question_answer

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%

Correct Answer: A

Solution :

Downs syndrome is the result of trisomy in which chromosome number 21st contains an extra copy of chromosome (2A + 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.