Breaking up is hard to do (especially for sex chromosomes)

As chromosomes go, X and Y make an unlikely pair. The X is large and contains thousands of genes critical for life. The Y, by contrast, is little more than a nub. Its main purpose is to provide the instructions for initiating male development and making sperm. Yet these two very different chromosomes must work together if they are to meet and pair up properly during meiosis—the special form of cell division that creates sperm and egg.