What evidence reveals that the jurors are biased ?

It is revealed that Juror 3 is biased toward the defendant, and because his own son had hit him and run away from home when he was fifteen.

"I've got a kid...when he was fifteen, he hit me in the face...I haven't seen him in three years. Rotten kid! I hate tough kids! You work your heart out (but it's no use)".

Juror 4 is biased against the defendant based up the area in which he was raised.

"We're missing the point here. This boy - let's say he's a product of a filthy neighborhood and a broken home. We can't help that. We're not here to go into the reasons why slums are breeding grounds for criminals. They are. I know it. So do you. The children who come out of slum backgrounds are potential menaces to society."