Physiology & Biochemistry · single question
What is the most common chromosomal anomaly / defect?
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Correct answer: B. Trisomy 21 (Down syndrome)
Trisomy 21 (Down syndrome) is the MOST COMMON chromosomal abnormality in live births, occurring in approximately 1 in 700 live births. It is caused by nondisjunction during meiosis, resulting in three copies of chromosome 21. Dental features: hypodontia, delayed eruption, macroglossia, bruxism, increased periodontal disease. Trisomy 18 and 13 are more severe but much less common. Turner syndrome affects females only.
- A. CORRECT. Trisomy 21 (Down syndrome) is the most common chromosomal abnormality in live-born children — approximately 1:700 live births.
- B. Incorrect. Trisomy 18 (Edwards syndrome) occurs in approximately 1:5,000 live births — much less common than Trisomy 21.
- C. Incorrect. Monosomy X (Turner syndrome, 45,X) affects 1:2,500 females — less common than Trisomy 21.
- D. Incorrect. Trisomy 13 (Patau syndrome) occurs in about 1:10,000–20,000 live births — much rarer than Trisomy 21.
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