Genes are not the reason Homer is stupid. He has a crayon crammed up his nose and into his brain. When it’s removed he is very intelligent.
“Shut up, brain, or I’ll stab you with a Q-Tip!”
“How is education supposed to make me feel smarter? Besides, every time I learn something new, it pushes some old stuff out of my brain - remember when I took that home winemaking course, and I forgot how to drive?”
“I hope I didn’t brain my damage!”
“I am so smart, I am so smart, S M R T, I mean S M A R T.”