Hidden Gene Explaination

[rabernet]

On another thread someone asked what a hidden gene is (like in the NERD line womas and RDR's Platty Daddy). It's one of those things that I understand on a very basic level, but can't explain well to someone else.

Can anyone who understands it better share their understanding of what exactly a hidden gene is?

Thanks in advance, I'm sure it will be interesting to many of us.