In a process as complex as meiosis, errors in chromosome duplication may occur. Sometimes, an error called nondisjunction occurs. Nondisjunction is failure of chromosomes or chromatids to separate during anaphase I or II of meiosis, resulting in an uneven distribution of genetic material. Nondisjunction results in aneuploidy, a condition in which the organism has an abnormal number of copies of a chromosome. The specific term for the condition of having more than two copies of a set of chromosomes is polyploidy. Many plants and some fish and amphibians are normally polyploids. There are no known human polyploids; however, in humans, variations in chromosome number are most often detrimental. Many aneuploid gametes produce nonviable offspring or individuals with recognizable syndromes.
Trisomy-21, the aneuploidy that results in Down syndrome, arises when a nondisjunction occurs with chromosome 21, most often after the anaphase I stage of meiosis. One of the two daughter cells produced thus has both of the duplicated homologous chromosomes. When this cell goes through meiosis II, it will produce two daughter cells that each have two copies of chromosome 21. If an ovum produced in this way joins with a normal sperm that contains one copy of chromosome 21, the resulting zygote will have three copies—called a trisomy—of this chromosome.
Aneuploidy in the sex chromosomes produces less-severe effects. Klinefelter's syndrome is a trisomy in the sex chromosomes resulting in XXY. The causes of the aneuploidy may be a nondisjunction of the sex chromosome at anaphase I, in either an ovum or a sperm. The ovum would proceed through meiosis II with XX homologous chromosomes (instead of just two copies of a single X chromosome), and the sperm cell would proceed through meiosis II with XY chromosomes (instead of just two copies of a single Y chromosome). An affected ovum (XX) fertilized by a normal sperm (Y) or a normal ovum (X) fertilized by an affected sperm (XY) would produce a zygote with the XXY trisomy, Klinefelter's syndrome. A nondisjunction at anaphase II in an ovum would also produce an affected ovum that would give rise to the XXY trisomy when fertilized by a normal sperm.
Aneuploidy can also refer to an abnormally low number of chromosomes. Turner's syndrome results from a type of monosomy in which a female individual has only one X chromosome. Nondisjunction causes both copies of a chromosome to move into one of two daughter cells, which means that the other daughter cell lacks a copy of that chromosome. Turner's syndrome results when either the ovum or the sperm lacks a sex chromosome and the other has an X chromosome. The resulting female zygote then has only a single X chromosome rather than XX.