Read the given paragraph and answer the question:
Humans have 23 pairs of chromosomes. 22 pairs are exactly same in both males and females; these are the autosomes. A pair of X-chromosome are present in the female, whereas the presence of an X and Y chromosome are determinant of the male characteristic.
A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes. Broadly, genetic disorder may be grouped into two categories – Mendelian disorders and Chromosomal disorders.
A haemophilic man marries a normal woman. What is the possibility that their child have haemophilia?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 0%
Haemophilia is an X-linked recessive disorder. Write the faulty allele as Xh. The man is XhY; the normal woman is XX.
| Egg \ Sperm | Xh | Y |
|---|---|---|
| X | XhX (carrier daughter) | XY (normal son) |
Every daughter gets her father's Xh but also a normal X from her mother, so she is an unaffected carrier. Every son gets his father's Y and his mother's normal X, so he is normal. None of the children has haemophilia.
Option (1) 75% — Incorrect. No child can show the disease in this cross.
Option (2) 25% — Incorrect. A son gets his X from the mother, who has no faulty allele.
Option (3) 0% — Correct. Haemophilia is X-linked recessive. The father (XhY) gives his Xh only to daughters, and the normal mother (XX) gives a normal X to every child. Daughters are carriers (XhX) and sons are normal (XY), so no child (0%) has haemophilia.
Option (4) 50% — Incorrect. Half the children (the daughters) carry the allele, but carriers do not have the disease.
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.