(a) Sickle cell anaemia in humans is a result of point mutation. Explain.
(b) Write the genotypes of both the parents who have produced a sickle cell anaemic offspring.

Explanation

(a) In sickle cell anaemia, due to point mutation there is a subsitution of a single nitrogen base at the sixth codon of the B globin chain of haemoglobin.
`underset("Glutamic acid ")(GAG )overset"Mutation"rarr underset("Valine")( GUG)`
(b) The genotype of both the parents would be `Hb^A Hb^s "and " Hb^A Hb^S` . The genotypes of both the parents who have produced a sickle Celled anaemic offspring it that the out of three possible genotypes only homozygous individuals for `Hb^S (Hb^S Hb^S)` show the diseased phenotype. Heterozygous `(Hb^A Hb^s)` individuals are apparently unaffected but they are carrier of the disease as there is 50 percent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cells trait.