Because Fry is now his own grandfather, he lacks a certain brainwave pattern, which is why he was immune to the Brainspawn in the episode The Day the Earth Stood Stupid, yet this information wouldn't be addressed until The Why of Fry.
Please read the following before uploading
Do not upload anything which you do not own or are fully licensed to upload. The images should not contain any sexually explicit content, race hatred material or other offensive symbols or images. Remember: Abuse of the TV.com image system may result in you being banned from uploading images or from the entire site – so, play nice and respect the rules!