I seem not to have a good understanding of this at all! Could I have clarification as to whether popular consensus is that this is a single gene trait or a polygenic trait? I know much has been made of recent work and some photos that apparently provide evidence supporting single gene involvement, but it's still not clear to me if this is generally accepted or not. (For those interested in answering this as a poll, the following may be of use:

http://forum.backyardpoultry.com/viewtopic.php?f=10&t=7973831 )

Many thanks,
Htul