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 :-

An affected mother has three copies of chromosome 21 (trisomy 21).
During meiosis, the three chromosomes 21 are distributed so that half of her eggs receive two copies of chromosome 21 and half receive one copy.
The normal father gives one chromosome 21 in every sperm.
Egg with two copies + sperm with one = trisomy 21 (Down's syndrome); egg with one copy + sperm with one = normal.
So about 50% of the offspring would be affected.