Note that we do not know, at this point, the order of the genes; we have arbitrarily put b in the middle.

The next step is to determine which of the test-cross progeny are nonrecombinants and which are double crossovers. The nonrecombinants should be the most-frequent phenotype; so they must be the progeny with phenotypes encoded by ch+ b+ cn+ and ch b cn . These genotypes are consistent with the genotypes of the parents, which we outlined earlier. The double crossovers are the least-frequent phenotypes and are encoded by ch+ b+ cn and ch b cn+.

We can determine the gene order by comparing the alleles present in the double crossovers with those present in the nonrecombinants. The double-crossover progeny should be like one of the nonrecombinants at two loci and unlike it at one; the allele that differs should be in the middle. Compare the double-crossover progeny ch b cn+ with the nonrecombinant ch b cn . Both have cherub wings (ch) and black body (b), but the double-crossover progeny have wildtype eyes (cn+), whereas the nonrecombinants have cinnabar eyes (cn). The locus that determines cinnabar eyes must be in the middle.

(b) To calculate the recombination frequencies among the genes, we first write the phenotypes of the progeny with the genes encoding them in the correct order. We have already identified the nonrecombinant and double-crossover progeny; so the other four progeny types must have resulted from single crossovers. To determine where single crossovers took place, we compare the alleles found in the single-crossover progeny with those in the nonrecombinants. Crossing over must have taken place where the alleles switch from those found in one nonrecombinant to those found in the other nonrecombinant. The locations of the crossovers are indicated with a slash:

single crossover nonrecombinant single crossover double crossover nonrecombinant single crossover single crossover double crossover

Next, we determine the recombination frequencies and draw a genetic map:





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