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?

The correct option is A 50%
Down's syndrome is caused due to trisomy of 21st chromosome.


In the above cross, the male gametes have 50% of chance to fuse with egg having normal number of chromosomes (22+X) and 50% of chance to fuse with egg having 24 chromosomes (23+X). The extra chromosome in the egg is due to non-disjunction (failure of separation of homologous chromosomes during meiosis).
The offspring produced by the fusion of male gamete with egg having 24 chromosomes will have 47 chromosomes and will suffer from Down's syndrome.
Hence, there is 50% chance of offspring to suffer from Down's syndrome in the above said cross.