question_answer

                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)                                                                                                                                                                                            50%                       

B)                 25%       

C)                 100%     

D)                 75%

Correct Answer: A

Solution :

                Down's 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.                                                 (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.