Question

Downs syndrome is caused by an extra copy of chromosome number $21$. What percentage of offspring produced by an affected mother and a normal father would be affected by this disorder?

• A. 0.5
• B. 0.25
• C. 1
• D. 0.75

Answer 1

The Correct Option is A

Down's syndrome is the result of trisomy in which chromosome number $21st$ contains an extra copy of chromosome $(2 A+1)$ Affected mother will produce $50 \%$ normal egg cells and rest $50 \%$ eggs are of abnormal type. In normal case

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$(n+1)$ - abnormal egg $n$-normal egg Fusion of normal egg with a normal sperm (comes from father) produces a normal offspring while fusion of abnormal egg with a normal sperm will produce an abnormal offspring.