Down's Syndrome is case of trisomy of ${21}^{\mathrm{s}\mathrm{t}}$ chromosome. In which ${21}^{\mathrm{s}\mathrm{t}}$ pair carries 3 chromosomes. During gamete formation 50% gametes carry normal number of chromosomes i.e. 23, but remaining 50% gametes carry 24 chromosomes (one more chromosome in ${21}^{\mathrm{s}\mathrm{t}}$ pair). So if gamete with 24 chromosomes is fused with male gemete (sperm) then again case of Down's syndrome is generated.