A couple, both of whom are carriers for the sickle-cell trait, wish to know the risk of their future child being affected by the disease. What is the percentage probability that their child will be affected, and what is the exact molecular defect the affected child will possess?

Step-by-step solution

Sickle-cell anaemia is an autosomal recessive genetic disorder. If both parents are carriers (heterozygous, Hb^A Hb^S ), the genetic cross is Hb^A Hb^S Hb^A Hb^S . The possible offspring genotypes are Hb^A Hb^A (25% normal), Hb^A Hb^S (50% carriers), and Hb^S Hb^S (25% affected). Thus, the probability of having an affected child is 25%. The molecular defect in an affected individual ( Hb^S Hb^S ) is the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta-globin chain of the haemogl