Ans: A non-essential amino acid is one which is synthesized in the body. Tyrosine is synthesized in the body from Phenylalanine, which is itself an essential amino acid. So, if a person does not take Phenylalanine in diet, his body will be unable to synthesize tyrosine. So, most books call tyrosine that it is a mis-classification.
Secondly, in patients suffering from Phenylketonuria (PKU), tyrosine becomes essential; because the reaction that converts Phe to Tyr is impaired in their body.
Due to the above two facts, Tyrosine should be placed under the category: semi-essential.
