Sorry, I disagree, any time two genes are on the same chromosome, there is an inherent linkage. Granted that it may be very weak and easily broken. The chicken genome is littered with the debris of cross-over events. I'm taking advantage of one such cross-over where the oocyanin gene has been separated from pea comb.
It might be worth delving into just how far apart porphyrin-Z and slow feathering-Z are located.
