A premutation is a situation in which there are an excess number of
repeats in a
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
that is at risk of increasing in length during
reproduction
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
but which does not cause disease in the person with the excess number of repeats.
Fragile X syndrome
Fragile X syndrome (FXS) is a genetic disorder characterized by mild-to-moderate intellectual disability. The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled. Physical features may ...
, a
trinucleotide repeat disorder
Trinucleotide repeat disorders, also known as microsatellite expansion diseases, are a set of over 50 genetic disorders caused by trinucleotide repeat expansion, a kind of mutation in which repeats of three nucleotides ( trinucleotide repeats) inc ...
, is a condition in which premutations may be present in the parents of affected people.
Huntington's disease
Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...
is another example of a trinucleotide repeat disorder in which individuals with a certain excess of repeats (27–39) of the CAG
codon
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
in the
Huntingtin
Huntingtin (Htt) is the protein coded for in humans by the ''HTT'' gene, also known as the ''IT15'' ("interesting transcript 15") gene. Mutated ''HTT'' is the cause of Huntington's disease (HD), and has been investigated for this role and also for ...
gene may not have the disease but are more likely to have a child with enough repeats (40+) to cause the disease.
[{{cite journal, vauthors=Walker FO, date=January 2007, title=Huntington's disease, journal=Lancet, volume=369, issue=9557, pages=218–28, doi=10.1016/S0140-6736(07)60111-1, pmid=17240289, s2cid=46151626]
References
Mutation