Verified answer

Recessive (or dominant) genes can become more common than the dominant allele by genetic drift.

Genetic drift is the random changes in the frequency of alleles in a population without the action of selective pressure.

So, for blue eyes, since there is no particular selective advantage or disadvantage to having them, they are not actually selected for or against. But, it is possible that random changes in gene frequency will result in that gene becoming more common.

Whether a gene is recessive or dominant plays no role in this process.

Huntington's disease is dominant, not recessive. It is an unusual disease - one of the "Trinucleotide repeat disorders" or "Polyglutamine diseases". They are unusual because they actually get worse over generations.

This happens because most people have around 9 repeats of the anticodon CAG in the gene for the Huntingtin protein (Htt); CAG codes for the amino acid Glutamine in the final protein. However, some people have more than this. Since this CAG repeat consists of multiple identical anticodons immediately after each other, it is more likely that DNA polymerase will "slip" when ciopying this gene, and will accidentally insert more CAGs (because the DNA pol. will land on an identical CAG, and be unable to tell that it has made an error).

As a result, if you have more CAG repeats, it gets progressively more and more likely that you will get more added: in other words, the CAG-repeat region grows.

Less than 35 CAGs results in a normal Htt protein, but more than 40 results in a protein that does not fold correctly (because the large region of Glutamines interferes with protein structure), and gives Huntington's disease.

Huntington's disease is *not* more common than the recessive "normal" phenotype (it only has an incidence of 1 in 10,000 to 1 in 1 million, depending on racial background).

But, in some families - with slightly more CAG repeats, it can suddenly arise, and get worse and worse as the generations proceed.