Down syndrome causes
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. ; Associate Editor(s)-in-Chief: Syed Hassan A. Kazmi BSc, MD 
Down's syndrome (DS) is caused due to the presence of an extra 21st chromosome (triplication of chromosome 21). 95 percent of DS cases are due to meiotic non-disjunction during meiosis I. 4 percent of DS cases arise due to Robertsonian translocation and very rarely DS may be caused a post-fertilization mitotic error (mosaicism).
Down's syndrome is caused by triplication of chromosome 21 (trisomy 21). The aneuploidy may occur in three possible ways:
- Meiotic nondisjunction of homologous chromosomes
- From improper division of chromosomes during meiosis I or meiosis II. One gamete will end up with extra 21st chromosome.
- Robertsonian translocation
- Individuals with normal number of chromosomes but an extra portion of 21st chromosome.
- Post-fertilization mitotic error
For a detailed description on the mechanisms leading to Down's syndrome, click here
- ↑ Antonarakis SE (March 1991). "Parental origin of the extra chromosome in trisomy 21 as indicated by analysis of DNA polymorphisms. Down Syndrome Collaborative Group". N. Engl. J. Med. 324 (13): 872–6. doi:10.1056/NEJM199103283241302. PMID 1825697.
- ↑ Antonarakis SE, Petersen MB, McInnis MG, Adelsberger PA, Schinzel AA, Binkert F, Pangalos C, Raoul O, Slaugenhaupt SA, Hafez M (March 1992). "The meiotic stage of nondisjunction in trisomy 21: determination by using DNA polymorphisms". Am. J. Hum. Genet. 50 (3): 544–50. PMC 1684265. PMID 1347192.
- ↑ Hook EB (October 1984). "Parental age and unbalanced Robertsonian translocations associated with Down syndrome and Patau syndrome: comparison with maternal and paternal age effects for 47, +21 and 47, +13". Ann. Hum. Genet. 48 (Pt 4): 313–25. PMID 6238567.
- ↑ Zhao WW, Wu M, Chen F, Jiang S, Su H, Liang J, Deng C, Hu C, Yu S (2015). "Robertsonian translocations: an overview of 872 Robertsonian translocations identified in a diagnostic laboratory in China". PLoS ONE. 10 (5): e0122647. doi:10.1371/journal.pone.0122647. PMC 4416705. PMID 25932913.
- ↑ Petersen MB, Adelsberger PA, Schinzel AA, Binkert F, Hinkel GK, Antonarakis SE (September 1991). "Down syndrome due to de novo Robertsonian translocation t(14q;21q): DNA polymorphism analysis suggests that the origin of the extra 21q is maternal". Am. J. Hum. Genet. 49 (3): 529–36. PMC 1683126. PMID 1831959.
- ↑ Hsu LY, Gertner M, Leiter E, Hirschhorn K (November 1971). "Paternal trisomy 21 mosaicism and Down's syndrome". Am. J. Hum. Genet. 23 (6): 592–601. PMC 1706744. PMID 4257130.
- ↑ Modi D, Berde P, Bhartiya D (June 2003). "Down syndrome: a study of chromosomal mosaicism". Reprod. Biomed. Online. 6 (4): 499–503. PMID 12831601.