The Seed Is Strong

Someone’s wife has 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.

Live-Action Television

 * The Trope Namer 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 Joseph 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.