Trisomy 21 (Down Syndrome) is usually due to the chromosomes not separating equally right after a normal egg is fertilized by a normal sperm.
I-Trisomy 21 (Down Syndrome) idla ngokudalwa kukungohlukani kwemisontwana esesibindini seseli yesihluma (chromosomes) ngokulinganayo emva nje kokuba iqanda eliqhelekileyo lizaliswe ngembewu yobudoda engenangxaki.