English: 1. Meiosis I 2. Meiosis II 3. Fertilization 4. Zygote

The left image at the blue arrow is nondisjunction taking place during meiosis II. The right image at the green arrow is nondisjunction taking place during meiosis I.

Nondisjunction is when chromosomes fail to separate normally resulting in a gain or loss of chromosomes. This can happen during mitosis or meiosis. Primary nondisjunction occurs during meiosis I and the result is both members of a homologous pair go into the same daughter cell. This has the eggs have one more or one less number of chromosomes. Once the sperm fertilizes the egg then there is an abnormal number of chromosomes. Secondary nondisjunction happens during meiosis II and this results when the sister chromatids fail to separate, ending up with both daughter chromosomes going into the same gamete. During this, one egg will have one more or one less chromosome. Once these eggs are fertilized there will be two zygotes with an abnormal number of chromosomes. It is less harmful to have secondary nondisjunction since you can still have two normal gametes while in primary nondisjunction there are no normal gametes.