Read the paragraph and answer the question.
Broadly, genetic disorders may be grouped into two categories - Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The trait may also be linked to the sex chromosome as in case of haemophilia. It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny.
What is the probability of having a haemophilic child to a carrier mother and a normal father ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 25%
Haemophilia is X-linked recessive. Write the faulty allele as Xh. Carrier mother XhX × normal father XY.
| Egg \ Sperm | X | Y |
|---|---|---|
| Xh | XhX (carrier daughter) | XhY (haemophilic son) |
| X | XX (normal daughter) | XY (normal son) |
Of the four equally likely children, one is a haemophilic son: 1/4 = 25%. (Among sons alone it would be 50%.)
Option (1) 50% — Incorrect. 50% is the chance for a son; for any child it is half that.
Option (2) 75% — Incorrect. No daughter can be haemophilic here, since the father's X is normal.
Option (3) 100% — Incorrect. Half the eggs carry the normal X.
Option (4) 25% — Correct. The mother is a carrier (XhX) and the father is normal (XY). Their children are XX, XY, XhX and XhY in equal numbers; only XhY is haemophilic, so the chance is 1 in 4 = 25 per cent.
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.