Editing Down syndrome (section)

===Trisomy 21===
Down syndrome (also known by the [[karyotype]] 47,XX,+21 for females and 47,XY,+21 for males)<ref name=Flet2007>{{cite book| vauthors = Fletcher-Janzen E, Reynolds CR |title=Encyclopedia of special education: a reference for the education of children, adolescents, and adults with disabilities and other exceptional individuals|year=2007|publisher=John Wiley & Sons|location=New York|isbn=978-0-470-17419-7|page=458|url=https://books.google.com/books?id=7n-tu1Q7jBsC&pg=PA458|edition=3rd|url-status=live|archive-url=https://web.archive.org/web/20170123083050/https://books.google.com/books?id=7n-tu1Q7jBsC&pg=PA458|archive-date=2017-01-23}}</ref> is mostly caused by a failure of the 21st chromosome to separate during egg or sperm development, known as [[nondisjunction]].<ref name=Rubin2013/> As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21.<ref name=Patt2009/><ref name=Rubin2013/> About 88% of cases of trisomy 21 result from nonseparation of the chromosomes in the mother, 8% from nonseparation in the father, and 3% after the egg and sperm have merged.<ref name=Zhang>{{cite book| vauthors = Zhang DY, Cheng L |title=Molecular genetic pathology|year=2008|publisher=Humana|location=Totowa, N.J.|isbn=978-1-59745-405-6|page=45|url=https://books.google.com/books?id=F_7QXO0ZBigC&pg=PA45|url-status=live|archive-url=https://web.archive.org/web/20170123083008/https://books.google.com/books?id=F_7QXO0ZBigC&pg=PA45|archive-date=2017-01-23}}</ref>