Genetic anticipation: A remarkable phenomenon in which a genetic disease appears earlier with each succeeding generation. Anticipation was once thought not to exist in genetics. It was chalked off as a meaningless statistical artifact. However, anticipation has now been proven to occur in a large number of important genetic disorders, including Huntington disease and myotonic dystrophy. In molecular terms, anticipation is due to the expansion of a trinucleotide repeat sequence in the DNA. This phenomenon also occurs in the fragile X syndrome, the most common inherited form of mental retardation.