Question

Recognise the figure and find out the correct matching.

• A. $c-\text{Glu},d-\text{Val},a-\text{normal~Hb~(A)~gene},b-\text{sickle~cell~Hb~(S)~gene}$
• B. $c-\text{Glu},d-\text{Val},b-\text{normal~Hb~(A)~gene},a-\text{sickle~cell~Hb~(S)~gene}$
• C. $d-\text{Glu},c-\text{Val},a-\text{normal~Hb~(A)~gene},b-\text{sickle~cell~Hb~(S)~gene}$
• D. $c-\text{Glu},d-\text{Val},b-\text{normal~Hb~(A)~gene},a-\text{sickle~cell~Hb~(S)~gene}$

Answer 1

The correct option is A $c-\text{Glu},d-\text{Val},a-\text{normal~Hb~(A)~gene},b-\text{sickle~cell~Hb~(S)~gene}$

Sickle cell anemia is an autosomal linked recessive trait that can be transmitted from parents to the offspring when both the parents are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele, $H{b}^A$ and $H{b}^S$. Out of the three possible genotypes only homozygous individuals for $H{b}^S\left(H{b}^{s}H{b}^s\right)$ show the diseased phenotype. Heterozygous $\left(H{b}^{A}H{b}^S\right)$ individuals appear apparently unaffected but they are carrier of the disease as there is $50$ per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait. The defect is caused by the substitution of glutamic acid $\left(Glu\right)$ by valine $\left(Val\right)$ at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from $GAG$ to $GUG$. 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.