Question

Assertion :Sickle-cell anemia is a genetically determined disorder affecting many new born babies. Reason: It is caused by heterozygosity for allele Hb${}^s$ producing a single amino acid substitution in the $\alpha$-chain of the normal haemoglobin molecule determined by allele Hb${}^A$.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
• C. Assertion is correct but Reason is incorrect.
• D. Both Assertion and Reason are incorrect.

Answer 1

The correct option is C Assertion is correct but Reason is incorrect.

Sickle-cell anemia is inherited from parents to offspring and hence is a genetic trait for which the newborns are screened. It is an autosomal recessive disorder which means both copies of the gene in each cell must have mutations to express the symptoms. The heterozygous individuals with one copy of affected allele and one wild-type allele will not express the symptoms but will serve as a carrier of the disease to the next generation. The homozygous recessive ($H{b}^s$$H{b}^s$) are not viable and hence the disease is transmitted by heterozygous individuals. Haemoglobin has four protein subunits, two $\alpha$-globin and two $\beta$-globin. The $H{b}^b$ gene codes the $\beta$-chain. A missense mutation, replacement of A by T at the 17${}^t$${}^h$ nucleotide of the $H{b}^b$ gene changes the codon GAG (glutamic acid) to GTG (which encodes valine). The mutated allele $H{b}^s$ encodes the abnormal haemoglobin molecules which stick to one another and cause stiffness and sickle shape of red blood cells. The sickle-shaped RBC block and damage the vital organs and tissue. Thus, the correct answer is option C.