Nondisjunction anytime you are splitting chromosomes (mitosis, meiosis) Meiosis occurs in the cells that give rise to germ cells (ovary, testes) Need to become haploid so that can become diploid when fertilized. Meiosis in ovary: prenatally, babies have all mitotic cells w precursors early on, then stops. Meiosis in testes: brief period of mitotic division, then mitotic arrest, after birth more mitosis, once puberty, cells are stimulated to undergo meiosis, continued mitosis & meiosis throughout lifetime. Result: one daughter cell receives extra copy of one chromosomes & other w no copy of chromosome. Nondisjunction can also occur in mitosis, but not affect entire organism to great extent (may become apoptic) lead to cancer. Developmental abnormalities: estimated 1/3 of all conceptions have miscarriage, approx of these occur prior to detection of pregnancy, 40-50% of time chromosomal abnormality, some chromosomal abnormality that lead to live birth, but w consequence.