Question

In Drosophila, purple eyes (pr) are recessive to wild type red eyes (pr+), and forked bristles (1) are recessive to wild type bristles (fº). A pure breeding purple eyed, forked bristled fly was mated to a pure breeding red eyed, wild type bristled fly. The resulting F1s were crossed to true breeding purple eyed, forked bristled fly, and the following offspring were observed: 322 red eyed, wild type bristles 318 purple eyed, forked bristles 78 red eyes, forked bristles 82 purple eyes, wild type bristles. The genetic distance between the genes controlling eye color and bristle shape is:

Answer 1

Given,

         eye color, purple eyes (pr)= recessive

                          red eyes(pr+) = dominant

          bristles shape, forked bristles (f) = recessive

                                   wild type (f+) = dominant

Cross between pr+pr+f+f+ $\times$ prprff gives, pr+prf+f genotype progenies i.e., red eyed wild type bristled progeny

now,

         test cross is performed between red eyed wild type bristled heterozygous with a pure purple eyed forked bristled fly

               which gives a ratio of 1:1:1:1

therefore, to find the distance between the genes controlling eye color and bristle shape, we need to find the recombinant frequency

recombinant frequency(%) = total recombinants / total offsprings *100

here, recombinants are

           red eyes forked bristles = 78 and purple eyes wild type bristles = 82

therefore, recombinant frequency = 78+82/322+318+78+82 *100

                                                      = 160/800 *100

                                                      = 0.2*100

                                                      = 20%

By defiintion, one map unit is equal to one percent recombinant phenotype

therefore, the distance between the genes is 20 map unit (m.u.)

THANK YOU!!