Three recessive traits are being followed in Drosophila thic

Three recessive traits are being followed in Drosophila: thick bristles, peach eyes and extra sex combs. Flies that are homozygous for thick bristles (tb/tb), peach eyes (p/p) and extra sex combs (esc/esc), are crossed with flies that are heterozygous for each trait (tb+/tb, p+/p and esc+/esc). A total of 4197 progeny fall into the following categories:

            1779 (tb, p, esc)

            1654 (tb+, p+, esc+)

            252     (tb+, p, esc)

            241     (tb, p+, esc+)

            131     (tb+, p, esc+)

            118     (tb, p+, esc)

            13       (tb, p, esc+)

            9          (tb+, p+, esc)

What is the map distance between tb and esc?

A: 18.7 cM

B: 5.9 cM

C: 6.5 cM

D: 12.3 cM

E: 11.7 cM

Solution

The answer is option D - 12.3 cM

Since the frequency of double recombinants will be less the gene esc is in the middle.

Hence the gene order is tb esc p; tb+ esc+ p+

For cross between tb and esc Recombinants between tb and esc has to be found out.

They are tb+esc p - 252; tb esc+ p+ - 241; tb+esc p+ - 9 and tb esc+ p - 13.

Map distance = 252+241+9+13/ 4197 x 100 = 12.3 cM.