Fragile X syndrome is caused by expansion of a CGG trinucleotide repeat in the 5' UTR of the FMR1 gene. A premutation (55-200 repeats) in a female carrier can undergo further expansion to a full mutation (>200 repeats) during female meiosis — this is called:

• A Genomic imprinting
• B Mitotic instability
• C Anticipation ✓
• D Loss of heterozygosity

Explanation

Anticipation refers to the phenomenon where certain trinucleotide repeat diseases worsen in severity and/or appear at an earlier age in successive generations due to intergenerational expansion of the repeat. In Fragile X, a premutation (55-200 CGG repeats) is unstable during maternal meiosis and expands to a full mutation (>200 repeats), causing FMRP deficiency and clinical disease. This maternal bias in expansion is unique to Fragile X; in myotonic dystrophy and Huntington's disease, anticipation also occurs. Genomic imprinting refers to parent-of-origin-specific gene expression.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.