"Why not?"
Don put down his pencil and turned to Mikey. His brother wasn't bugging him, he was being serious. He wanted to encourage this behavior.
"Are you ready to be a dad, Mikey?"
"What?"
"Even if we mutated a fully grown turtle like mom, they would still need to be taught everything. Splinter was extremely lucky in that he was able to learn about humans from observing Master Yoshi. But mom probably doesn't know anything. Not to mention that turtles just aren't as intelligent as rats. It would be like having a little kid around."
Mike looked crestfallen.
"Oh."
"You know… she's just a turtle."
Raph snapped out of his reverie, pinning his brother with his gaze.
"…what?"
"She may be genetically related to us, but that turtle in there is not our mother."
Leo had his arms folded over his plastron. He didn't want to be saying these words any more than Raph apparently wanted to hear them, but they needed to be said.
Raph seemed to be slowly inflating with rage. Leo wondered if he'd gone too far.
"Speak for yourself, Leonardo," was all his brother said. Somehow, it was worse than a punch in the face.
I have no idea if turtles are actually less intelligent than rats. How do you measure that kind of thing anyway? But it seems true.