Triplet (CGG) expansion at the FMR-1 locus in Fragile-X Syndrome

    Unaffected individuals typically have from 6 ~ 54 copies of the CGG repeat. Slipped-mismatch repair in the female germline gradually increases the number of CGG repeats in some families.  Normally-transmitting males & their daughters are phenotypically unaffected.  Unequal crossing-over between high-CGG-copy X chromosomes further increases the copy number. At a threshhold around ~200 copies, affected offspring will show the Fragile-X Syndrome of mental retardation.  The phenotype is less common in females, where the alternative low-CGG-copy X chromosome apparently masks expression.  Repeat regions create a cytologically-detectable "satellite" at the end of the long arm of the  X-chromosome that is subject to breakage (a "fragile X")

Figure © 2000 by Griffiths et al. ; text © 2011 by Steven M. Carr