The Seed Is Strong

Someone’s wife has (at least presumably) cheated on him. The wife gets pregnant and the child looks a lot like the child’s true father, often revealing the crime. This can be especially obvious if the child has a racial background different from the supposed parents. Bonus points if the husband picks a fight with the father.

Subtrope of Your Cheating Heart. See also Cheating with the Milkman and Pizza Boy Special Delivery.

Live-Action Television

 * The Trope Namer (of at least the working title) is Game of Thrones, in which Robert Baratheon’s wife cheats on him. Robert Baratheon doesn’t realize this. His King’s Hand Jon Arryn catches on and sends a letter saying “the seed is strong”.
 * Rowan and Martin's Laugh-In had the fabulous Farkle Family. Parents Frank and Fanny were both brunettes with good eyesight, but their dozen or so children were all freckled redheads with glasses -- just like "friend and trusted neighbor" Ferd Berfel.  Played for Laughs in that Frank clearly never suspected Fanny of dallying with Ferd.

Western Animation

 * In The Amazing World of Gumball Margret Robinsin switches the names of 2 babies. The fathers see the babies and the names they have and then proceed to fight (implying they came to this conclusion).
 * In King of the Hill the Caucasian Dale Gribble, despite being a paranoid Conspiracy Theorist, never realizes his Caucasian wife's child Joesph has strong Native American features due to being conceived from her 14 year long affair with John Redcorn. Everyone except Dale and Joseph know it.

Web Animation

 * The Cyanide and Happiness YouTube account has this video.