What has kept the HD allele from being eliminated?
Huntington’s disease is a lethal condition that is not yet curable. But if the HD allele is so lethal, why hasn’t it “died out” with those who carry it and disappeared from human populations?
Part of the answer may lie in the fact that HD is a late-onset disorder. Symptoms normally arise in the fourth or fifth decade of life, usually after a person has already started a family. A parent could therefore pass on the HD allele to his children before he even realized that he carried the allele. In this way, the HD allele is able to survive, even though its human carrier will not.
This explanation, however, doesn’t fully answer the question. After all, not all cases of HD are late-onset. Sometimes symptoms arise during the reproductive years. Other times, symptoms arise as early as childhood.
Currently, there are two hypotheses that attempt to explain how the HD allele has reached and been maintained at such high frequencies, especially in Europe:
Mutation
Early on, researchers did not think that new mutations could account for the global persistence of HD because, in most cases, mutations have little effect on large populations. However, as described in Part 3, the new mutation rate may be much higher than previously thought.
It is estimated that anywhere from 1%-14% of persons with HD did not inherit the HD allele from a parent, but instead acquired the HD allele from a newly formed mutation. Therefore, despite its deleterious nature, HD might be maintained in high frequencies due to the relatively high rate of new mutations.
Increased Fertility
As mentioned in Part 7 above, there is evidence in some populations that presymptomatic people with HD have a higher average fertility than the general population. If generally true, this increased fertility could also account for the persistence of HD in our world. However, this hypothesis presently lacks conclusive evidence.
Last Modified: 05/22/2009
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