Summary
Diseases such as Huntington's caused by unstable expansion of three-base repeats.
In depth
Repeat sequences expand during replication or meiosis. Huntington's involves CAG repeats coding polyglutamine (36+ repeats cause disease); fragile X involves CGG repeats silencing FMR1. Anticipation means that disease appears earlier and more severely in successive generations as repeats expand.
The repeat tract is unstable and tends to expand across generations, so symptoms appear earlier and more severely in later generations, called anticipation. In Huntington disease expansion occurs mostly in paternal transmission; in fragile X syndrome it occurs through maternal transmission, and over about 200 CGG repeats the gene is methylated and silenced.
Other examples include myotonic dystrophy (CTG), Friedreich ataxia (GAA) and several spinocerebellar ataxias (CAG). Mechanisms include slipped strand mispairing during replication and repair that mis-processes hairpin structures.
Frequently asked questions
What is Trinucleotide Repeat Disorders?
Diseases such as Huntington's caused by unstable expansion of three-base repeats.
Which area of biology covers Trinucleotide Repeat Disorders, and how advanced is it?
Trinucleotide Repeat Disorders is one of the 54 topics in Genetics. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Genomic Imprinting, Mitochondrial Inheritance, Epigenetics.
Why does Trinucleotide Repeat Disorders matter?
There are tests that count repeats, and therapies that silence the mutant gene (antisense oligonucleotides) are in clinical trials.
Can you give an example of Trinucleotide Repeat Disorders?
Huntington's disease usually appears at 30 to 50 years of age with more repeats leading to earlier onset.
What should I learn before Trinucleotide Repeat Disorders?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Trinucleotide Repeat Disorders. Advanced topics assume the Intermediate vocabulary.
What topics are related to Trinucleotide Repeat Disorders?
The closest topics to Trinucleotide Repeat Disorders are Genomic Imprinting, Mitochondrial Inheritance, Epigenetics, Frameshift Mutation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Trinucleotide Repeat Disorders?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Trinucleotide Repeat Disorders. To practise the whole of Genetics, use the Genetics quiz.
How is Trinucleotide Repeat Disorders different from Genomic Imprinting?
Trinucleotide Repeat Disorders: Diseases such as Huntington's caused by unstable expansion of three-base repeats. Genomic Imprinting: Parent-of-origin-specific gene expression caused by epigenetic marks, as seen in IGF2 and H19. Read both side by side and note what each one explains.
How should I study Trinucleotide Repeat Disorders step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Genomic Imprinting and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.