nondisjunction
Mistakes may occur during meiosis that result in nondisjunction . This is the failure of replicated chromosomes to separate during meiosis the animation at the link below shows how this happens . Some of the resulting gametes will be missing a chromosome, while others will have an extra copy of the chromosome. If such gametes are fertilized and form zygotes, they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders. Table below lists several genetic disorders that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome. Look back at the X and Y chromosomes and you will see why. The X and Y chromosomes are very different in size, so nondisjunction of the sex chromosomes occurs relatively often. http://learn. genetics. utah. edu/content/begin/traits/predictdisorder/index. html.