Also, I would not be so quick to criticise our pre-eminent avian experts who determined that Pastel (Euro Yellow) hens carry only one copy of this gene. Hens can only carry one copy, which is all that's needed to produce a visually yellow bird, whereas cocks require two copies to display the same colour. It is entirely correct to refer to hens as 'single factor'. (...which reinforces the point - systematic naming helps. Calling a bird a 'Single Factor Pastel hen' tells us everything we need to know; calling it a 'yellow hen' tells us almost nothing other than it's a very pretty bird
