Question

In mice, coat colour and tail length are controlled by two linked genes: B = Black,...

In mice, coat colour and tail length are controlled by two linked genes:

B = Black, b = brown; and T = long tailed, t = short tailed.

A testcross was carried out using a heterozygous black, long tailed mouse, and the following progeny were observed:

7 Black, long-tailed

18 Black, short-tailed

20 Brown, long-tailed

5 Brown, short-tailed

a)      What is the recombination frequency between the B and T genes?

b)      What is the map distance between the B and T genes?

c)      Are the B and T genes showing coupling or repulsion?

d)      The heterozygous black, long-tailed mouse in the original question had true breeding parents. What were there genotypes and phenotypes?

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Answer #1

Progenies :

Black long = BT/bt = 7 = recombinant

Black short = Bt/bt = 18 = parental

Brown long = bT/bt = 20 = parental

Brown short = bt/bt = 5 = recombinant

A. Recombination frequency = number of recombinants/total number of progenies × 100 = 7 + 5 / 18 + 20 + 7 + 5 × 100 = 12/50 × 100 = 24%

B. Gene distance = recombination frequency = 24 mu

C. Repulsion, we will tell this in accordance with the genotype of parental progenies

D. BT/BT (black long) × bt/bt (brown short)

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