Assume you are mapping genes on the second chromosome of Drosophila melanogaster using three different true?breeding mutant strains: strain 1: flies have black bodies (wild type body colour is tan) and bright red eyes (wild type eye colour is dull or brick red); strain 2: flies have tan bodies and brown eyes; strain 3: flies have black bodies and white eyes.
You cross males from strain 1 to females of strain 2 and the resulting F1 flies are completely wild type (tan bodies and dull red eyes). You then cross F1 females to males from strain 3 and obtain the following phenotypes amongst 1000 F2 flies:
Number of flies
Black bodies; bright red eyes 285
Tan bodies; brown eyes 300
Black bodies; brown eyes 25
Tan bodies; bright red eyes 31
Black bodies; wildtype eyes 10
Tan bodies; white eyes 14
Black bodies; white eyes 175
Tan bodies; wildtype eyes 160
TOTAL 1000

Note: There are two separate eye pigment pathways in Drosophila: one produces red pigments and the other produces brown pigments; together, these give the wild type eye colour (if both pathways are non?functional, no eye pigment is produced).

a. State the number of linked genes that account for the phenotypes observed and explain the basis of the bright red, brown, and white eye phenotypes.

b. Using genetic symbols appropriate to your explanation in part a, give the genotypes of the strains 1, 2, and 3 flies and the F1 flies.

c. Draw a linkage map showing the order of the genes involved and distances between them in map units.