Q1. [4]
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types – 'X' chromosomes and 'Y' chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
Answer the following sub-parts based on the given passage about chromosomes and sex determination.
- (a) What are chromosomes?
- (b) Why is the pair of sex chromosomes in human males called mismatched pair?
- (c) Show with the help of a flow chart that the statistical probability of getting a boy or a girl is 50 : 50.
Previously asked in CBSE board exam
2025 31/5/1 Q38
Generated by claude-sonnet-4-6 · 2026-06-15 06:53 · grounding stimulus
Model Answer
(a) Chromosomes are thread-like structures present in the nucleus of a cell that carry genetic information (genes) from parents to offspring.
(b) In human males, the sex chromosomes are X and Y. Since X and Y chromosomes are of different sizes and shapes (not perfectly paired), they are called a mismatched pair.
(c) Flow chart showing 50:50 probability:
```
Parents: Mother (XX) × Father (XY)
↓ ↓
Gametes: X X X Y
↓
Zygotes: XX XX XY XY
(Girl)(Girl)(Boy)(Boy)
```
∴ Probability of girl : boy = 50% : 50%
Source: Heredity and Evolution, Sex Determination
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Explanation
- (a) is 1 mark — one line definition is enough.
- (b) is 1 mark — key term is "mismatched" because X and Y differ in size/shape.
- (c) is 2 marks — examiners expect a proper cross/flow chart showing gametes from both parents and resulting combinations. Writing all four zygote possibilities (XX, XX, XY, XY) and stating the 50:50 ratio is essential for full marks.
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