Question

Pleiotropic effect is found in

• A. Scurvy
• B. Night blindness
• C. Haemophilia
• D. Sickle cell anaemia

Answer 1

The correct option is D Sickle cell anaemia

Human skin colour is a polygenic trait governed by more than one gene where dominant allele of each gene express only a part of trait and the full trait is expressed only in the presence of dominant alleles of all multiple genes. It controls skin pigmentation only, no other phenotypic trait is affected by these genes.

Night blindness is impaired ability to see in dim light; it is an autosomal disorder. Haemophilia is an inherited bleeding disorder characterized by lacking or low levels of clotting factors resulting in the inability or impaired ability of blood clotting.

When a single gene mutation affects more than one phenotypic traits, it is called as pleiotropy. Sickle cell anaemia is caused by a mutation in Hb${}^b$ gene, that codes the $\beta$-chain, causes replacement of A by T at the 17th nucleotide of the Hbb gene changes the codon GAG (glutamic acid) to GTG (which encodes valine). The mutated allele Hb${}^s$ encodes the abnormal haemoglobin molecules which stick to one another and cause stiffness and sickle shape of red blood cells which in turn has two major phenotypic effects. The sickled cells are destroyed by the liver, causing anemia which is manifested as physical weakness, slow development, and hypertrophy of the bone marrow and the “tower skull”. The sickled cells interfere with capillary blood flow, stick together and damage every major organ which in turn causes pain, heart failure, rheumatism, and other ill effects. Thus, sickle cell anemia shows pleiotropy; the correct answer is D.