Down's syndrome is an autosomal aneuploid condition caused by:
Answer and explanation
Correct answer: B. Pericentric inversion of the heterochromatic region on the long arm of chromosome 9
- Step 1: Down's syndrome was first clinically described by Langdon Down in 1866.
- Step 2: Cytogenetic analysis shows an extra copy of chromosome 21 ( or ), arising predominantly from maternal meiotic non-disjunction during oogenesis.
- Step 3: Clinical signs include short stature, round face with flat nasal bridge, protruding furrowed tongue, broad palm with single simian crease, and congenital heart defects.
Answer
Trisomy of chromosome 21 caused by meiotic non-disjunction.
Why not the others: Monosomy of chromosome 21 is lethal in utero; Deletion of short arm of chromosome 5 causes Cri-du-chat syndrome; Inversion of chromosome 9 is a benign structural variant.
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