What happens during meiosis that ultimately results in a defect characterized by the addition of chromosomes?

During meiosis the chromosomes line up along the metaphase plate (also known as the equatorial plate). A problem arises when nondisjunction occurs in Meiosis II and the sister chromatids do not divide properly leaving the gametes has extra genetic material, when these gametes they fuse with another gamete at fertilization this results in trisomy of a chromosome as in Down Syndrome (trisomy 21) and Patau Syndrome (trisomy 13) or an extra X or Y chromosome. Most trisomy are incompatible with life and the zygote/fetus does not survive to birth.