Assertion : Only a boy child could be born with a substitution of glutamic acid by valine on $6^{\text {th }}$ codon of beta-chain of haemoglobin. Reason : The gene for the above mutation is found on Y-chromosome.

Sickle cell anaemia is an autosomal recessive hereditary disorder in which the erythrocytes become sickle-shaped under oxygen deficiency as during strenuous exercise and at high altitudes. The disorder or disease is caused by the formation of an abnormal haemoglobin called haemoglobin-S. As found out by Ingram (1958), haemoglobin-S differs from normal haemoglobin-A in only one amino acid $-6^{\text {th }}$ amino acid of $\beta$-chain, glutamic acid, is replaced by valine due to substitution (transversion) of $\mathrm{T}$ by A in the second position of the triplet codon (CTC) which is changed to CAC in the $\beta$-haemoglobin gene situated on chromosome 11.