Genetic inheritance and variations
Read the following passage carefully and answer the given questions.
While studying the pattern of inheritance in pea plants of contrasting characters, Mendel proposed the principles of inheritance, which are today referred to as ‘Mendel's Laws of Inheritance’. He proposed that some characters are dominant over others. The Punnett square can be effectively used to understand the independent segregation of the two pairs of genes during meiosis and the production of eggs and pollen in the plants. 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. In Sickle cell anaemia the mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure. In thalassemia the formation of abnormal haemoglobin molecules resulting into anaemia which is characteristic of the disease. Thalassemia can be classified according to the affected chain of haemoglobin molecule.
Identify the disease, where affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Phenylketonuria
(1) Thalassemia — Incorrect. In thalassemia too few globin chains are made, so haemoglobin is abnormal. No enzyme of phenylalanine is involved.
(2) Phenylketonuria — Correct. This is an inborn error of metabolism, inherited as an autosomal recessive trait. The missing enzyme cannot convert phenylalanine into tyrosine. Phenylalanine builds up and is changed into phenylpyruvic acid. When these collect in the brain, they cause mental retardation.
(3) Haemophilia — Incorrect. Haemophilia is a sex-linked disorder. A protein needed for blood clotting is affected, so even a small cut keeps bleeding.
(4) Sickle-cell anaemia — Incorrect. Here valine replaces glutamic acid in the β globin chain, and the red blood cells become sickle-shaped.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Mendelian Disorders – …
