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When chromosomes fail to separate during meiosis I or II, non-disjunction can result. It can occur in meiosis I when homologous chromosomes fail to separate or in meiosis II when sister chromatids fail to separate. When there is an additional chromosome enclosed in a gamete, at pair number 21, this causes a condition of 2n+1 which is the diploid number of chromosomes(46) plus an additional chromosome at pair 21. This is called trisomy 21. The third chromosome at this position causes Down Syndrome. This condition was named after John Langdon Down, who described it in 1866. It is more common with older parents where 88% of the time, the extra chromosome is present in the mother's egg and 8% from the father's sperm. Older parents are exposed longer to environmental agents that may play a role in non-disjunction during meiosis.
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