Explanation

Sickle cell anaemia is a genetic disease reported from Negroes due to molecular mutation (mis-sense mutation) of gene $Hb^{A}$on chromosome 11, which produces the $\beta$-chain of mature haemoglobin. The mutated gene $HB^{s}$ produces sickle cell haemoglobin. The sixth amino acid, in $\beta$-chain of normal haemoglobin is glutamic acid which is replaced by valine in sickle cell haemoglobin. The sequence of DNA in mutated $\beta$-globin chain is CAC/GAG which on transcription results in codon GUG which produces valine in place of glutamic acid.