Sperm and egg cells contain only 23 chromosomes. When the sperm and egg cells unite, the resulting fetus inherits half of its DNA recipe from its mother and half from its father. Two of these 46 chromosomes determine the sex of a person. A girl inherits two X-chromosomes, one from her mother and one from her father.
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).
Making the world better, one answer at a time. Nondisjunction can occur during Anaphase in mitosis, or either Anaphase I or II in meiosis. This is the step of each aformentioned process when the chromosomes are separated into different regions within the dividing cell, so they can be segregated further by cytokenesis.