Answer:
When the crossing over results in both mutant alleles on one chromosome and the other chromosome will be mutation free.
Explanation:
During the meiosis chromosomes are present in homologous form in a pair.
They exchange their genetic material with each other. This exchange is random and usually genetic material on the ends/telomere changes with the homologous chromosome.
In given condition the exchange is in such manner that the mutant allel of one chromosome shifts to other during crossing over. While other chromosome also contains its own mutant allele and does not exchange it during crossing over.
