Meiotic Nondisjunction

**Semantic type:** Cell or Molecular Dysfunction

**Definition:** The failure of the chromosomes to disjoin and move to opposite poles, may affect as many as 25% of all ova and 2% of all sperm. Half of these abnormal gametes are nullisomy, half are disomy. Two copies of a chromosome pass into the same gamete, leaving the other gamete as nullisomy. 95% of the Down Syndrome birth were result of meiotic disjunction.