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Meiotic nondisjunction

failure of two members of a chromosome Pair to separate (disjoin) during meiosis so that both go to one daughter cell and none to the other. This mechanism is responsible for the extra chromosome 21 in trisomy 21 (down Syndrome) and for extra and missing chromosomes causing other birth defects and many spontaneous abortions (miscarriages). Trisomy 18 (Edwards' Disease) and trisomy 13 (Patau Syndrome) are some other chromosomal abnormalities.