allerySex of a human child is determined by the chromosome contributed by:
AMother only
BFather only
CBoth parents equally
DNeither parent (it is environmental)
Answer & Solution
Correct answer: B. Father only
1. Females have two X chromosomes (XX), so every egg carries an X chromosome.
2. Males have one X and one Y, so sperm carry either X or Y.
3. If a Y-carrying sperm fertilises the egg, the child is male (XY).
4. If an X-carrying sperm fertilises the egg, the child is female (XX).
5. The child's sex therefore depends entirely on which sperm fertilises the egg.
6. So the father determines the sex of the child.
_Source: syllabus Science Ch 9 'Heredity' (learncbse.in extract)_
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