B Locus: Pigeon Base Colors
Pigeon base colors are on the sex chromosomes. There are three main base colors: Blue (black), Ash Red, and Brown.
This is the same locus as chocolate chickens and chocolate ducks, and chocolate turkeys and roux quail and probably buff geese as well.
To begin with I should explain something about pigeons. There are three ways pigment is deposited: smooth spread, coarse spread, and clumped pigment.
Smooth spread includes the wing tips and tail bars. Since the pigment is spread evenly it appears black to us on a usual blue (black) pigmented pigeon. It has a nice gradient which is why it's called "smooth."
Coarse spread makes up the pattern area of the bird on the wing shield. Coarse spread areas are also spread evenly, which appears as black. However, it isn't known as "smooth" spread because there are no gradients.
"Clumped" pigment is all the non-spread pigmented areas of the bird. On a "blue" pigeon these are the blue areas. These areas don't necessarily have less pigment, but the pigment is clumped together in islands. The effect of the microscopic black islands and the white areas in between appears as grey to the naked eye. (Think about how they make grey color in graphic novels: black dots on a white background.)
The first color I'll start with is wild type. Blue: b+ (Sometimes pigeon people don't even bother to write the "b" and just symbolize wild type with a "+" symbol.)
This color is often called "blue" but it really has black pigment. (The blue appearance is merely the clumping of black pigment.
This pigeon has black smooth and coarse spread areas and "blue" (grey) clumped pigment.
Blue Bar: (wild type) b+
Feral Blue Rock Pigeon by Vedant Kasambe DSCN0149 (16).jpg
Blue Check
Ash Red: B^A
An ash red pigeon produces pheomelanin pigment in the coarse spread areas and produces very little eumelanin pigment. This creates a red pattern on the neck and wing shield. The bird will produce far less clumped pigment and is lighter overall. The bird produces no "smooth spread" pattern, so it lacks the dark tail bar and wing gradient.
Homozygous and hemizygous ash reds look like this:
Ash Red Bar: B^A/B^ or B^A/-
Italian owl(mealie).jpg by Jim Gifford
However, it's only mostly dominant. Males with a copy of blue or brown will have blue or brown flecks.
B^A/b+: Ash red cock heterozygous for blue, note the flecks in his tail
Brown: b
This is actually a rare color, probably because it's unpopular. Most "brown" pigeons you see are actually red. Brown is sex-linked recessive, so only homozygous males and hemizygous females can express brown pigment.
Brown changes the shape of eumelanin pigment, making it brown rather than black. Brown pigment fades easily in the sunlight.
Brown also usually gives a "false pearl eye" meaning the bird produces no yellow pigment in the eye. So instead of a yellow or orange eye, it will have a red or white eye.
Brown Bar: b
This photo is from this article about the Brown gene, which goes into much more depth than I have.