If one parent has sickle cell disease and the other is a sickle cell disease carrier, what percentage of their children may be sickle cell disease carriers?

Question

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Correct — B, 50%. Sickle cell disease is an autosomal recessive condition. A parent with the disease is SS (both copies of the β-globin gene carry the sickle allele) and a carrier — sickle cell trait — is AS. The affected parent can only pass on S; the carrier parent passes A or S with equal chance. The cross SS × AS therefore gives, on average, 50% AS children (carriers) and 50% SS children (disease). No child of this couple can be AA, i.e. completely free of the sickle allele.

Why the other options are wrong — (a) 25% is the affected (SS) share in a different cross — carrier × carrier (AS × AS), which gives 25% AA : 50% AS : 25% SS. It is not the carrier share when one parent is already SS.  (c) No two-parent cross of this type yields 75% carriers. 75% appears in AS × AS as the proportion of children who are NOT affected (AA plus AS) — a different quantity.  (d) All children would be carriers only if one parent were SS (diseased) and the other AA (fully normal). Here the second parent is AS, so half the children inherit S from both sides and have the disease rather than the trait.

Concept: Sickle cell disease comes from a single point mutation in the HBB gene on chromosome 11: glutamic acid is replaced by valine at the sixth position of the β-globin chain, producing haemoglobin S. When deoxygenated, HbS polymerises and distorts red cells into a rigid sickle shape, causing haemolytic anaemia and painful vaso-occlusive crises. Inheritance is autosomal recessive — two copies (SS) give the disease, one copy (AS) gives sickle cell trait, which is usually symptom-free.

State PCS 2026 Q23 — Science & Technology PYQ with Answer

State PCS 2026 Q23 — Science & Technology PYQ with Answer