Why does a mutation in the genetic code change the structure of a haemoglobin molecule?

The mutation alters the base sequence which therefore changes the sequence of Amino Acids coded for (the primary structure). Now the sequence of Amino Acids has been changed when bonds form and folding occurs to form the secondary structure an alternate shape will be created. Thus when further folding occurs to form the tertiary structure and then the joining of the two polypeptides (the quaternary structure of the final haemoglobin molecule), it will be of a different structure.