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Most embryos cannot survive with a missing or extra autosome (numbered chromosome) and are spontaneously aborted.

The most frequent aneuploidy in humans is trisomy 16, although fetuses affected with the full version of this chromosome abnormality do not survive to term (it is possible for surviving individuals to have the mosaic form, where trisomy 16 exists in some cells but not all).

For many of the autosomal trisomies, only mosaic cases survive to term.

However, mitotic aneuploidy may be more common than previously recognized in somatic tissues, and aneuploidy is a characteristic of many types of tumorigenesis (see below).

During meiosis, when germ cells divide to create sperm and egg (gametes), each half should have the same number of chromosomes.

Multipolar spindles: more than two spindle poles form.Merotelic attachment occurs when one kinetochore is attached to both mitotic spindle poles.One daughter cell would have a normal complement of chromosomes; the second would lack one.The most common aneuploidy that infants can survive with is trisomy 21, which is found in Down syndrome, affecting 1 in 800 births.Trisomy 18 (Edwards syndrome) affects 1 in 6,000 births, and trisomy 13 (Patau syndrome) affects 1 in 10,000 births.

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