Man who is sufferer of the disease marries a woman who is a carrier o…

Read the given passage and answer the question given below.

The sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this in an affected individual a simple cut will result in non-stop bleeding. The heterozygous female (carrier) for the disease may transmit it to sons. The possibility of a female becoming a sufferer of disease is extremely rare because mother of such a female has to be at least carrier and the father should be sufferer. The family pedigree of Queen Victoria shows a number of suffering descendents as she was a carrier of the disease.

Man who is sufferer of the disease marries a woman who is a carrier of this disease. What percentage of children born as carrier of this disease?

Answer & explanation

Correct answer: option 3

The correct answer is Option (3) → 25

The disease in the passage (haemophilia) is X-linked recessive. Let X^h stand for the X chromosome carrying the gene for the disease.

Father (sufferer) = X^hY; his sperms carry X^h or Y. Mother (carrier) = X^hX; her eggs carry X^h or X.

The four equally likely children are: X^hX^h, a daughter who is a sufferer; X^hX, a daughter who is a carrier; X^hY, a son who is a sufferer; XY, a normal son.

A carrier has one X^h and one normal X and does not show the disease. Only X^hX is a carrier: 1 of 4 children = 25 per cent. Sons have a single X, so they are either sufferers or normal, never carriers.

Option (1) 100 — Incorrect. 100 per cent would need every child to be X^hX; sons can never be carriers.

Option (2) 50 — Incorrect. 50 per cent is the share among the daughters only, not among all the children.

Option (3) 25 — Correct. In this sex-linked recessive disorder, the affected man (X^*Y) gives X^* to every daughter and Y to every son; the carrier woman (X^*X) gives X^* or X. The children are X^*X^*, X^*X (carrier), X^*Y and XY, so 1 in 4, or 25%, are carriers.

Option (4) zero — Incorrect. The daughter X^hX is a carrier, so the answer cannot be zero.

Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.