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Identical twins differ from non-identical (fraternal) twins in their origin because identical twins arise when:
ATwo eggs are each fertilised by a different sperm
BTwo sperms fuse before entering a single egg
COne egg is fertilised by two separate sperms
DA single fertilised egg divides into two foetuses
Answer & Solution
Correct answer: D. A single fertilised egg divides into two foetuses
1. Non-identical (fraternal) twins form when ovaries release two eggs, each fertilised by a different sperm.
2. Identical twins form when a single egg is fertilised and then divides into two foetuses.
3. So the single fertilised egg splitting describes identical twins.
4. Two eggs with two sperms is the fraternal case, so it is wrong.
5. One egg with two sperms or two sperms fusing first are not the described mechanisms, so they are ruled out.
6. Hence a single egg splitting in two gives identical twins, the only matching mechanism.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 17 Reproduction in Plants and Animals "Parturition (Child Birth)", p.260_
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