RNA and Protein Evolution

a) A silent, or synonymous, mutation has occurred in the codon for residue 2 (CAC to CAT).
b) A missense, or nonsynonymous, mutation has occurred in the codon for residue 6 (GAG to GTG). This is the sickle-cell mutation.
c) A nonsense mutation has introduced a stop signal after the codon for residue 7 (AAG to TAG), terminating the chain prematurely.
d) A frameshift mutation has occurred by deletion of a single T residue. The rest of the chain, with a completely altered sequence, will continue to be produced until a stop signal is encountered in the new frame. Both (c) and (d) would result in b-thalassemia.