The most single gene diseases cause that a gene has been changed in an easy way (replacement of one base to another, loss of or deposit of an extra base, etc.) then inherited in its altered from unchanged within the same family species as the disease itself uniform. Some diseases with simple dominant inheritance have shown considerable variation within the family. From the late l980's was the reason for this finding. Just outside the gene or paid within the gene is found a repeat of three bases.
The number of repeats in the normal population varies, but when the number rises significantly, it will disrupt the plant genetic function, increasing the number of repetitions wearing. It is therefore, a proliferation number of triplet repeats, which exceeds the threshold of disease. Under inheritance, it will rarely occur changes in the number of repetitions under normal conditions but when the repetition count is high and associated with disease symptoms, occurs more often changes in the number of repetitions between the generations. Such triplet-change has been called dynamic mutations, mutations that constantly change.
In Huntington's disease is the base sequence (triplet ten) found repeated from 36 to over 121 times, while normally only find that repetitions are less than 34 at a normal repeat length jumps up to over 36 is not actually registered (that is extremely rare). While those with low symptoms and late onset of Huntington has a "premutation" between 36 and 50 repetitions easier / more likely to jump up in higher numbers to the next generation (less often, they are shorter) and therefore, give serious Huntington with an earlier onset. The table shows a similar relationship with the dominant myotone muscular dystrophy, and by the sex-linked fragile X syndrome (mental retardation in males).
The number contains discovered dominantly expressed triplet diseases is increasing, and has so far found mostly in the nervous system genetic disease, but also a recessive neurological disease (Friedreich's ataxia) is due more frequently a triplet-repeat than normal point mutations.