Population Genetics
Part 1

An examination of the origin and frequency of HD

The Genetics of HD

It is important to understand the basic genetics behind Huntington’s disease (HD) before learning about its population genetics. This section is simply a brief refresher – to learn more, please visit The Basics of Huntington’s Disease and The Inheritance of HD.

• The Huntington gene is responsible for encoding the huntingtin protein. Although the biological function of the huntingtin protein is still unknown, it is known that the alteration of this protein ultimately results in HD.
• There are multiple versions, or alleles, of the Huntington gene. Some alleles, known as HD alleles, will result in Huntington’s disease. Other alleles, known as non-HD alleles, will not. Because Huntington’s disease is a dominant genetic disorder, a person only needs one copy of an HD allele to develop HD. For more on alleles or genetic dominance, click here.
• Specifically, the number of CAG codon repeats in the Huntington gene determines whether an allele will be an HD allele or a non-HD allele. Alleles with fewer than 35 CAG codon repeats are non-HD alleles; alleles with more than 40 CAG codon repeats are HD alleles. Alleles with between 35 and 40 CAG codon repeats are intermediate – sometimes they will result in HD, sometimes they will not.

Last Modified: 04/12/2007

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