Q1. [4]
The mechanism by which the sex of an individual is determined is called sex-determination. In human beings, sex of a newborn is genetically determined, whereas in some others it is not. There are 46 (23 pairs) chromosomes in human beings. Out of these, 44 (22 pairs) control the body characters and 2 (one pair) are known as sex chromosomes. The sex chromosomes are of two types — X chromosome and Y chromosome. At the time of fertilisation, depending upon which type of male gamete fuses with the female gamete, the sex of the newborn child is decided.
Read the passage and answer the following questions:
- (a) Why is a pair of sex chromosomes in human beings called a mismatched pair in terms of type and size ? [1]
- (b) Out of male or female, which of them has a perfect pair of sex chromosomes ? In case of a perfect pair, will the gametes produced be of the same kind or of a different kind ? [1]
- (c) Choose one of the following alternatives: [2]
Previously asked in CBSE board exam
2022 31/1/1 Q14
Generated by claude-sonnet-4-6 · 2026-06-15 06:53 · grounding stimulus
Model Answer
(a) In males, the pair of sex chromosomes consists of one X chromosome and one Y chromosome, which differ in both type and size (Y is much smaller than X). Hence, it is called a mismatched pair.
(b) Female has a perfect pair of sex chromosomes (XX). Since both chromosomes are identical, all eggs produced will carry one X chromosome — so gametes will be of the same kind.
(c) (Note: The alternatives for part (c) are not printed in the question. Based on the passage, the standard answer is:)
The sex of a child is determined by the father, not the mother. All eggs from the mother carry X chromosome. The father produces two types of sperms — 50% carrying X and 50% carrying Y. If a sperm with X fertilises the egg → girl (XX); if a sperm with Y fertilises the egg → boy (XY). Thus, the father's sperm determines the sex of the child.
Source: Chapter 8 – How do Organisms Reproduce?, Sex Determination
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Explanation
- For (a), the key point is that X and Y differ in both type and size — examiners expect both words.
- For (b), remember females are XX (homogametic) → one type of gamete; males are XY (heterogametic) → two types. This directly follows from the passage.
- For (c), since the alternatives were missing, the standard NCERT-based answer about father determining sex is used. If alternatives are printed, choose the one stating the father/male parent is responsible for the child's sex.
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