The Inheritance of HD
Part 3

An introductory guide to the genetics of Huntington’s disease.

How do genes determine physical traits? What does it mean to say that the HD gene is dominant?

Since we inherit one complete set of DNA from each parent, chromosomes occur in pairs called “homologues.” (Click here for a picture of homologues.) Hence, a gene that is found on a given chromosome actually has a partner on its matching, or homologous, chromosome. This means that a person actually has two copies of every gene, one allele on each of two homologous chromosomes. This feature raises some important questions about how alleles interact and relate to each other.

Alleles can be thought of as having different “strengths.” If two different alleles are present together, the “stronger” one will influence the trait under consideration. This phenomenon is called dominance. A dominant allele influences the resulting trait whether an individual has one or two copies of that allele. In contrast, in order for recessive alleles to be expressed, an individual must always have two copies of the “weaker” allele. (See Table C-1.)

HD is called a dominant trait because individuals with just one copy of the HD allele typically develop HD symptoms. The HD allele (with many CAG repeats) is dominant over the non-HD allele (with few CAG repeats). Again, an individual need have only one copy of the HD allele to inherit the disease. There is also no exact cut-off point for when the number of CAG repeats is considered “abnormal.” (Click here for a table of repeats and their effects.) Occasionally, an individual will have an allele whose CAG codon count falls within a small “gray area” for which the repeat number is slightly higher than normal, but not quite “abnormal.” This version of the allele has a medium strength. Its dominance is said to be “incomplete,” and individuals with this allele may or may not develop the disease.

Last Modified: 12-23-02

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