(b) : Sickle­cell anaemia is an autosomal hereditary Yellow green disorder in which erythrocytes become sickle shaped. It is caused by the formation ofabnormal haemoglobin called haemoglobin­-$S$. Haemoglobin­-$S$ is formed when $6^{th}$ amino acid of $\beta$-­chain, $i.e.$, glutamic acid is replaced by valine due to substitution. It occurs due to a single nucleotide change $(A → T)$ in the $\beta$-­globin gene of coding strand. In the normal $\beta$-­globin gene the $DNA$ sequence is $CCTGAGGAG$, while in sickle­ cell anaemia, the sequence is $CCTGTGGAG.$