Basic Trope: A couple has a child that looks very different from either parent, implying that he/she was fathered by someone else.
Alice is married to Bob, but has an affair with Jamal. Nine months later, she gives birth to a black baby.
Shaniqua gives birth to a white baby, despite being married to Jamal (presumably after an affair with Bob).
Exaggerated: Alice has several children, all of whom are different races and look nothing like Bob.
Downplayed: Alice's baby has an eye color that would be extremely rare if it were also Bob's child.
Alice has been cheating.
Bob had unprotected sex with Charlotte, who had had unprotected sex with Jamal just hours before (or they had a threesome), then Bob went and had sex with Alice, which gave her both his own sperm and that of Jamal.
Inverted: Alice and Bob are married but have split up. Alice has moved in with Jamal. When she is pregnant, everybody thinks Jamal is the father. When the baby is born, it is white and Bob-like.
Alice and Bob used donor eggs and/or sperm to conceive, and they came from someone with very different looks.
Enforced: "Okay, we already know Alice cheats. How do we show it without her getting in trouble with Bob?" "Let's have her give birth to a black baby. Bob is Too Dumb to Live, so he'll think it's his child."
Lampshaded: "That's our baby? He looks nothing like me!"
Alice finds herself in bed with Jamal, but they have no protection.
Alice knows that Bob is Too Dumb to Live, and cheats on him just to see if he'll notice.
Agent Charlie notices this and blackmails Alice into being an accomplice.