These photos are in part collected here for my own records, but I hope they can also be educational when others are pondering the changes they may see between chick fluff and adult feathers. They grow up so quickly...
0001
Born on St. Paddy's Day, I was very tempted to keep this little one, my first successful 2020 hatchling.
CHICKENS & EGGS
Chicken Egg Color Inheritance
There is nothing more delightful than a carton full of eggs in every hue, from bright blues to rich olives to deep chocolatey browns. But where do all those colors come from? How can you build a flock ​​​that will lay a colorful array of eggs? Let's explore.​​​
​
Sections: Pigments | Steps to Pigmentation | Genetics | Egg Color by Breed
​
Pigments involved in Egg Shell Coloring
First let's quickly touch on the different pigments involved in egg shell coloration.​​
Protoporphyrin IX​
-
Responsible for brown, reddish-brown, and speckled coloration.
-
Derived from hemoglobin and deposited on the eggshell during its formation.
Biliverdin
​
-
Responsible for blue and green eggshells.
-
Produced from the breakdown of hemoglobin.
-
Incorporated throughout the whole egg shell.
Biliverdin-Zinc
Chelate
-
Formed when biliverdin binds with zinc ions.
-
Can influence the shade and stability of the blue or green color in eggshells, though its specific role is less well-documented.
Uroporphyrin
​
-
A derivative of porphyrin, contributing to pinkish or reddish tints in eggshells.
-
Found in smaller amounts and often combined with other pigments.
​​
The Two Processes involved in Egg Shell Coloring
To understand how our eggs end up such a lovely array of colors, it's important to understand the two separate processes that color our eggs.
STEP
1
Blue Pigment
Eggshells form around the egg in the egg's last days inside a hen. If the hen carries any blue shell genetics, a blue pigment will be incorporated into the shell as it forms. This is why blue and green eggs are blue on the inside of their shell.
STEP
2
Brown Pigment
Brown pigment is added to an egg after the egg has fully finished forming. This is why it only appears on the outside of an egg shell. Brown pigment over a blue shell can result in eggs which appear minty green or even dark olive.
The Genetics of Egg Shell Coloring
To understand how our eggs end up such a lovely array of colors, it's important to understand the two separate processes that color our eggs.
​
​
The Inheritance of Blue Egg Shell Coloring
The rest of this article will assume you have a very basic working knowledge of inheritance and genetics. If you need a bit of a refresher, click here. If you'd like to learn more about genetics, my friend Jess has authored and illustrated an incredible website on genetics. She focuses on canine color genetics, but it can help improve the understanding of matters of inheritance in any species.
​​
The blue egg color in chickens is primarily determined by a specific gene called Oocyan. This gene is responsible for the deposition of a pigment called biliverdin in the eggshell, giving it a blue or greenish-blue color. The inheritance of blue egg color follows a pattern of simple dominance, with blue (O) being dominant over white (o).
Since the first "layer" of color produced on an egg is either blue or white, we'll discuss this first.​​​
OO x Oo
If one parent is homozygous for blue eggs (OO) and one parent is heterozygous (Oo) for blue eggs, the Punnett square would look like this:
o
O
O
OO
Oo
O
OO
Oo
Genotypes:
-
two OO blue egg layers
homozygous dominant -
two Oo blue egg layers
heterozygous
Phenotypes:
-
4/4 blue egg layers (OO & Oo)
Probability:
-
100% chance the offspring will be blue egg layers
Oo x Oo
If both parents are heterozygous (Oo), meaning they carry one allele for blue eggs and one for white eggs, the Punnett square would look like this:
O
o
O
OO
Oo
o
Oo
oo
Genotypes:
-
one OO blue egg layer
homozygous dominant -
two Oo blue egg layers
heterozygous -
one oo white egg layer
homozygous recessive
Phenotypes:
-
1/4 blue egg layers (OO & Oo)
-
3/4 white egg layers (oo)
Probability:
-
75% chance the offspring will be blue egg layers
-
25% chance the offspring will be white egg layers
Oo x oo
If one parent is homozygous for white eggs (oo) and one parent is heterozygous (Oo) for blue eggs, the Punnett square would look like this:
o
o
O
Oo
Oo
o
oo
oo
Genotypes:​
-
two Oo blue egg layers
heterozygous -
two oo white egg layers
homozygous recessive
Phenotypes:
-
2/4 blue egg layers (Oo)
-
2/4 white egg layers (oo)
Probability:
-
50% chance the offspring will be blue egg layers
-
50% chance the offspring will be white egg layers
What About The Greens and Olives and Browns?
The "base" color of an egg shell is either blue or white, as determined by Oocyan (explained above). This color is a part of the entire eggshell, which is why the shells of blue eggs ​appear blue on the inside as well as the outside.
​
Other colors are produced by the application of porphyrin to the egg after it has finished forming. You can think of it almost as a spray finish, with a varying shades and thicknesses of brown being applied to the shell.
​
Hens whose genes instruct them to apply a very thick/dark layer of brown can produce the deep chocolatey eggs of Copper Marans, while olive eggs are produced by a thinner layer of brown over a blue shell. Most of the brown eggs we are familiar with from the grocery store are a lighter application of brown applied on a white egg.​​
Multiple genes determine the exact amount and shade of brown that a hen will produce, so determining the shade of an egg is not a matter of simple dominance. (If you are here to learn how to produce olive eggers, here's the short and sweet cheat: use darker brown layers.)
​
To complicate things, some of the genes involved are sex-linked, meaning that
​
​
​​​​
