Final answer:
A non-trinucleotide repeat expansion in the coding region of a gene would most likely result in a frameshift mutation, which disrupts the reading frame and leads to a nonfunctional protein.
Step-by-step explanation:
If non-trinucleotide repeat expansions were present in the coding region of a gene, the type of mutation that would be most likely is a frameshift mutation. This is because if the repeat expansion is not in multiples of three (which is the size of a codon), the insertion or deletion of nucleotides would shift the reading frame of the gene. Consequently, this would alter every amino acid sequence downstream from the mutation, leading to the production of a nonfunctional protein. Trinucleotide repeats maintain the reading frame since they are in multiples of three, while expansions that are not would disrupt this frame.
A missense mutation would result in a change of a single amino acid, and a nonsense mutation would introduce a premature stop codon. Both types of mutations can significantly affect protein function but do not usually cause a reading frame shift. A silent mutation does not change the amino acid sequence of the protein at all. Therefore, a frameshift mutation is the most severely disruptive mutation type when repeat expansions are not in multiples of three.