Final answer:
In myotonic dystrophy, the increasing severity in successive generations is known as anticipation, a phenomenon observed in certain genetic disorders.
Step-by-step explanation:
In myotonic dystrophy, the phenomenon where the severity of the disease increases with each succeeding generation is known as (A) anticipation. This is a genetic effect that can occur with certain genetic disorders, leading to earlier onset and increased severity in successive generations. Disorders such as Huntington's disease also display this pattern. Anticipation is distinct from other concepts like incomplete penetrance, genomic imprinting, and variable expressivity, which describe different aspects of how genetic traits can manifest.