Rhw

In this case, we know that ro is the middle locus because the genes have been mapped. Eight classes of progeny will be produced from this cross:

e+ ro+ bv+ nonrecombinant e ro bv nonrecombinant ro bv single crossover between e and ro ro+ bv+ single crossover between e and ro ro+ / bv single crossover between ro and bv ro / bv+ single crossover between ro and bv ro / bv+ double crossover ro+ / bv double crossover

To determine the numbers of each type, use the map distances, starting with the double crossovers. The expected number of double crossovers is equal to the product of the single-crossover probabilities:

Note that the g-to-e distance (18 m.u.) is shorter than the sum of the g-to-c (12 m.u.) and c-to-e distances (8 m.u.), because of undetectable double crossovers between g and e.

3. Ebony body color (e), rough eyes (ro), and brevis bristles (bv) are three recessive mutations that occur in fruit flies. The loci for these mutations have been mapped and are separated by the following map distances:

expected number of double crossovers :

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