Several Y genes that have decayed, or whose function has become limited, still have X homologs that escape X inactivation. In other words, Y degeneration or divergence appears invariably to precede the expansion of X inactivation. Decay or divergence of genes on the Y drives the acquisition of X inactivation, not the other way around. Yet, the story of the Y is not solely a story of decay. Genes shared with the X have tended to be lost, but about half of the genes on the human Y arrived there relatively recently.
Chromosomes are labile enough that genes can be transferred piecemeal from different places.
Once something lands on the Y, genes tend to be amplified in copy number and rearranged. The new residents of the Y may be most likely to survive on the rogue chromosome if they confer male-specific advantages. Indeed, some of the newcomers to the human Y appear to help in sperm formation. This is yet another example of similar, independent evolutionary trajectories. The collection of genes on the human Y is not related to the collection on the fruitfly Y, but in each case, the Y appears to be a bastion of male-fertility factors.