The agouti hair pattern often seen in many wild animals is characterised by a sm

Question

The agouti hair pattern often seen in many wild animals is characterised by a small band of light pigment near the tip of the hair. The width of this lightly pigmented band is genetically determined. Purebreeding agouti rabbits with a wide band at the tip of the hair were crossed to purebreeding rabbits with black fur. All of the offspring of this cross had agouti fur with the narrow band at the tip of the hair. These offspring were test crossed to purebreeding black rabbits. The offspring of the second cross produced the following results below. Propose a detailed genetic hypothesis to account for these results. Show clearly how you arrived at your conclusion.

Phenotype and Number of offspring for each Phenotype

Black fur: 151

Agouti fur with the wide band: 127

Agouti fur with the narrow band: 22

Explanation / Answer

In case of black rabbits, you will not see the band due to epistasis and test cross has to be with homozygous recessive.

So the cross will be BbNb X bbnn

But the progeny we are getting is not showing independent assortment as the ratio is not 1:1:1:1. So the possibility is that these genes are showing linkage. As there are only 3 phenotypes this could be because the presence of B allele masks the effect of band width locus so the black population consists of black wide and black narrow.

From the data, it appears that agouti with a wide band is a non-recombinant parental class. The allelic combination in heterozygote must be B w/ b W.

The proportion of recombinants (agouti narrow) bW/bw is 22 in addition to this the black fur class approximately 22 will be the recombinants with black wide fur (Bw/bw).

therefore the map distance will be: (22+22) X 100/300= 14.7

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