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Down's syndrome in humans is caused by the presence of an extra copy of which chromosome?
AChromosome 18
BThe X chromosome
CThe Y chromosome
DChromosome 21
Answer & Solution
Correct answer: D. Chromosome 21
1. Down's syndrome is an aneuploid condition with one extra chromosome.
2. The extra chromosome is a third copy of chromosome 21, called Trisomy 21.
3. This adds to the usual pair, giving three copies of chromosome 21.
4. So the affected chromosome is number 21, option D.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 18 Genetics "Down's syndrome", p.277_
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