And so now if we take a sperm cell, so this is our sperm cell that also has let's suppose we have a normal sperm cell and that means we have a normal amount of chromosome number one. We only have one. When this sperm cell, normal sperm cell combines with Xcel number two, we're going to form a Zygote that has the following genotype distribution for chromosome 21. 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.