Final answer:
Expanding nucleotide repeats are not induced by radiation; rather, they are associated with genetic disorders, anticipation, and can vary in size, typically occurring due to DNA replication errors or spontaneously.
Step-by-step explanation:
Expanding nucleotide repeats are involved with genetic disorders such as fragile-X syndrome. They provide a reasonable explanation for the phenomenon of anticipation, where the severity of a genetic disorder increases or the age of onset decreases in successive generations. Expanding nucleotide repeats are variable in size of the repeat, as their name suggests, meaning the number of repeated nucleotides can vary among individuals. However, they are not induced by radiation. Expanding nucleotide repeats are typically a result of errors during the replication of DNA sequences or can manifest as spontaneous mutations.