and it would be expected that the parental type Cb and cB genes to dominate themselves; Also it must be noted that cb and CB are formed as a recombinant gametes and thus they are present in lesser level.

Also, in the second (F2) generation the parental gametes would be represented by the Ccbb (brown) and ccBb (albino) Phenotypes.