So recall that female individuals produce all their ex cells before birth. So as the female individual ages, the Xcels also age. And the greater the age of the X cells, the more likely that those X cells will actually undergo nondisjunction during meiosis. Now, to see exactly what we mean by that, let's take a look at the following diagram. So this is one example of how tricemy 21 can actually take place, can actually occur. So let's suppose we're dealing with a normal female individual.