Board Thread:TV Show Discussion/@comment-29740399-20160825233628/@comment-29740399-20160827025601

I don't know how it cheapens the complex relationship in the series but The World of Ice and Fire says "It has been reliably reported, however, that King Aerys took unwonted liberties with Lady Joanna's person during the bedding ceremony, to Tywin's displeasure." Also Tywin and Joanna were cousins, and Jamie and Cersei have the blonde hair and green eyes. There's just not much room in the gene pool. Tyrion has almost white hair and heterochromia goin on..which are not Lannister traits, but rather Targaryen.

A Dance with Dragons does provide a thread of support for the theory that Tyrion is not Tywin’s. Barristan Selmy reports the following to Daenerys:

“Prince Aerys. . . as a youth, he was taken with a certain lady of Casterly Rock, a cousin of Tywin Lannister. When she and Tywin wed, your father drank too much wine at the wedding feast and was heard to say that it was a great pity that the lord’s right to the first night had been abolished. A drunken jape, no more, but Tywin Lannister was not a man to forget such words, or the. . . the liberties your father [Aerys] took during the bedding.”

He couldn't be Cersei and Jamie's father cuz Tywin and Joanna were married in 263 AC and the twins were born 266 AC after Joanna had been at Casterly Rock, away from Aerys. Joanna rarely visited Kings Landing and Aerys sometimes visited Casterly Rock. I'm just sayin nothing should surprise AWOIAF fans. GRRM can be crazy!