Huntington's disease is caused by:

Huntington's disease is caused by:
  a. a rare recessive allele.
  b. a repeat of a nucleotide triplet in a gene.
  c. a monosomic condition.
  d. a dominant X-linked allele.
  e. a deletion of part of chromosome 4.

Question 2

What factor allows the dominant mutant allele for Huntington's disease to persist in human populations
  despite its devastating effects?
  a. Heterozygote advantage.
  b. The fact that its symptoms do not typically show until after the individual has had
  children.
  c. Its effects are not expressed in heterozygous individuals.
  d. Its effects are only expressed in homozygous recessive individuals.
  e. Its effects are only expressed in homozygous dominant individuals