We now know the locations of all the crossovers; their locations are marked with a slash in Figure 7.14.

Calculating the recombination frequencies Next, we can determine the map distances, which are based on the frequencies of recombination. Recombination frequency is calculated by adding up all of the recombinant progeny, dividing this number by the total number of progeny from the cross, and multiplying the number obtained by 100%. To determine the map distances accurately, we must include all crossovers (both single and double) that take place between two genes.

Recombinant progeny that possess a chromosome that underwent crossing over between the eye-color locus (st) and the bristle locus (ss) include the single crossovers ( st+ / ss e and st / ss+ e+ ) and the two double crossovers ( st+ / ss / e+ and st / ss+ / e ); see Figure 7.14. There are a total of 755 progeny; so the recombination frequency between ss and st is:

st-ss recombination frequency =

The distance between the st and ss loci can be expressed as 14.6 m.u.

The map distance between the bristle locus (ss) and the body locus (e) is determined in the same manner. The recombinant progeny that possess a crossover between ss and e are the single crossovers st+ ss+ / e and st ss / e+ , and the double crossovers st+ / ss / e+ and st / ss+ / e . The recombination frequency is:

ss-e recombination frequency =

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