A mutation in a gene that causes the protein to acquire an abnormal activity, such as aggregating within the cell, will usually cause a dominant inherited disorder.

Question

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Since the additional function acquired by the mutant allele causes the syndrome, the one normal allele is not enough to give the individual a normal phenotype, and so a heterozygote would also exhibit the syndrome. Thus, the mutant allele would usually be dominant. This is the case for the Huntington disease allele.

A Mutation in a Gene That Causes the Protein to Acquire