"Never arrange to mate things within your own family." -- Prince Charles, in that Today programme interview. It was ages ago, but I had to wait until Pick of the Week before I was sure I'd heard right.
To be fair, he's talking about rare pigs; and his concern is prompted by his sister's boar ripping his sows' ears, rather than genetic disorders or hereditary porphyria or anything. Still, how unlike the home life of our own dear etc, etc.