An introductory guide to the genetics of Huntington’s disease.
What are alleles? How many alleles can there be for a gene and how many copies does each individual have?
Suppose we choose a particular chromosome from two different people and examine the DNA from the same spot on both chromosomes. We will find that the pattern of the bases (A’s, C’s, T’s, and G’s) is similar, but it is often not exactly the same, even if the region is a protein-coding gene. How can a gene code for a product if the pattern is not the same in every person? The answer is that there can be many different versions or variants of a given gene. These different versions of the gene are called alleles. Different alleles of a gene code for the same trait, but they may manifest themselves in different ways. The gene for eye color contains the instructions governing eye pigment, for example, but the specific color is determined by the particular alleles one has. Everyone has the same number of chromosomes and genes, but each person’s genetic code has a unique combination of alleles. This potential for variation explains why we all have similar genomes, yet we still have people of different heights, weights, and faces.
The way in which the Huntington gene varies among individuals is by the number of repeated C-A-G codons it contains. In other words, different alleles of the Huntington gene contain different numbers of CAG codons. It is important to understand that everyone has the Huntington gene, but individuals with Huntington’s disease have a many-CAG version of the gene, one that does not function normally. “Having the HD allele” is somewhat loose terminology, but it is used often and usually implies “having one of the multiple-CAG alleles on the Huntington gene that causes HD.” Within this site, the allele on the Huntington gene with the normal number of CAG repeats (the allele that does not result in HD) is referred to as the non-HD allele. The allele of the Huntington gene with the extra CAG repeats (the allele that does result in HD) is described as the HD allele.
Last Modified: 9-13-02
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