Nondisjunction
（重定向自Meiotic nondisjunction）
![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.](/Images/godic/202501/26/Nondisjunction_Diagrams.svg1734.png")
![Nondisjunction of sister chromatids during mitosis:
Left: Metaphase of mitosis. Chromosome line up in the middle plane, the mitotic spindle forms and the kinetochores of sister chromatids attach to the microtubules.
Right: Anaphase of mitosis, where sister chromatids separate and the microtubules pull them in opposite directions.
The chromosome shown in red fails to separate properly, its sister chromatids stick together and get pulled to the same side, resulting in mitotic nondisjunction of this chromosome.](/Images/godic/202501/26/Mitotic_nondisjunction1734.png")
![Karyotype of X monosomy (Turner syndrome) This condition is characterized by the presence of only one X chromosome and no Y chromosome (see bottom right corner).](/Images/godic/202501/26/45,X1734.jpg")
![Karyotype of trisomy 21 (Down syndrome)
Note that chromosome 21 is present in 3 copies, while all other chromosomes show the normal diploid state with 2 copies. Most cases of trisomy of chromosome 21 are caused by a nondisjunction event during meiosis I (see text).](/Images/godic/202501/26/Down_Syndrome_Karyotype1734.png")
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).