Principles of Inheritance and Variation Question 179
179. Down’s 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) 25%
(b) 100%
(c) 75%
(d) 50%
Show Answer
Answer : d
Hints & Solution
(d) 50% of ova will have $(n+1)$ chromosome which would, on fertilisation, yield abnormal zygotes $(n+1)+(n)=2 n+1$.
