If he had more Jenova cells then human cells he would look like Jenova only certain parts of him got the Jenova cells not all of him. And I was just proving that you said he grew up thinking Jenova was his mother, he didn't when he jumped to conclusions he did, but he didn't go...

*Que Flashback*
5 year old Sephy: My moms Jenova which means I'm the chosen one! I love my mommy!