New mutations from non-HD alleles to HD alleles occur more frequently on chromosomes with a high number of CAG repeats. Therefore, people with CAG repeat lengths in the high range of normal (close to 35) or in the intermediate range (35-40) are more likely to develop an HD allele in their germ cells and pass this HD allele on to their children. In other words, a person whose parent has a higher than normal frequency is more likely to develop HD than a person whose parent has fewer CAG repeat lengths.
Also, it seems that paternal transmission of the Huntington gene is more prone to expansion of the CAG repeats than maternal transmission. Therefore, a person whose father has a high number of CAG repeats is more likely to develop HD than a person whose mother has a high number.
Interestingly, chromosomes with a high number of CAG repeats tend to be marked by other features, including seven repeats of the CCG codon next to the CAG repeats and a missing GAG codon within the Huntington gene. It has been hypothesized, though not proven, that these markers might contribute to the instability and expansion of the CAG repeats.
Last Modified: 05/22/2009
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