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I have to disagree that using a Rhode island verses a New Hampshire, that it doesn't make a difference. Because it most certainly will. In the expression of red and intensity of yellow and red not only in the shanks but also in possible expression of other traits in the plumage. And if either are crossed and the bird is sold as a "Delaware", someone is going to get a lot of surprises when trying to breed that bird back to a genotype correct Dels. But no, that isn't what I was getting at with "white" legs.
There are lines that have had a line of CX crossed into them for growth rate and overall size. Which is why some are seeing birds with white legs pop up in following generations when line bred. There isn't a "single gene" that controls yellow legs expression. It's a combination of many. Some more apparent then others in expression and some sex linked some not. Some linked closely with other Loci and harder to breed to segregate them. And it isn't just people on BYC breeding non standard or "standard" crosses. Which is what happens when the introduction of white into the background, and may not be apparent until segregated breeding is done. And I do have birds on the ground that show all of what I'm talking about.
Right now I also have line of birds, more apparent in the roos, that show white legs and yellow legs from the very same parents. Which is why I also brought up yellow skin. One of the roos that shows yellow legs also isn't homozygous for yellow skin and another is. And just to look at them (phenotype), you wouldn't be able to tell the difference until test bred against genotype correct hens. That's when all the "fun" begins for someone who doesn't know or expects at least their line of birds to be genotype and phenotype correct. Not necessarily "show birds" but at least correct enough that they can breed towards that if they so choose. And not fighting with genetics that should be there, against genetics that should not. And that's assuming they know enough about them to be able to make a "informed" decision as to what to breed for and what has to be fixed and what can be left till later.
And many will not know or may not care that they are also introducing genes, that may not be as readily apparent, as the major ones, into their lines by doing that. Even if they're outcrossing to "standard" stock ie: Barred rocks or New Hampshires. Which in itself isn't a bad thing, depending on the reasoning as to why they're doing it. Now whether or not they realize how to fix some of the "problems" introduced by doing that is another matter. Hatcheries and breeders alike.
From what I've learned, there are many Barred rocks and Delawares who are B/B(B/-) and id+/id+(id/-)(green legs that may yellow up with age, but hens will be pale). Which in itself will cause many of the problems that people are seeing when out crossed onto ID/ID(ID/-) birds. Which when looking at only phenotype, will produce anything from green legged birds or pale yellow that may/may not yellow up as they get older to birds which you can't tell the difference until you start breeding them and "anomolies" start showing up. Depending on what else is in their genetic background.
Which is why I posed those questions to you. It may not be a DQ for pale yellow legs, but it certainly is going to be an indicator that something is genetically amiss.
Now after doing more studying into the background of both Barred rocks and New Hampshires, with differing "historical" lines of each. Reason being I was confronted with many of the same problems that people on here are having. Apparently no one seems to have the answers or they pass it off as "the alphabet soup" or they believe that genetic anomalies happen much more often then they actually do. Or they think it's all caused by feed or environment. Which some is, though not at the level most are thinking. So I took it upon myself to learn as much as I can about what interactions in genes have and what can come up in phenotype and not be a direct indicator of genotype. Will I claim to be an "expert", no, not even close, but I will back up what I"m talking about, as I've not only researched it, I've also been actively breeding it to test those very interactions.
What I've posed here is only a small part of the overall "genotype vs phoentype" problems that many are seeing. It isn't just one set of gene interactions, and it isn't going to be "fixed" in all lines in a couple of generations. And that's not taking into account any out crossing which is being done for various reasons.
edited for added insights and clarity