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 p?

A: 18.7 cM

B: 5.9 cM

C: 6.5 cM

D: 12.3 cM

E: 11.7 cM

Solution

1779 (tb, p, esc) - Parental

            1654 (tb+, p+, esc+) - Parental

            252     (tb+, p, esc) - Single cross over

            241     (tb, p+, esc+) - Single crossover

            131     (tb+, p, esc+) - Single crossover

            118     (tb, p+, esc) - Single crossover

            13       (tb, p, esc+) - double crossover

            9          (tb+, p+, esc) - double crossover

map distance between tb and p :

13+ 9 + 252 + 241 / 4197 = 0.1227 x 100 = 12.27 = 12.3 cM