Genetics of Huntington's Disease (Cont.)

 
Table 1
No. of CAG repeats
Outcome
<28
Normal range; individual will not develop Huntington's disease
29-34
Individual will not develop Huntington's disease but the next generation is at risk
35-39
Some, but not all, individuals in this range will develop HD; next generation is also at risk
>40
Individual will develop Huntington's disease
 

Genetics of Huntington's Disease: How Is HD Inherited?

Huntington's disease is an autosomal dominant condition, which means that only one parent must have this gene in order for a child to inherit Huntington's disease. In a small number of cases, an individual with Huntington's disease will not have a parent with the disorder.
 
Each parent has two copies of every chromosome but gives only one copy to each child. Therefore, each child of a person with Huntington's disease has a 50-50 chance of inheriting the Huntington disease gene. A person who inherits the Huntington disease gene, and survives long enough, will sooner or later develop Huntington disease.
 
If a child does not inherit the Huntington disease gene, he or she will not develop Huntington's disease and cannot pass it to his or her children.
 
In some families, all the children may inherit the Huntington disease gene; in others, none do. Whether one child inherits the gene has no bearing on whether others will or will not share the same fate.
 
A small number of cases (1 to 3 percent) of Huntington's disease are sporadic, meaning they occur even though there is no family history of HD. These cases are thought to be caused by a new genetic mutation -- an alteration in the Huntington disease gene that occurs during sperm development.
 

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Written by/reviewed by: Arthur Schoenstadt, MD
Last reviewed by: Arthur Schoenstadt, MD