Statement I: Phenylketonuria is an inborn error of metabolism and is inherited as an autosomal recessive trait. In this disorder the affected person shows mental retardation.
Statement II: In phenylketonuria, the affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine, so phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Both Statement I and Statement II are correct.
Phenylketonuria (PKU) is a Mendelian disorder caused by a single faulty gene on an autosome. The affected person cannot convert the amino acid phenylalanine into tyrosine, so phenylalanine builds up and is changed into phenylpyruvic acid and other derivatives. These collect in the brain and cause mental retardation.
Statement I: Phenylketonuria is an inborn error of metabolism and is inherited as an autosomal recessive trait. In this disorder the affected person shows mental retardation. — True. NCERT calls PKU an inborn error of metabolism inherited as an autosomal recessive trait, and the build-up of the derivatives in the brain causes mental retardation.
Statement II: In phenylketonuria, the affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine, so phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives. — True. This is the cause of the disorder as given in NCERT.
NCERT Extract:
Mendelian Disorders – “… Phenylketonuria : This inborn error of metabolism is also inherited as the autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.”