Question:
Down's syndrome is most commonly caused by:
Down's syndrome is most commonly caused by:
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About 95 percent are nondisjunction, and the egg is the usual source.
Updated On: Jun 24, 2026
- Maternal nondisjunction
- Paternal nondisjunction
- Translocation
- Mosaicism
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The Correct Option is A
Solution and Explanation
Step 1: Down's syndrome is trisomy 21, meaning the patient has three copies of chromosome 21. The mechanisms that produce this extra chromosome differ in frequency.
Step 2: About 95 percent of cases result from nondisjunction, where the two copies of chromosome 21 fail to separate during meiosis, giving a gamete with an extra chromosome 21.
Step 3: This nondisjunction occurs far more often in the egg than in the sperm, mostly during meiosis I, and its frequency rises sharply with advancing maternal age. So maternal nondisjunction is the single most common cause.
Step 4: Why the others are wrong. Paternal nondisjunction accounts for only a small minority of nondisjunction cases. Robertsonian translocation (commonly 14;21) causes about 4 percent of cases and is the type that can be inherited. Mosaicism accounts for only about 1 to 2 percent.
Step 5: Therefore the most common cause is maternal nondisjunction.
Answer: Option A (Maternal nondisjunction).
Step 2: About 95 percent of cases result from nondisjunction, where the two copies of chromosome 21 fail to separate during meiosis, giving a gamete with an extra chromosome 21.
Step 3: This nondisjunction occurs far more often in the egg than in the sperm, mostly during meiosis I, and its frequency rises sharply with advancing maternal age. So maternal nondisjunction is the single most common cause.
Step 4: Why the others are wrong. Paternal nondisjunction accounts for only a small minority of nondisjunction cases. Robertsonian translocation (commonly 14;21) causes about 4 percent of cases and is the type that can be inherited. Mosaicism accounts for only about 1 to 2 percent.
Step 5: Therefore the most common cause is maternal nondisjunction.
Answer: Option A (Maternal nondisjunction).
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