The tendency for some genetic disorders to manifest at an earlier age and/or to increase in severity with each succeeding generation.
The mechanism of anticipation is associated with a particular type of DNA change - areas of DNA within or close to genes which contain multiple repeats of a three letter DNA sequence - a "trinucleotide repeat". The number of trinucleotide (triplet) repeats can increase (or sometimes decrease) during the production of gametes (eggs/sperm). The disease manifests itself when the number of repeats has increased above a particular number. For some of these conditions the age of onset or severity is associated with the number of trinucleotide repeats.
Disorders caused by this type of mutation include myotonic dystrophy, fragile-X syndrome and Huntington's disease.
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