How many of the given statements are incorrect?

I. Thalassemia is caused due to mutation in only chromosome 21
II. Mutation on either chromosome 11 or 16 may cause sickle cell anaemia
III. At the 6${}^{\mathrm{th}}$ position beta globin chain, glycine is replaced by glutamic acid in case of sickle cell anaemia
IV. Phenylketonuria is caused due to lack of enzyme phenylalanine tyrosinase

The correct option is A 4
Thalassemia is caused due to mutation in the genes encoding the different globin chains of haemoglobin. Haemoglobin is made up of alpha and beta-globin chains. The gene encoding the alpha-chain is located on chromosome 16. The chromosome 11 contains the gene encoding beta-chain. Mutation in the gene of chromosome 16 leads to alpha thalassemia and mutation in gene on chromosome 11 leads to beta thalassemia. Thalassemia could be caused due to mutation in either chromosome 11 or 16 or both of them.

Sickle cell anemia is caused due to mutation in chromosome 11 only. It is related to the mutation in the gene encoding the beta-globin chain of haemoglobin. But this mutation is different from the one caused in thalassemia. Here, the amino acid glutamic acid at the 6${}^{\mathrm{th}}$ position of the beta-globin chain is substituted by valine.

Phenylketonuria is an inborn, autosomal, recessive metabolic disorder in which the homozygous recessive individuals lack the enzyme phenylalanine hydroxylase that is needed to change phenylalanine to tyrosine.

Hence, all statements are incorrect.