GMS1: Genetics of Egg Color

One day in early spring, I got 9 eggs in one day from my nine pullets.  Looking over the variety of colors and shapes, I decided I ought to write up a bit about egg shell colors and do a bit of research.  This led me to muse about all of the other chicken genetics topics which I know a little to not-so-much about, and so I’ve decided to do a mini-series on chicken genetics interspersed with my regular posts.  As a teacher, I know that I never learn anything so well as when I have to explain it to someone else.  So, I’ll write up genetics information here, those who know better can chime in, and I will provide references so that you can read further on the topics with interest you.  As to not overwhelm with technical jargon, I’ll try to weave the vocabulary into the articles as we go.  My goal is to post Genetics Mini-Series (GSM) posts on Sundays throughout the summer.

Genetics of Egg Shell Color Part 1:

Genetics Mini-Series Article #1

White Egg Shells (o, wild type)

Most of us grew up with white table eggs.  Egg shell is made almost entirely from calcium carbonate (CaCO₃), which is also the main component in the shells of marine animals, pearls, and chalk.  About 5% of egg shell is made up from other minerals such as calcium phosphate, magnesium carbonate, and various proteins.¹ White eggs are the wild type, meaning that it is “the normal, non-mutated version of a gene common in nature.”²  The junglefowl from which domestic chickens descend lay white eggs.  When breeders discuss the genetic composition of a chicken, they often refer to gene symbols.  The genetic symbol for a the white egg shell is o.  (You can find a fairly comprehensive list of chicken gene symbols here.  More about gene symbols: Dominant traits are designated by a capital letter and recessive traits by a lowercase letter.  Sometimes the gene symbols for the wild type are shown with a “+” symbol, as in o+, so that you can identify not only whether it is dominant or recessive but also whether is it the original or a mutation.)

Egg color is often (not always) tied to chicken earlobe color (great article here).  Chickens who lay white eggs tend to have white earlobes.  You can see the white ears on the rose combed Brown Leghorn above.  Mediterranean breeds such as the Leghorn are bred to lay white eggs.³

Chicken Breeds Laying White Eggs: Leghorn, Ancona, Hamburg, Polish, Silkie, Icelandic, many others

Brown Egg Shells

In contrast, Continental (other areas of Europe) and American breeds tend to have red earlobes and lay tinted (off-white or tan) eggs or brown eggs.  Controlled by 13 or more genes⁴, the varying levels of brown pigmentation seen on tinted and brown eggs is actually a coating applied during the last few hours in the hen’s uterus and isthmus⁶. (You can see a diagram and read an article about the hen’s reproductive system here.)

This brown pigment is protoporphyrin, which is derived from hemoglobin or blood.  A little brown pigment over a white egg will produce a slightly tan egg, often called tinted; a lot of this pigment will produce the deep chocolate brown seen in Marans eggs.  Because this pigment is not part of the shell but more like a paint on the surface, it can actually be washed from the shell.  It can also be scratched, and some nesting materials, like wood chips, can mar the surface of a brown egg when the “paint” is still wet.  Because it is controlled by so many genes, brown egg color does not always act in a consistent way.  Generally, if you cross a light egg layer with a dark egg layer, the resulting generation will lay an intermediate color.  However, some breeds have a gene which can inhibit (lessen or block entirely) the expression of brown pigment, such as pr, a recessive sex-linked gene.^{7, 8}  (More on sex-linkage in a much later article.)

A chicken will carry the genetic code which instructs its body to create a certain amount of brown coating for its eggs.  This genetic information is passed down from both the cock and the hen, although some have noted a stronger influence from the father’s side in some breeds.^9(?)  However, possessing a specific code for a certain amount of brown pigment does not mean that the hen will lay this color consistently.  Eggs are generally at their darkest in the fall and winter when hens lay fewer eggs.  In late spring and summer, they are laying more heavily and more of their “paint” is used up.¹⁰  Older hens will also create less pigment; yet, their genetic material remains unchanged, and they can pass the genetics for the brown pigment they had in their youth onto their daughters (if the cock’s genetics support this as well.)  Thick coatings of brown pigment fill in some of the approximately 7,500 pores which allow an egg’s shell to be permeable.  Thus, dark eggs typical of the Marans hens actually have a lesser incidence of salmonella.  (It can also reduce evaporation from the egg during incubation, which is why Renee Caldwell recommends sanding the outside of shipped dark eggs prior to incubation, explained here, #5.)

Chicken Breeds Laying Brown Eggs: Euskal Oiloa, New Hampshire, Australorp, Cochin, Wyandotte, Plymouth Rock, many others

Chicken Breeds Laying Dark Brown Eggs: Marans, Barnevelder, Welsummer, Penedesenca

Blue Egg Shells (O)

Some bird from South America, such as the Araucana, carry a dominant gene for blue egg shells.   In addition to calcium carbonate and the other typical minerals, blue egg shells contain oocyanin (thus the genetic symbol O; note the root “cyan”) which is a byproduct from the body’s production of bile.  This blue pigment is not a coating like brown but is throughout the egg’s shell.  Thus, the blue egg is also blue inside (before the inner membranes dry to a papery white) and is more difficult to see into when candling.  However, like brown pigment, the amount of oocyanin will also lessen throughout a hen’s laying cycle, and she will lay progressively lighter blue eggs.  Interestingly, the gene for blue egg shells is closely related to the gene for a pea comb.  The genes for these two traits are relatively close together on the same chromosome.  A pea comb is a good predictor of the presence of the blue egg shell gene, but a chicken can have a pea comb and have white-shelled eggs like my cream-egg-laying Easter Egger pullet, Little White.  Interestingly, there is now a theory that blue-green pigments in egg shells help to protect the growing embryo from solar radiation, which may be why these pigments are often found in more breeds and species from tropical regions.¹¹

Chicken Breeds Laying Blue Eggs: Araucana, (true) Ameraucana, Cream Legbar, some Easter Eggers, rare South American breeds

Green and Olive Eggs

When a brown coating overlays a blue egg shell, the egg appears green.  If the brown coating is very dark, the egg appears olive.  Punnett observed this interaction when he conducted his initial experiments which concluded that blue egg shells were dominant. (You can read the original article by Punnett on my Google Docs.)  Today, many breeders are creating their own strain of mixed breeds to produce “olive eggers.”  Generally, this is a cross between an Ameraucana and a Marans.

Chicken Breeds Laying Green Eggs: Isbar, Easter Egger (mutt), Olive Egger (mutt), rare South American breeds

In the next installment of the Genetics Mini-Series, I’ll discuss the basic genetics of breeding using the topic of how to breed for blue egg color.