The Y chromosome is a dominant gene, because... / People with down SyNDrOmE have YXX when male, and XXX when female, and any ''multiple X'' syndrome only has a plurality of X's, while one Y chromosome always (except for with XY females) makes male babies.
A Sudden Realization Ralph meme.
Sudden Realization Ralph is Ralph Wiggum from The Simpsons, eyes wide as something finally clicks. Captions describe moments of sudden understanding - realizing a joke years later, noticing the obvious hiding in plain sight, working out that the snooze button means less sleep. It's the meme for epiphanies that arrive late.