The Inheritance of HD
Part 6

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

There has never been any history of HD in my family...how could the HD allele just suddenly appear?

Remember that HD alleles are distinguished by the number of CAG codons they contain. The number of repeats is not fixed between generations, and it is possible that the number of repeats changes when cells divide. Usually this change results in a larger number of repeats, although occasionally the number of repeats decreases. No one is sure exactly what causes the number of repeats to multiply, but there is some evidence that codon numbers expand as a result of DNA copying inaccuracies during sperm formation. When DNA is copied, it is reproduced in small sections that are strung together later to make the long, continuous strands of DNA in chromosomes. There is some speculation that codon repeats could expand if these pieces are not hooked together correctly. (For more information about mutations, click here.)

A person who has a “normal allele” with a borderline number of repeats (typically between 36 and 39 copies of CAG) may produce a sperm or egg cell that contains an allele with a few additional codons. On rare occasions, these extra codons may be just enough to cause the child inheriting the allele to have an abnormal repeat number. One researcher speculated that about 10% of HD cases are caused by such changes in repeat number. Although in most cases the HD allele is not inherited in this way, this possibility explains how HD sometimes seems to just “appear” in a family.

Last Modified: 12-23-02

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