Pyruvate kinase deficiency is an inherited metabolic disorder in humans caused by mutations in the $PKLR$ gene. This gene encodes the enzyme pyruvate kinase, which is vital for maintaining ATP production via glycolysis in red blood cells.

Define the term gene mutation and explain how a mutation in the $PKLR$ gene can:

• have no effect on the primary structure of the encoded pyruvate kinase polypeptide
• lead to a non-functional enzyme that reduces the survival rate of red blood cells.