The codon for the sixth amino acid is GAG .the sixth codon GAG mutates to GAA as a result of mutation 'A' and into GUG as a result of mutation 'B'. haemoglobin structure did not change as a result of mutation 'A' whereas haemoglobin structure changed because of mutation 'B' leading to sickle shaped RBC's . explain giving reasons how could mutations 'B' change the haemoglobin structure and not mutation 'A'?