That's called a chromosal mutation. And it can cause varying mutations such as Down's Syndrome. This means that the person is born with 47 chromosomes instead of 46 and causes the child to be born with a flattened nose and eyes slanting upward and blah blah blah.
@Jongin If chromosomes do not divide properly during mitosis, the daughter cells can end up with an abnormal number of chromosomes. Hypothesize the effect of the abnormal number of chromosomes on the functioning of the daughter cell. Put forward evidence to support your answer.
@Jongin Because you always make me choose places and I don't know anything about dating. **grabs my biology book and starts reading** did you study for your quiz?