. Trisomic condition of Down's syndrome arises due to
b. crossing over
d. dicentric bridge formation
In meiosis, the pairs of chromosomes are supposed to split up and go to different poles in the dividing cell; this event is called "disjunction." However, occasionally one pair doesn't divide, and the whole pair goes to one pole. This means that in the resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes. This accident is called "non-disjunction." Due to this one more X chromosome moves to the one pole and trisomic condition arises.