Chapter 18, Problem 10P

Drosophila researchers have collected many strains that carry a single recombinant P element containing a wild-type white gene (a P[w⁺] transgene) inserted into a known genomic location. These strains can be used to map the location of any mutant gene in the fly genome.

Investigators performed a testcross to map a recessive mutation rough (ro), which causes rough eyes, relative to a P[w⁺] element on chromosome 3. Females heterozygous for the P[w⁺] on one chromosome 3 and a ro− mutation on the other, homologous chromosome 3 were crossed to ro−/ro− males, and the progeny in the following list were obtained. In both the parents and the progeny, the endogenous white gene is nonfunctional—the flies have red eyes only if they contain the P[w⁺] transgene.

a.	Are ro and the P[w+] linked? If so, how many map units separate them?
b.	The data in part (a) do not indicate on which side of the P[w+] (toward the centromere or telomere) the ro gene is located. How could the experiment be modified to reveal that information?
c.	Suppose you map the ro mutation to a genomic region between two different P[w+] elements that are 5000 bp apart. Describe some experimental approaches that would allow you to identify the ro gene at the molecular level.
d.	How could you use the DNA sequence of the ro gene to determine the function of the protein it encodes?