Hand-drawn diagram of meiosis I and II nondisjunction leading to trisomy 21 (Down syndrome), showing chromosome separation errors.

Black and white, hand-drawn classroom style diagram of nondisjunction in meiosis leading to Down syndrome (trisomy 21). Panel A: Meiosis I nondisjunction — homologous chromosome 21s fail to separate, producing gametes with 2 or 0 copies. Panel B: Meiosis II nondisjunction — sister chromatids fail to separate, producing gametes with 2, 0, or normal 1 copy. Final step: Fertilization with a normal gamete (1 copy) produces a zygote with trisomy 21. Simple stick chromosomes, outlined cells, arrows showing process, with handwritten labels: "Meiosis I nondisjunction", "Meiosis II nondisjunction", "Fertilization → Trisomy 21". See more