In Drosophila, a heterozygous female for the X-linked recessive traits a, b, and c was c...

In Drosophila, a heterozygous female for the X-linked recessive traits a, b, and c was crossed to a male that phenotypically expressed a, b, and c. The offspring occurred in the following phenotypic ratios.

No other phenotypes were observed.

(a) What is the genotypic arrangement of the alleles of these genes on the X chromosome of the female?

(b) Determine the correct sequence and construct a map of these genes on the X chromosome.

(c) What progeny phenotypes are missing? Why?

HINT: This problem involves a three-point mapping experiment where only six phenotypic categories are observed, even though eight categories are typical of such a cross. The key to its solution is to be aware that if the distances between the loci are relatively small, the sample size may be too small for the predicted number of double crossovers to be recovered, even though reciprocal pairs of single cross-overs are seen. You should write the missing gametes down as double crossovers and record zeros for their frequency of appearance.