As explained by DevBio, nondisjunction is what happens when a pair of chromosomes do not separate properly during meiosis. According to Indiana University-Purdue University Indianapolis (IUPUI), nondisjunction can occur during either anaphase I or anaphase II of meiosis.
To understand the two different forms of nondisjunction, it is important to consider how meiosis happens normally. As explained by the University of Arizona, in meiosis I, the homologous replicated chromosomes (sister chromatids) line up on the metaphase plate. During anaphase I, the homologous chromosomes are supposed to separate. During anaphase II, the sister chromatids of the already-separated homologous chromosomes separate.
Nondisjunction in anaphase I of meiosis is a result of homologous chromosomes failing to separate and nondisjunction in anaphase II of meiosis is a result of sister chromatids failing to separate, as IUPUI describes. Both types of nondisjunction have the same result: gametes that have an incorrect number of chromosomes, called aneuploidy.
In most situations, aneuploidy in human gametes result in non-viable zygotes. However, there are a few situations in which embryos with gross chromosome abnormalities survive to term. According to IUPUI, the forms of aneuploidy that permit survival are trisomy 21 (Down syndrome), trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome) and differing numbers of sex chromosomes X and Y.