Question

5. In Drosophila, males from a true-breeding stock with raspberry-colored eyes were mated to females from a true-breeding stock with sable-colored bodies. In the F1 generation, all the females had wild-type eye and body color, while all the males had wild-type eye color but sable-colored bodies. When F1 males and females were mated to each other, the F2 was composed of: 216 females with wild-type eyes and wild-type bodies 223 females with wild-type eyes and sable bodies 191 males with wild-type eyes and sable bodies 188 males with raspberry eyes and wild-type bodies 23 males with wild-type eyes and bodies 27 males with raspberry eyes and sable bodies Are the genes controlling body color and eye color linked? If yes, which chromosome are they on?

Answer 1

The parents are from true breeding stocks: raspberry eye color male x sable body color female

F1: wild type eye and body color (female) x sable body color( male )    if no mention is made of the eye color, then it is assumed to be wild type

F2: 216 wild type female

223 sable female

191 sable male:

   188 raspberry male

23 wild type male

   27 raspberry sable male

1. The phenotypes seem to be controlled by 2 genes, one for eye color and the other for body color.

2. The F1 female progeny show that the wild type allele is dominant for body color (s+ > s) and the wild type allele is also dominant for eye color (r+ > r).

3. Sable body color is seen in the F1 males but not in the F1 females, so the s gene is X-linked.

4. In the F2 generation the raspberry eye color is seen in the males but not in the females, so r is also an X-linked gene.

We can now assign genotypes to the true breeding parents in this cross:

r s+/ Y (raspberry male) x r+s / r+s female

F1    r s+/ r+s female (wild type)    x r+s / Y (sable male)

Gametes: heterozygous F1 female can make the following gametes:

(parentals) r s+,, r+s & (recombinants) r s, r+s+;

F2 generation has 6 different phenotypes.The recombinations are of (27+23)/429*100 i.e 11.7 % which is less than 50%.

Since the cross between F2 generation is not in the ratio of 9:3:3:1 the genes are linked to each other.That is recombinants. This is present on chromosome number 3.