What does nondisjunction refer to in genetics?

Nondisjunction specifically refers to the failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to cells with an abnormal number of chromosomes. This can result in conditions such as aneuploidy, where a gamete ends up with one extra or one less chromosome than normal. When fertilization occurs with these gametes, it produces offspring with either too many or not enough chromosomes, which can lead to various genetic disorders.

Understanding that nondisjunction is related to the unequal distribution of chromosomes during division clarifies why it primarily causes an abnormal number of chromosomes in cells, confirming that having too many or not enough chromosomes is the key outcome of this process.