There is something important about the type of mutations that drive Huntington's.
Huntington's is caused by an expanded repeat composed of (CAG)n with n>40 repeats. Due to the instability of repeat regions, new mutations arise in this gene at a relatively high rate. So, even if there were strong selection against expanded repeats the disease would continue to exist. However, as mentioned by others here, strong selection is unlikely due to the late age of onset of the disease. So mutation continues to introduce the disease and selection has been too weak to remove it.
In contrast, BRCA mutations differ from the Huntington mutations. BRCA mutations are single "point" changes in the sequence, known as SNPs, that disrupt the protein function. Novel point mutations are exceptionally rare.
So, unlike with BRCA, when investigating why Huntinton's disease exists, one must consider BOTH mutation and selection.
Huntington's is caused by an expanded repeat composed of (CAG)n with n>40 repeats. Due to the instability of repeat regions, new mutations arise in this gene at a relatively high rate. So, even if there were strong selection against expanded repeats the disease would continue to exist. However, as mentioned by others here, strong selection is unlikely due to the late age of onset of the disease. So mutation continues to introduce the disease and selection has been too weak to remove it.
In contrast, BRCA mutations differ from the Huntington mutations. BRCA mutations are single "point" changes in the sequence, known as SNPs, that disrupt the protein function. Novel point mutations are exceptionally rare.
So, unlike with BRCA, when investigating why Huntinton's disease exists, one must consider BOTH mutation and selection.