In a cross between a male and female, both heterozygous for sickle cell anaemia gene, what percentage of the progeny will be diseased?
Answer: (A) 25%. Answer: (A) 25% Solution: Sickle-cell anaemia is an autosomal recessive disorder; only homozygous HbSHbS individuals show the disease.
- A.25%✓
- B.100%
- C.50%
- D.75%
Correct Answer
(A) 25%
Solution & Explanation
Answer: (A) 25% Solution: Sickle-cell anaemia is an autosomal recessive disorder; only homozygous HbSHbS individuals show the disease. A cross between two heterozygous (HbAHbS) carriers gives genotypes in a 1:2:1 ratio, i.e., 1 HbAHbA : 2 HbAHbS : 1 HbSHbS. Only the 1/4 (25%) homozygous HbSHbS progeny are diseased. NCERT Reference: NCERT Class XII, Ch 4, p.74, lines 021-024: "The disease is controlled by a single pair of allele, HbA and HbS. Out of the three possible genotypes only homozygous individuals for HbS (HbSHbS) show the diseased phenotype. Heterozygous (HbAHbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50 per cent probability of transmission of the mutant gene to the progeny"