What chromosomal abnormality causes Down syndrome (mongolism) in humans?
Answer and explanation
Correct answer: D. trisomy of autosome 21 caused by non-disjunction during meiotic gametogenesis
- Step 1: Down syndrome results from having three copies of chromosome 21 (trisomy 21, total 47 chromosomes). Step 2. It occurs when homologous chromosomes 21 fail to separate during meiotic anaphase (non-disjunction). Answer: trisomy of autosome 21 caused by non-disjunction during meiotic gametogenesis. Why not the others: Monosomy X causes Turner syndrome; XXY causes Klinefelter syndrome; triploidy is lethal.
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