DNA that stutters
Most of the mutations we have met are static — one letter changed, and it stays changed. But a handful of genes contain a short sequence of three bases repeated over and over, like CAG CAG CAG. A modest number of repeats is normal and harmless. A trinucleotide repeat expansion is what happens when that run grows too long — and crucially, it can keep growing each time the DNA is copied and passed on.
Why does it grow? During DNA replication, the copying machinery can slip on these repetitive runs and accidentally add extra copies — like a needle skipping on a scratched record and replaying the same groove. The longer the run already is, the easier it slips, so expansions tend to snowball across generations. This makes the repeat a dynamic mutation: not a fixed typo, but one that changes size as it descends a family tree.
Anticipation: earlier and worse each generation
Because a longer repeat usually means earlier onset and more severe disease, families with these disorders can show anticipation: the condition appears at a younger age, and hits harder, in each successive generation. A grandparent might develop Huntington disease in their fifties; a grandchild who inherited a longer repeat might show signs decades sooner.
Huntington disease — CAG repeats in the HTT gene
Repeat count Outcome
------------ ---------------------------------
< ~27 normal, stable
~27 - 35 intermediate; may expand in sperm
~36 - 39 reduced penetrance (may or may not develop disease)
>= ~40 full penetrance; disease will develop
Expansion bias: repeats often grow most when passed through the FATHER
(in sperm), so the largest jumps — and earliest-onset cases — tend to
come down the paternal line.
Generation 1 (grandfather): 41 repeats -> onset ~age 55
Generation 2 (father): 48 repeats -> onset ~age 42
Generation 3 (child): 62 repeats -> juvenile onset
^ the repeat GREW each transmission = anticipationKnowledge, choice, and care
Because the repeat count can be measured, Huntington disease can be tested for before any symptoms appear. That raises hard, personal questions — would you want to know? Genetic counseling exists precisely for moments like these: trained counselors help people understand the odds, weigh testing, and plan, without pressure. The science here is honest about uncertainty (reduced-penetrance ranges, variable onset), and good care meets people where they are. This guide is education, not medical advice — real decisions belong to families and their clinicians.
Step back and the whole track fits together. The number of faulty copies needed (one or two), the chromosome the gene sits on (autosome or X), and whether the mutation is static or a growing repeat — these few questions decide how a single-gene disorder is inherited, who is a carrier, and what the recurrence odds are. From cystic fibrosis to Huntington disease, the same handful of rules quietly does the work.