• The genes r, s, and t reside in the middle of the Drosophila x chromosome; r is 15 M to the left of, and t is 20 cM to the right of s. in this region, the coefficient of coincidence (c) is 0.2. A geneticist wishes to create an X chromosome that carries the recessive mutant alleles of all three genes. One stock is homozygous for rand t, and another stock is homozygous for By crossing the two stocks, the geneticist obtains females that are triple heterozygotes, r s+ t/r+ s t+. These females are then crossed to wild-type males.
 If the geneticist examines 10,000 sons from these females, how many of them will be triple mutants, r s t?
1. For small map intervals (<20 cM), the map distance equals the frequency of a single crossover interval. 

2. The coefficient of coincidence equals the observed frequency of double crossovers/expected frequency of double crossovers.

3. The expected frequency of double crossovers is calculated on the assumption that the two crossovers occur independently.

4. Males inherit their X chromosome from their mothers.