Q1. [4]
Sex of an individual is determined by different factors in various species. Some animals rely entirely on the environmental cues, while in some other animals the individuals can change their sex during their life time indicating that sex of some species is not genetically determined. However, in human beings, the sex of an individual is largely determined genetically.
Answer the sub-questions based on the case below. Part (a) and (b) are compulsory; an internal choice has been provided in Part (c).
- (a) In what way are the sex chromosomes 'X' and 'Y' different in size ? Name the mismatched pair of sex chromosome in humans.
- (b) Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male / female) perfect pair / pairs of sex chromosomes are present ?
- (c) Citing two examples, justify the statement "Sex of an individual is not always determined genetically".
Previously asked in CBSE board exam
2022 31/4/1 Q15 (main)
Generated by claude-sonnet-4-6 · 2026-06-15 06:54 · grounding stimulus
Model Answer
(a) The 'Y' chromosome is shorter/smaller in size compared to the 'X' chromosome. The mismatched pair of sex chromosomes in humans is XY (found in males).
(b) There is 1 pair of sex chromosomes in human beings. In the female (XX), a perfect/matched pair of sex chromosomes is present.
(c) Two examples showing sex is not always genetically determined:
- Snails – some individuals can change their sex during their lifetime depending on environmental or social conditions.
- Reptiles (e.g., lizards/turtles) – the temperature of the environment during egg incubation determines the sex of the offspring, not genes.
Source: Heredity and Evolution, Sex Determination
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Explanation
- (a) Examiners expect mention of relative size difference (Y smaller than X) and identification of XY as the mismatched pair.
- (b) Key facts: only 1 pair of sex chromosomes; females have XX (perfect/matched pair), males have XY.
- (c) Two distinct examples are required. The passage directly states that "some animals rely on environmental cues" and "some can change their sex during lifetime" — link your examples clearly to these two points. Snails (sex change) and reptiles (temperature-based determination) are standard NCERT examples. Award is typically 1 mark per valid example with brief justification.
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