Biology · Principles of Inheritance and Variation · NEET
Phenylketonuria is an AUTOSOMAL RECESSIVE trait. NCERT states clearly: 'This inborn error of metabolism is also inherited as the autosomal recessive trait.' Autosomal means the gene is NOT on the sex chromosome (X or Y), so boys and girls are affected equally. Recessive means a person shows the disease only when BOTH copies of the gene are faulty (homozygous). A person with one faulty copy is a healthy carrier. NEET loves to give a wrong option saying 'Phenylketonuria - autosomal dominant' (NEET 2018/2020) - that option is FALSE.
The missing enzyme is phenylalanine hydroxylase. Normally this enzyme converts the amino acid phenylalanine into another amino acid called tyrosine. In PKU the gene coding for this enzyme is mutated, so the enzyme is absent or not working. Remember: phenylalanine hydroxylase -> phenylalanine becomes tyrosine. No enzyme means this step is blocked.
Because phenylalanine cannot be changed to tyrosine, phenylalanine accumulates in the blood. The extra phenylalanine is then converted into phenylpyruvic acid and other derivatives. These build up in the brain and cause mental retardation. They are also thrown out in urine because the kidney absorbs them poorly. So the two key products to remember are phenylalanine (piles up) and phenylpyruvic acid (harmful derivative).
PKU is a MENDELIAN disorder. Mendelian disorders are caused by mutation (change) in a SINGLE gene, and they pass to children by the same rules Mendel found. NCERT lists Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria and Thalassemia as common Mendelian disorders. Chromosomal disorders (like Down's syndrome, Klinefelter's, Turner's) are different - they come from a whole extra or missing chromosome (aneuploidy), not a single gene.
Pleiotropy means ONE gene affects MANY traits. In PKU, a single mutated gene (for phenylalanine hydroxylase) causes several different effects at once: mental retardation, and a reduction in hair and skin pigmentation. Skin and hair get lighter because tyrosine (needed to make the pigment melanin) is not formed. Because one gene changes multiple phenotypes, NCERT uses PKU as the standard example of a pleiotropic gene. NEET matching questions (2016, 2023, 2026) repeatedly match 'Pleiotropy' with 'Phenylketonuria'.
Both are autosomal recessive, but the defect is different. In PKU an ENZYME (phenylalanine hydroxylase) is missing, so an amino acid (phenylalanine) piles up - it is a metabolic disorder. In sickle cell anaemia a POINT mutation changes glutamic acid to valine at the 6th position of the beta-globin chain of haemoglobin. So if a question mentions 'Glu to Val in beta-globin', the answer is sickle cell, NOT phenylketonuria.
Select the correct match.
Pick out the correct statements: (i) Hemophilia is a sex-linked recessive disease (ii) Down's syndrome is due to aneuploidy (iii) Phenylketonuria is an autosomal recessive gene disorder (iv) Sickle cell anaemia is an autosomal recessive gene disorder
Match List I with List II: A. Incomplete dominance B. Co-dominance C. Pleiotropy D. Polygenic inheritance I. Human skin colour II. Inheritance of flower colour in Antirrhinum sp. III. Phenylketonuria disease in humans IV. ABO blood groups
Try the real previous-year questions from this chapter — each with the answer and a full solution.
PKU stands for Phenylketonuria. It is called this because phenyl-ketone bodies (like phenylpyruvic acid) are formed and appear in the urine.
Phenylalanine cannot be converted into tyrosine because the enzyme phenylalanine hydroxylase is missing. So phenylalanine builds up in the blood and brain.
Tyrosine is the raw material for melanin, the pigment in hair and skin. Since PKU blocks the making of tyrosine, less melanin is made, so hair and skin are lighter. This is also why PKU is an example of pleiotropy - one gene affects both brain and pigmentation.
Yes. NCERT calls it an 'inborn error of metabolism'. It is a mistake in a metabolic (chemical) pathway caused by a single faulty gene that codes for an enzyme.
No. PKU is recessive, so a child needs TWO faulty copies (one from each parent) to be affected. If only one parent carries it, the child can be a carrier but will not show the disease.
