Think about the dna coding sequence of a gene. if an a were swapped for a t, what kind of mutation could it cause and why? think about the dna coding sequence of a gene. if an a were swapped for a t, what kind of mutation could it cause and why? it could cause a silent, missense, or nonsense mutation because those are the types that can be caused by a nucleotide-pair substitution like this one. it could cause a silent mutation because a and t are complementary to each other so it is not really a substitution mutation. it could cause a nonsense mutation because the sequence would no longer be the same, so the protein would be shorter and non-functional. it could cause a frameshift nonsense or frameshift missense mutation because it would change the reading frame of the codon triplet.