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.
Heterozygous individuals which are carriers of the disease Sickle-cell anaemia can be represented by the genotype:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → HbAHbS
Sickle-cell anaemia is an autosomal recessive disorder. It is controlled by one pair of alleles: HbA (normal) and HbS (mutant).
(1) HbSHbS — Incorrect. This person has two mutant alleles and shows the disease.
(2) HbAHbS — Correct. This heterozygous person looks healthy but is a carrier. There is a 50 per cent chance of passing HbS to each child.
(3) HbAHbA — Incorrect. This person has two normal alleles and is neither sick nor a carrier.
(4) HbSHbC — Incorrect. This is not the carrier genotype. A carrier has one normal allele (HbA) and one mutant allele (HbS).
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 – …
