Final answer:
The parent of origin effect in trinucleotide expansion diseases influences the severity and onset of disorders based on whether the expanded allele is inherited from the mother or father, with some expansions being more significant during maternal transmission.
Step-by-step explanation:
The parent of origin effect for trinucleotide expansion disease refers to the phenomenon whereby the severity and age of onset of certain genetic disorders can be influenced by the parental origin of the allele with the trinucleotide repeat expansion. These diseases, such as Huntington's disease and myotonic dystrophy, involve the repetition of three nucleotide bases beyond the normal number, leading to the production of abnormal proteins or toxic RNA, which in turn causes disease symptoms. Expansions may occur during gametogenesis and can be more severe when transmitted by one parent compared to the other. For instance, the FMR1 gene associated with Fragile X syndrome often expands more significantly during oogenesis (in females) rather than spermatogenesis (in males). Consequently, a child is more likely to inherit a larger expansion, and thus a more severe form of the disease, if the mutated allele is from the mother rather than the father.