Non-clinical sciences/Genetics · Inheritancelow yield

Trinucleotide-repeat anticipation

What it means

Unstable repeats expand each generation → earlier onset and greater severity in successive generations (anticipation). Huntington (CAG, chr 4; worse with paternal transmission), fragile X (CGG, FMR1), myotonic dystrophy (CTG, DMPK), Friedreich ataxia (GAA, AR).

Worked example

A father with late-onset Huntington's has a son with juvenile-onset disease — paternal CAG expansion.

In the exam

A neurodegenerative condition appears at progressively younger ages down the generations because an unstable triplet repeat lengthens at each transmission.

What settles it

Anticipation is the hallmark of expanding triplet repeats; it is not seen in stable point-mutation Mendelian disease.

Classically confused with

Autosomal dominant

Source: StatPearls