Read the following passage carefully and answer the given questions.
Mendelian Disorders
Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive. By pedigree analysis one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome as in case of haemophilia. It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny.
Phenylketonuria affected individual lacks an enzyme that converts the amino acid phenylalanine into -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Tyrosine
Phenylketonuria is an inborn error of metabolism, inherited as an autosomal recessive trait. The affected person lacks the enzyme that converts the amino acid phenylalanine into tyrosine. So phenylalanine builds up and is changed into phenylpyruvic acid and other derivatives. Their build-up in the brain causes mental retardation, and they are also passed out in the urine.
(1) Threonine — Incorrect. Threonine is a different amino acid, not formed from phenylalanine.
(2) Tryptophan — Incorrect. Tryptophan is another aromatic amino acid, but it is not made from phenylalanine.
(3) Tyrosine — Correct. Phenylalanine is normally converted into tyrosine; this step is blocked in phenylketonuria.
(4) Serine — Incorrect. Serine has no link with this pathway.
Note: This passage is taken from NCERT as it is. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Mendelian Disorders – …
