Due to the repetitive nature of the DNA sequence in these regions, 'loop out' structures may form during DNA replication while maintaining complementary base pairing between the parent strand and daughter strand being synthesized.
If the loop out structure is formed from sequence on the daughter strand this will result in an increase in the number of repeats.
However, if the loop out structure is formed on the parent strand a decrease in the number of repeats occurs.
This property results in the characteristic of anticipation seen in trinucleotide repeat disorders.
Anticipation describes the tendency of age of onset to decrease and severity of symptoms to increase through successive generations of an affected family due to the expansion of these repeats.