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. 50%
• B. 25%
• C. 100%
• D. 75%

Answer 1

Answer: (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



$(n+1)$ - 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.