Answer:
If there were a new mutation to the HTT gene that causes a reduction to the CAG trinucleotide repeat, the allele frequency for Huntington's disease would decrease in South Africa. HD is caused by too many repeats of the CAG segment causing an abnormally long version of the huntintin protein which becomes deadly. so if the number of repeats is reduced then the person is more likely to not have Huntington's disease. If they do still develop Huntington's disease, it will be more likely to be adult onset and they will still have an opportunity to reproduce offspring which may carry the trait for reduced CAG.