When we breed for fawn, the goal is to get it as "clean" as possible. The above is an English Angora junior, the color is as clean as it could be.
"Clean" means that there's very little or preferably no black or gray tips on the face or the wool. The traditional thought is that the presence of the "wide band" gene would clean out the black or gray tips. However, the fawn/cream/red are classified as "Wide Band Group" in the SOP, then listed as faults: "Any smut, ticking, ear lacing, or faint grayish band in wool". My thought is that if they are all wide band and that wide band gene could clean out the smut and gray, then there should not be such faults. How to explain the clean vs smut fawn (and cream and red)? In my opinion it's the B- or bb gene that determine the appearance.
In tort, the regular or black tort is aaB-C-D-ee, chocolate tort is aabbC-D-ee. The difference in color is that the black tort has a dark face and "Wool is to be dark fawn over the back", in most cases the wool has darker tips, while chocolate tort has a brown face and "Wool is to be fawn over the back". (the quotation marks are used because these are the words used in the SOP). That sounds almost like the smutty fawn and fawn except that tort and chocolate tort are self aa while the fawns are agouti A-. Therefore in my opinion, the smutty fawn is just the agouti version of the black tort and the clean fawn is the agouti version of the chocolate tort.
Smutty fawn: A-B-C-D-ee
Clean fawn: A-bbC-D-ee
In order to breed clean fawn, the best way to achieve the goal is to use two chocolate based fawn or a chocolate based fawn bred with a chocolate tort.
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