Suppose a person has a mutation in their dna and the first triplet for the insulin gene reads TAT. The normal gene reads TAG. What amino acid does the mutant dna and the normal dna code for and will the person with this mutation be diabetic?

The right answer is The TAT codon (UAU for RNA) codes for tyrosine (an amino acid) whereas the TAG (UAG for RNA) codon is a stop codon.

Yes, the person will become diabetic because the subject has been mutated nonsense.

So, there was a nonsense mutation (conversion of an amino acid codon to a stop codon). Therefore, the synthesis of insulin will be impossible because the first codon is a premature stop codon, so there will be no insulin and a development of insulin-dependent diabetes.