When I look at the bird in person, not just in a picture, she seems to be more white with black than black with white. I'm *assuming* that coloration is the primary way large hatcheries differentiate them; that they throw the birds who are more dark in with the Ancona breeders and the ones who are lighter in with the Exchequer breeders. There is such wild variation with other genetics when it comes to hatchery birds that I'm less comfortable relying on those things.
For example, how many hatchery 'dominiques' have single combs because there's not been an effort to make all the parent birds homogeneous for rose comb gene? And thinking of the wild size variations I've seen in different hatchery birds of the same breed, I'm not comfortable judging on that, either.
An aside: It is temperament that really is turning me off from large hatcheries (my cockerel from a show line is better with the hens than ANY of the half-dozen hatchery cockerels I've had), but the loose way that different breeds from hatcheries are so similar as to be indistinguishable is a close second reason.
Sorry, didn't mean to go on a rant, there!
The mottling gene is essentially just the white spots that appear, and the amount of spots will vary by bird and can increase with age/molt.
Mottled Ancona, Exchequer Leghorn, Pita Pinta Asturiana, Mottled Cochin & Calico Cochin, Speckled Sussex, etc. They're all the mottling gene.
"White with black spots" would be a paint (dominant white gene with color leakage. Paint silkie, ermine/erminette) and doesn't look anything like mottling.
As far as combs- I don't know how all those genetics work. BUT, I did look into it when I was looking up Dominiques a few years ago.
Both the Dominique and the Barred Plymouth Rock had BOTH comb types. Eventually, it was a "let's decide this one has this comb, that one has that comb" and that's why they have the combs they do today. But, the single comb can still pop up in the Dominique occasionally.
Hatcheries should be keeping their "breeds" separate. It's not impossible that different colors or breeds get mixed together by accident, or the wrong ones sent entirely. (Not exclusive to hatcheries, based on various posts I've seen.)
But that doesn't mean they're just throwing birds together based on how many spots they show at a few months.
The two breeds look similar because they ARE similar. For whatever reason, their breed standards and body type are extremely similar. Perhaps it was human preference, perhaps it was what did best in the environment. They likely had similar ancestors, or one might have been used to develop the other (The Dominique helped develop the Barred Plymouth Rock, for example.)