Which among the following is a mode of inheritance of sickle-cell anaemia?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Autosomal recessive.
A disorder is autosomal when its gene is on a body chromosome (not X or Y), and recessive when it shows only with two copies of the faulty allele. Sickle-cell anaemia is controlled by the alleles HbA and HbS on an autosome. Only HbSHbS persons are ill; HbAHbS persons are healthy carriers.
Option (1) Autosomal dominant. — Incorrect. In an autosomal dominant disorder one faulty allele is enough to cause disease. Here the heterozygote stays healthy, so the trait is not dominant. The book’s example of an autosomal dominant trait is myotonic dystrophy.
Option (2) Sex linked recessive. — Incorrect. Sex-linked recessive disorders, such as haemophilia and colour blindness, have their gene on the X chromosome. The sickle-cell gene is on an autosome.
Option (3) Sex linked dominant. — Incorrect. A sex-linked dominant trait would also be on the X chromosome and would show with one copy. Neither part fits sickle-cell anaemia.
Option (4) Autosomal recessive. — Correct. The gene is on an autosome, and the disease shows only in the homozygous HbSHbS person. Two carrier parents can therefore have an affected child.
NCERT Extract:
Mendelian Disorders – “… Sickle-cell anaemia : This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). 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.”