Drosophilia Genetics- maping linked genes!?

Question

In Drosophilia, the genes st( scarlet eyes), ss (spineless bristles), and e (ebony body) are located on same chromosome. with map positions

st- 44 --------------- ss-58------------------------e- 70

Each of these mutations is recessive to the wild type allele. (st+, reddish-brown, ss+, smooth bristle, and e+, gray body)

Phenotypically wild type females (st ss e+/ st+ ss+ e) were crossed with triply recessive males

a. what is the probability that the female will produce a gamete with the allele combination st+ ss+ e+? Assume no interference... (hint work it backwards step by step)

b. predict phenotypes of the progeny of the parents given and the frequencies with which they will occur assuming a level of interference at .4

I know how to do a 3-point test but don't understand the work backwards part... ANY help greatly appreciated!

Answer 1

A. the gamete type st+ ss+ e+ assumes there is a crossover between ss locus and e locus thus the probability of this gamete type from the female is (.86)(.12)/2. .86 is the probability that there is no crossover between st locus and the ss locus. This is divided by two because there are two possible SCO (single crossovers) between these two loci.

B. I am a little fuzzy here but interference only affects DCO (double crossovers) as the definition of interference it that it is a chance the first crossover prevents the second crossover.

So I think you figure the probabilities of each crossover event as done for A. Then for the DCO's you would also multiply by the interference.

I am not as sure on this aspect because the frequencies do not add up to one.

Hope this helps somewhat sorry I could not be more sure of part B, I am in school now for a BS in Mol gen.