The E locus in Chickens!
The E extension locus is the locus that determine the base pattern in chickens. I will show you all some examples of E locus alleles from most dominant to most recessive. The E locus controls the extension of black eumelanin and gold pheomelanin on a bird. Some genes extend black over more of the bird and some restrict the black to certain areas and extend gold. Not all of these pictures are stellar. I'm doing my best.
E: Extended Black. Extended Black is the most dominant and darkest of the base patterns. It has been debated what an extended black would look like without additional melanizers. Black birds that you see at shows are solid black like this hen. These birds are usually E based, but black can actually be created on most base patterns with enough melanizers.
It is theorized that males without melanizers might have some gold leakage in the hackle and saddle feathers like this guy.
Photo taken from raph on Backyard Chickens. Click on the link for more of the discussion.
Extended black also lends epidermal melanin in the beaks and shanks.
E^R: Birchen is the second most dominant and the second darkest of the E alleles. It causes a mostly black bird. The hen has gold edging on the hackle feathers, along with down the front of the neck and breast.
On males it is much the same except the gold also extends to the shoulders and saddles as well:
This guy is lacking the breast lacing. I think a melanizing gene may be responsible.
There are a lot of amazing Birchen-based patterns. With the addition of a few other genes, you can get spangled birds, laced birds, and autosomal barred birds all with fully patterned tails. Also, in conjunction with id+ and a few other genes, Birchen can enable dark facial skin and eyes.
e+: Duckwing. Gold duckwing is the wildtype color, and it proves that wildtype is anything but basic.
Hens are a stippled brown with salmon breasts and gold necks with black striping
A male will have a black breast and tail and a gold neck and saddle. His wing is what "gold duckwing" gets its name for. He has red shoulders/bows, a black wing bar and a bay-colored wing bay.
e^Wh: Wheaten. There is thought to be more than one Wheaten mutation. Recessive Wheaten (e^Y), however, isn't commonly found in public hands, so we won't be focusing on it. In my experience Dominant Wheaten (e^Wh) is incompletely dominant with Duckwing. It is the lightest of all of the colors on the e locus. I think it looks very cool and exotic and has an interesting level of sexual dimorphism. Blue wheaten is one of my favorite colors.
The females are a lovely creamy color like this Ameraucana from the Ameraucana Alliance page (because apparently my only Wheaten photos are blue wheaten or red wheaten.)
Also from the Ameraucana Alliance page, Susie Winder's Wheaten male. Wheaten males and duckwing males can be hard to distinguish unless you know their background.
Since Wheaten has the most gold extension, it is the best base for creating patterns like buff and black-tailed red.
e^b: Partridge. The most recessive of the e alleles. Darker than e+ but lighter than E^R. It is hard to find one without the pattern gene. Partridge takes pattern well but always has a black tail.
Females are the same color as gold duckwing but with brown stippled breasts instead of salmon breasts. Here is a Partridge Drents hen from Kippenpagina.
Partridge males are much the same as gold duckwing, except with a touch more black in the hackle. Also from Kippenpagina.