Abstract
Huntington disease (HD), a dominant single-gene trait, was the first major neurological disorder for which the DNA sequence of the culprit gene (HTT) was determined. The sequence uncovered some big surprises. The gene contained a long series of repeats of the three bases CAG that encode glutamine in the huntingtin protein, and people with >39 repeats almost always developed symptoms if they lived long enough. The more repeats there were, the earlier in life did symptoms appear. Furthermore, when there were >35 repeats, the number of repeats tended to increase in the next generation, until finally, there were so many repeats that significant symptoms appeared. Because symptoms often do not appear until after 40 or 50years, carriers of the large allele often decline to have predictive testing done and instead have families of their own, which in 50% of cases will pass the gene to the next generation.
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