If Huntington's disease is due to a dominant trait, shouldn't three-fourths of the population have Huntington's while one-fourth has the normal phenotype?

Question

anonymous · Answer

Genotypic dominance does not equate trait commonality. In other words, the recessive trait of non-Huntington's disease can be common in a given population.

anonymous · Answer

@getlit @cardinal_carlo It depends on the selection.  If there is no selection difference between the dominant and recessive trait, then the ratios of the two (distribution in the population) will fluctuate very little.  However, if the selection is strong in one direction it will tend to move towards fixation given enough time.

Huntington's disease is very deadly so that the selection against it is very strong, though modern health care is lessening that.  This keeps it at a low rate in the population.