So instead of having two, as in a normal case, we have three copies. And this is what we call tricemy 21. So if a normal sperm cell like the one shown here, that contains a single copy of chromosome 21, combines with Xcel number two that contains an extra copy of chromosome 21, we're going to produce a Zygote, an individual that has trisomy 21. Now, notice in this particular case I've discussed nondisjunction taking place during anaphase two of meiosis. But nondisjunction can also take place during anaphase one of meiosis. And in both cases we can basically produce a Zygote that contains trisomy 21.