I wonder how this genetic error could propagate wide enough for the disease to get a name.
It's clearly maladaptive, isn't it?
Of course, same question could be asked for BRCA.
It's clearly maladaptive, isn't it?
Of course, same question could be asked for BRCA.
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.
I wish I could find the excerpt, but the book The Gene talks about the original research and it's an interesting story.
The book is worth reading in general (Siddhartha Mukherjee is a great writer and it's really good).
http://www.simonandschuster.com/books/The-Gene/Siddhartha-Mu...
Orphans are not a very good evolutionary tactics. But you definitely have a point otherwise. Had it earlier onset age, it would have no way to propagate.
I'm not arguing that this is necessarily the case, but it is not clearly maladaptive.