This is an example of trisomy 21 (47, XY, +21) also known as Down syndrome. Additions or deletions of genetic material are generally lethal in utero, but Trisomy 21 is an example of one form of addition in which the fetus may occasionally survive to term and beyond. The overall incidence is 1 in 1000 livebirths, but the non-dysjunctional event in meiosis that produces this anomaly increases in incidence with increasing maternal age, particularly over age 40.

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