# Trinucleotide Repeat Disorders > Trinucleotide repeat expansion disorders in MBBS Pathology: anticipation, fragile X CGG counts, Huntington CAG, myotonic dystrophy CTG and Friedreich GAA. - Canonical URL: https://prepelephant.com/topics/mbbs/pathology/trinucleotide-repeat-disorders - Exam / course: MBBS · Subject: Pathology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Trinucleotide Repeat Disorders", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/trinucleotide-repeat-disorders ## Direct answer Anticipation — disease onset earlier and more severe in successive generations — is the signature of trinucleotide repeat expansion disorders, in which CAG, CGG, CTG or GAA repeats amplify during gametogenesis until they cross a pathological threshold. The four every exam expects are fragile X syndrome (CGG expansion silencing FMR1), Huntington disease (CAG in huntingtin), myotonic dystrophy type 1 (CTG in DMPK) and Friedreich ataxia (GAA in frataxin). Repeat length, not merely presence, determines phenotype, and the parent of origin determines whether expansion occurs at all. ## What you must remember - **Mechanism:** repeats above an intermediate length become meiotically unstable and lengthen in offspring; longer repeats mean earlier onset, faster progression — anticipation. - **Fragile X:** normal up to about 44 CGG repeats, premutation 55–200, full mutation over 200, which methylates and silences FMR1; long face, large ears, macro-orchidism, intellectual disability — the most common inherited cause of intellectual disability in boys. - **Fragile X parent of origin:** premutations expand to full mutations through maternal transmission; premutation carriers themselves get fragile X-associated tremor/ataxia syndrome (older men) and premature ovarian insufficiency (women). - **Huntington disease:** CAG 36 or more (fully penetrant at 40 and above; normal 26 or fewer); polyglutamine huntingtin, caudate atrophy, chorea, dementia, behavioural change; anticipation rides on paternal transmission. - **Myotonic dystrophy 1:** CTG 50 or more in DMPK; myotonia, early cataract, cardiac conduction disease, insulin resistance, frontal balding; the congenital floppy-infant form comes from a mother. - **Friedreich ataxia:** intronic GAA expansion in FXN — autosomal recessive, the exception among the classics; hypertrophic cardiomyopathy, scoliosis, pes cavus, areflexia, onset before 25. - **Other CAG diseases:** spinobulbar muscular atrophy (Kennedy disease, androgen receptor) and most spinocerebellar ataxias. - **Rule of thumb:** expanded CAG in coding regions produces toxic polyglutamine protein; untranslated expansions (CGG, CTG, GAA) act through gene silencing or RNA toxicity. ## A pedigree worth the numbers Take a family referred because a ten-year-old boy has intellectual disability and the paediatrician noticed large ears and a long face. The mother is entirely well; her father, however, died with a late-life tremor and unsteadiness that was never diagnosed. The connecting thread is a fragile X premutation in the grandfather — about 90 CGG repeats, enough to cause fragile X-associated tremor/ataxia syndrome in him, too small to make FMR1 mRNA toxic... rather, in the premutation range FMR1 mRNA itself accumulates and injures neurons. His daughter inherited the premutation; her oocytes, passing through the meiotic bottleneck, amplified some alleles beyond 200 repeats, methylation switched FMR1 off, and her son expresses the full syndrome. Her premature ovarian insufficiency at 38, if it occurs, belongs to the same premutation story. Now contrast a Huntington family. Three generations, onset at 58 in the grandfather, 44 in the father, 29 in the son — each step the CAG count climbed, and each step through a father, because spermatogenesis expands large CAG arrays far more than oogenesis does. Myotonic dystrophy mirrors this with the sexes swapped: the severe congenital form, with feeding weakness and developmental delay, arrives almost only through mothers carrying thousands of CTG repeats. Friedreich stands apart as recessive — both parents carry expanded alleles, and anticipation, though present, is modest. Writing these three parent-of-origin facts side by side is the single highest-scoring paragraph this topic can produce. ## Where students slip The recurring error is treating anticipation as a property of dominant disease in general. Most dominant disorders do not anticipate; only unstable repeats do, so "increasing severity over generations" in a viva should prompt the candidate to name a repeat disorder, not to praise the family's history-taking. The second slip is quoting a single threshold for fragile X — the boundaries (45–54 intermediate, 55–200 premutation, over 200 full) are the marks. Third, Friedreich is routinely mislabelled dominant; it is the autosomal recessive exception, and its GAA repeat sits in an intron rather than the coding sequence, a favourite distinction question. ## Frequently asked questions ### How many CGG repeats constitute a full fragile X mutation? Over 200 CGG repeats in FMR1; this triggers methylation and transcriptional silencing. The premutation range is 55–200 repeats. ### Which trinucleotide and gene are involved in Huntington disease? CAG repeat expansion in the huntingtin gene — 36 or more repeats causes disease, 40 and above is fully penetrant, with anticipation through paternal transmission. ### Which trinucleotide repeat disorder is autosomal recessive? Friedreich ataxia — intronic GAA expansion in the frataxin gene FXN, with hypertrophic cardiomyopathy as the main cause of death. ### Why is congenital myotonic dystrophy always maternally transmitted? Massive CTG expansion in DMPK to over roughly 1000 repeats happens in oogenesis; the congenital floppy infant with feeding and respiratory weakness therefore arises through mothers. ### What is fragile X-associated tremor/ataxia syndrome? A late-onset gait ataxia, intention tremor and cognitive decline in older men carrying premutation-range CGG repeats, caused by toxic FMR1 messenger RNA rather than silencing. ### Which physical sign is virtually specific to fragile X in boys? Macro-orchidism, alongside the long face and large ears; it reflects FMR1's role in testicular growth regulation.