Trisomies are chromosomal anomalies which usually occur due to non-disjunction. The vast majority of affected fetuses being spontaneously aborted, often very early during gestation. Only three are compatible with extra-uterine life (T13, T18, T21), and only one beyond early infancy (T21).
In order of frequency they are:
Individuals with other rare trisomies may survive beyond infancy only if they have mosaic forms.
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