Coat colours in dogs depends upon the action of at least 2 genes. At one locus a

Question

Coat colours in dogs depends upon the action of at least 2 genes. At one locus a dominant epistatic inhibitor of coat colour pigment (I_) prevents the expression of colour alleles at another independently assorting locus, producing white coat colour. When the recessive condition exists at the inhibitor locus (ii), the alleles of the hypostatic locus may be expressed, iiB_ producing black and iibb producing brown. When dihybrid white dogs are mated together, determine:

a)the phenotypic proportions expected in the progeny;

b)the chance of choosing, from among the white progeny, a genotype homozygous at both loci.

Explanation / Answer

Dihybrid cross between white dogs means that both dogs are heterozygous for both the allele I and B

The genotype of white dog is IiBb

Parents : IiBb x IiBb

Gametes produced by each parent are IB, Ib, iB,ib

iiBB (black)

The expected phenotypic ratio of progeny is 12 white : 3 black: 1 brown which is similar to the phenotypic ratio in dominant epistasis. In dominant epistasis dominant allele at one locus inhibits the expression of alleles at second locus irrespective of whether they are dominant (B) or recessive (b)

Out of the white progeny one two individuals are homzygous at both loci .These are IIBB and IIbb

IB Ib iB ib IB IIBB (white) IIBb (white) IiBB (white) IiBb (white) Ib IIBb (white) IIbb (white) IiBb (white) Iibb (white) iB IiBB (white) IiBb (white)

iiBB (black)

iiBb (black) ib IiBb (white) Iibb (white) iiBb (black) iibb (brown)

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