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")