An RNA interference-like phenomenon has been implicated in the regulation of transposable elements. In Drosophila, two of the key proteins involved in this regulation are encoded by the genes aubergine and piwi. Flies that are homozygous for mutant alleles of these genes are lethal or sterile, but flies that are heterozygous for them are viable and fertile. Suppose that you have strains of Drosophila that are heterozygous for aubergine or piwi mutant alleles. Why might the genomic mutation rate in these mutant strains be greater than the genomic mutation rate in a wild-type strain?