Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. Trisomy 21 (47,XY,+21) is caused by a meiotic nondisjunction event. The extra Y chromosome is usually a result of nondisjunction during paternal meiosis II. Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. Mosaicism syndromes can be caused by mitotic nondisjunction in early fetal development. Mitotic nondisjunction after conception would lead to mosaicism, and is discussed later. Aneuploidy occurs when nondisjunction at a single chromosome results in an abnormal number of chromosomes. In "nondisjunction", sister chromatids fail to separate during anaphase. An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes. This causes chromosomes to fail to properly segregate in a process called nondisjunction. Loss of part of a chromosome can also cause sterility due to nondisjunction. There are two main types of nondisjunction that occur&colon; trisomy and monosomy. Meiotic nondisjunction has been well studied in "Saccharomyces cerevisiae". This may be due to a nondisjunction in the father. This is due to a mis-segregation (nondisjunction) during gametogenesis.