Given below are two statements:
Statement I: Phenylketonuria is caused by mutation in the gene that codes for the enzyme phenylalanine hydroxylase.
Statement II: In phenylketonuria, a single gene mutation shows multiple phenotypic expression, such as mental retardation and a reduction in hair and skin pigmentation.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both Statement I and Statement II are true
Pleiotropy is the case where a single gene shows more than one phenotypic expression. In most cases the gene acts on a metabolic pathway, and that pathway affects several traits. Phenylketonuria in humans is the NCERT example.
Statement I: Phenylketonuria is caused by mutation in the gene that codes for the enzyme phenylalanine hydroxylase. — True. Phenylketonuria is caused by a mutation in the gene for phenylalanine hydroxylase, the enzyme that converts the amino acid phenylalanine into tyrosine. It is a single gene mutation.
Statement II: In phenylketonuria, a single gene mutation shows multiple phenotypic expression, such as mental retardation and a reduction in hair and skin pigmentation. — True. This one faulty gene shows its effect in more than one trait: mental retardation and a reduction in hair and skin pigmentation. That is why phenylketonuria is given as an example of pleiotropy.
NCERT Extract:
Pleiotropy – “We have so far seen the effect of a gene on a single phenotype or trait. There are however instances where a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.”