The red eye color of wild-type Drosophila is due to a mixture of scarlet and bro

Question

The red eye color of wild-type Drosophila is due to a mixture of scarlet and brown pigments. These pigments are both synthesized from a white precursor according to the following biochemical pathway. The genes (cn and bw) whose products catalyze the biochemical steps are also indicated. Mutations in these genes are all recessive.
bw cn
Scarlet pigments ? White precursor ? Brown pigments

What would be the eye-color phenotype of individuals with the following genotypes: 4 pts

a. cn+ bw- / cn- bw+

b. cn- bw+ / cn- bw-

c. cn+ bw- / cn+ bw-

d. cn- bw- / cn- bw-

The cn and bw genes are linked. To determine the map distance between the genes, I testcrossed females that were heterozygous for mutations in both genes. The phenotypes and frequencies of the progeny are shown below.

Phenotype Genotype Frequency Parental or recombinant

Red eyes 97

Scarlet eyes 410

Brown eyes 404

White eyes 89
Total 1000

e. Complete the table above to include the diploid genotypes of the progeny, and state whether they represent parental or recombinant classes. 4 pts

f. Draw the genotype of the female parent, and state whether she was cis- or trans- heterozygous arrangement. 2 pts

g. Calculate the recombination frequency between cn and bw. 2 pts

Explanation / Answer

a) Red

b) Brown

c) Scarlet

d) White

e)

g) Recombination frequency : Divide the no. of recombinant gametes by total no. of progeny.

Recombination frequency = (97+89)/1000 = 0.186

Phenotype Genotype Frequency Parental/recombinant Red cn+cn-/ bw+bw- 97 Recombinant Scarlet cn+cn-/ bw-bw- 410 Parental Brown cn-cn-/ bw+bw- 404 Parental White cn-cn-/ bw-bw- 89 Recombinant

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