Connie, you are right on about the influence of modifiers. They certainly make for a challenging puzzle! I have a serious mixed up soup of genes floating around my farm but of course that makes it fun:)
Ultimately I feel there is no singe genotype that should be isolated and developed to meet the standard. There are several ways to arrive at the bird described in the standard. We designed the SOP to be broad enough to include as many variations of the landrace as possible, but in some cases we had to choose from the variables that existed historically.(Slate vs Willow legs), and eliminate flaws that appeared to be common in the breed (Squirrel tail). I think this is the natural progression when having to seek recognition for a breed. You must be able to pick an Iowa Blue out of a lineup of similar looking breeds. That being said, The APA states a breed must produce true to type 50% of the time. For this reason, I would NOT cull based strictly on leg color, white on the earlobes, comb points, and other cosmetic problems while trying to build a flock of good breeders. If you don't have 50 birds to choose from like I do, select for consistent size and shape, and don't sweat the small stuff. You still should be able to produce some birds that fit the profile for showing.