
Description:
EB is the mutation that causes both Rosetta (one copy of the gene) and Tibetan (two copies of the gene). It is often called "brown" in early scientific papers, but is not to be confused with actual wildtype (sometimes called jumbo brown) or sex-linked brown. It was originally discovered in a lab at some point before 1978, though exactly by whom is unknown due to the different naming conventions.
The EB gene increases the eumelanin pigment in the bird's body, the same pigment that makes human hair and eyes brown. It does the same for quail--in both Rosetta and Tibetan, it will make the entire bird a dark brown color, including the eyes, beak, and skin on the feet. Each feather will have slight black horizontal barring, giving the bird an even darker look. There is a wide range of natural color for Tibetan and Rosetta, from a reddish brown to a deep chocolate, but in almost all cases, the darker brown is preferred.
EB is often considered to be a base pattern because it is on the E locus, and changes a significant amount of both patterning and color on the feather. It can be crossed with Fawn to yield Fawn-Enhancement, which creates a yellow horizonal barring pattern on each feather.
Rosetta:
In Rosetta (heterozygous EB), the bird will be almost entirely reddish brown, but will have light yellow shafts going
vertically up each feather (1). They will tend to be a slightly lighter color than Tibetan, and more of the facial markings may be present. Toes will usually be a mottled pink and brown, with more pink than on a Tibetan bird.
These will not breed true, as they have one copy of EB (heterozygous).

Tibetan
In Tibetan, the entire bird will be a dark brown, and the shaft of each feather will be brown instead of yellow. They can be a very dark, almost black color, but are not to be confused with Recessive Black or TOF Blacks. Feet will be a dark brown, with some pink toes possible, but all brown is preferred. Facial markings will still be present but often hard to see.

These will breed true.
Breeding with Extended Brown:
Rosetta x Rosetta = 25% Pharaoh, 25% Tibetan, and 50% Rosetta.
(EB/eb+ x EB/eb+ = 25% eb+/eb+, 25% EB/EB, 50% EB/eb+)
Rosetta x Pharaoh = 50% Rosetta, 50% Pharaoh.
(EB/eb+ x eb+/eb+ = 50% EB/eb+, 50% eb+/eb+)
Tibetan x Tibetan = 100% Tibetan offspring.
(EB/EB x EB/EB = 100% EB/EB)
Tibetan x Pharaoh = 100% Rosetta offspring.
(EB/EB x eb+/eb+ = 100% EB/eb+)
Tibetan x Rosetta = 50% Rosetta and 50% Tibetan offspring.
(EB/EB x EB/eb+ = 50% EB/eb+, 50% EB/EB)
References & Further Reading
Truax, R.E., & Johnson, W.A. (1979). Genetics of Plumage Color Mutants in Japanese Quail. Poultry Science, 58, 1-9.
Somes, Ralph G. Jr., "International Registry of Poultry Genetic Stocks" (1988). Storrs Agricultural Experiment Station. 29. https://digitalcommons.lib.uconn.edu/saes/29
Nadeau NJ, Minvielle F, Mundy NI. Association of a Glu92Lys substitution in MC1R with extended brown in Japanese quail (Coturnix japonica). Anim Genet. 2006 Jun;37(3):287-9. doi: 10.1111/j.1365-2052.2006.01442.x. PMID: 16734695.
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