Sickle-cell anaemia is

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(B) $Sickle-cell$ anaemia is an autosomal recessive genetic disorder in which erythrocytes become sickle-shaped.It is caused by the formation of an ab

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caused by a change in a single base pair of $DNA$

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(B) $Sickle-cell$ anaemia is an autosomal recessive genetic disorder in which erythrocytes become sickle-shaped.It is caused by the formation of an abnormal haemoglobin called $Haemoglobin-S$ $(HbS)$.$Haemoglobin-S$ is formed when the $6^{th}$ amino acid of the $\beta$-globin chain, $i.e.$, glutamic acid, is replaced by valine due to a point mutation.This occurs due to a single nucleotide change ($GAG \rightarrow GTG$ at the $DNA$ level, corresponding to $A \rightarrow T$ substitution) in the $\beta$-globin gene.In the normal $\beta$-globin gene, the $DNA$ sequence is $CCTGAGGAG$, while in $sickle-cell$ anaemia, the sequence is $CCTGTGGAG$.

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