They contain one chromosome 21 each. But what happens in this case? Because the age of this XL is high, the likelihood that nondisjunction takes place is also high. And so what that means is what might happen is this fiber doesn't actually attach onto this chromosome. This is the chromatid here. And so what happens is this entire pair of cystochromatids, basically segregates, moved to one side and nothing moves to the other side.