Why Are Chicken Eggs Different Colors?

From white and brown to pinkish and blue, chicken eggs come in all kinds of colors, especially if you shop at the overpriced organic food store. Despite what you’ve heard, it’s not directly associated with the color of the hen (when was the last time you saw a blue chicken?).

The main factor behind eggshell coloration is genetics, although a few environmental elements play a role too.

Scientists have discovered at least seven genes that are involved in chicken eggshell color: CPOX , FECH , BCRP , HRG1 , FLVCR , SLCO1A2 , and SLCO1C1 .

When it comes to brown and white eggs, their pigments are primarily influenced by the production of an organic pigment called protoporphyrin. The pigment is derived from heme, an iron-containing compound in the blood's hemoglobin. Two other pigments, biliverdin and biliverdin-zinc chelate, can also sway a chicken’s egg color, but protoporphyrin is typically deemed to be the most influential.

Generally speaking, white eggs have very little protoporphyrin, while brown eggs are abundant in it. A high expression of the CPOX gene results in more protoporphyrinogen and, therefore, a brown shell color. On the other hand, high expression of the FECH gene results in lower protoporphyrinogen levels and a lighter eggshell color.

However, genetics is rarely this straightforward forward and the other five genes – BCRP , HRG1 , FLVCR , SLCO1A2 , and SLCO1C1 – also impact the coloration by influencing the heme transporters, which deliver heme to the various parts of a biological cell.

Different breeds of chickens can be associated with specific eggshell colors because they harbor a particular collection of genes. Leghorn chickens, for example, lay white eggs, while Rhode Island Reds lay brown eggs due to the genetic variation within the population. Marans chickens, a fancy French breed revered for their exceptionally dark brown eggs, are the product of selective breeding that has optimized the expression of genes that pump out protoporphyrin.

Along with genetics, egg color can be shaped by numerous environmental factors, including the hen's age, stress levels, disease, and diet. Iron levels in their food are particularly important for chickens that lay brown eggs, as are certain probiotics, specifically Bacillus subtilis.

Stress, old age, and illness can also result in less pigmentation in eggs, although white eggs don’t necessarily mean that a hen was “unhappy” or sick. Likewise, shell color doesn't explicitly affect an egg's flavor. If you are mindful of taste, then you're better off focusing on the farm's quality, rather than the color of eggs they produce.

Blue eggs are a bit of an exception to all the genetic rules mentioned above. Just a few breeds – including the Araucana chicken from Chile, plus the Dongxiang and Lushi chickens from China – lay blue eggs and it’s all thanks to a gene called oocyan .

Remarkably, the gene’s expression results from a historic infection by EAV-HP, a retrovirus that managed to integrate into the genome of the chicken breed and became passed down through generations via their DNA. The infection most likely occurred hundreds of years ago and was spotted by keen-eyed farmers who selectively bred the chickens to keep the phenotype rolling.

Rest assured, the virus is harmless and blue eggs are perfectly safe to eat (if cooked correctly). Just like chickens, the human genome is filled with examples of endogenous retroviruses from historic infections; up to 8 percent of the human genome is made from sequences of viral origin. Unfortunately, none of them make us produce blue eggs, at least to our knowledge.