Dave Holderread also did research on alleles and their affect on duck color (it's in Storey's Guide to Raising Ducks, and I can't find the study online). I have Cascade ducks from Holderread that are sex-linked; basically if I breed a drake with another breed, the ducks will be sexed at birth by bill color.
I think Oregon State did a study as well, but I just got off from work and my brain is mushing. I'll check when I've decompressed (today I hate my job), because I usually nerd out on this stuff.
See where your hand is? Not there. It's next to this tiny ad: