alleryDown's syndrome is caused by:
AMonosomy of chromosome 21
BAn extra Y chromosome
CLoss of an X chromosome
DTrisomy of chromosome 21 (an extra copy → 47 chromosomes)
Answer & Solution
Correct answer: D. Trisomy of chromosome 21 (an extra copy → 47 chromosomes)
**Down's syndrome (Langdon Down, 1866) = trisomy 21** — 47 chromosomes due to non-disjunction.
**Phenotype:** short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic crease, retarded physical/mental development.
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