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. Bonus points if the husband picks a fight with the father. Subtrope of Your Cheating Heart

Live-Action Television
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”.

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).

Web Animation
The Cyanide and Happiness youtube account has this video: