Suppose the genes a and b were 20 cM apart and recombination

Suppose the genes a and b were 20 cM apart, and recombination occurred in females, but not in males. If a b/+ + females were crossed to a b/+ b males, the following phenotypes would be observed, ranked from most to least frequent:

The answer is either

(++) > (+b) > (ab) > (a+)

or   

(++) > (a+) > (+b) > (ab)

Solution

The observed phenotype is

(++) > (a+) > (+b) > (ab)

++ is the dominant gene. The a+ genes are more dominant compared to b+. ab is the phenotype that the offspring has in its gene.