User blog comment:Heinz84/Could Dr. Doofenshmirtz be Phineas and Candace's Biological Father?/@comment-1647442-20100721032219

It really couldn't be. As you stated, Candace and Vanessa are half-sisters. Well, Vanessa's one year older than Candace, so she was born first (obviously) and for Doof to be Phineas and Candace's father, he would've left Charlene and gone with Linda, yet how come he keeps in touchwith Charlene but not Linda? And you'd think he'd keep more in touch with Linda, because if this theory is true, then he was with Linda for enough time for her to have two kids. So, in conclusion, I'd have to say no, it's not possible.