page_title: Nondisjunction
section_title: Down syndrome (trisomy 21)
hierarchical_section_title: Nondisjunction / Consequences / Autosomal trisomy / Down syndrome (trisomy 21)
caption_attribution_description: Down Syndrome Karyotype. from en: with same file name
context_page_description: 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.
  Calvin Bridges and Thomas Hunt Morgan are credited with discovering nondisjunction in Drosophila melanogaster sex chromosomes in the spring of 1910, while working in the Zoological Laboratory of Columbia University.
context_section_description: Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. The majority of cases results from nondisjunction during maternal meiosis I.  Trisomy occurs in at least 0.3% of newborns and in nearly 25% of spontaneous abortions.  It is the leading cause of pregnancy wastage and is the most common known cause of mental retardation.  It is well documented that advanced maternal age is associated with greater risk of meiotic nondisjunction leading to Down syndrome. This may be associated with the prolonged meiotic arrest of human oocytes potentially lasting for more than four decades.
caption: 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).
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