Imagine that four genes on a single chromosome in a mutant phenotype of Drosophila cross over at half the normal rate as in the wild-type phenotype. How, if at all, would genetic maps of that chromosome differ between this mutant and the wild-type fly

We know that frequency of recombination is proportional with the distance between the genes on the chromosome. Therefore when the recombination rate is higher that means the distance between the genes on the chromosome is bigger. If the recombination rate is lower that means the genes are closer to each other on the chromosome. In this case the cross over rate is half the normal rate in the wild-type. That means that on the genetic map the distance between the two genes on the wild type will be twice bigger than the genes of the mutated Drosophila.