During the process of non-disjunction the XX (mom) or XY (dad) fail to separate during meiosis. As you can see in the diagram posted, the failure to separate occurs during either Meiosis I or II, the end result of the gamete determines at which stage it occurs. During the process of fertilization (the egg and sperm from each parent coming together), if a sperm, having both an X and a Y chromosome fertilizes an egg having a single X chromosome, or a normal Y- bearing sperm fertilizes an egg having two X chromosomes, an XXY male is conceived. (Diagram: a-3 and b-1,2). Hope this helps. Feel free to contact me if you have any questions or need further clarification.