An aneuploid cell has the wrong number of chromosomes. In the context of human genetics, most discussions of aneuploidy center around 'meiotic aneuploidy', in which aneuploidy is the result of misapportionment of chromosomes during meiosis in oogenesis. Most meiotic aneuploidy results in miscarriage, but certain aneuplodies result in birth defects. Examples include the monosomies leading to Turner syndrome (X0) and cri du chat syndrome, which results from deletion of part of chromosome 5. Several human trisomies can result in viable birth including trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), trisomy 8 (Warkany syndrome 2). Trisomy involving sex chromosomes in humans includes, XXX (Triple X syndrome), XXY (Klinefelter's syndrome), and XYY (XYY syndrome).

Most meiotic aneuploidies are the result of chromosomal misalignment in oogenesis (congression failure). Human eggs stop in prophase of meiosis I while the female herself is still an embryo. Meiosis continues only after ovulation and fertilization. As the woman ages, nondisjunction becomes more and more likely.

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