Recall Holliday-Whitehouse Model of DNA recombination (animation)
Consider a pair of
chromosomes,
each with a (CAG)4 repeat
Suppose repeat
regions
of homologous strands align "out
of register"
Recombination at
the "nick"
produces a (CAG)6 and a (CAG)2
chromosome
Further slippage in the (CAG)6 chromosome
may
lead to chromosomes with multiple (10s ~ 1,000s) of repeats.