Green/Blue Eggshell Coloration in Chickens

Eggshell coloration in chickens results from the deposition of pigments during egg formation in the oviduct. Green/blue coloration is primarily caused by the pigment biliverdin, which is produced during the breakdown of heme. Unlike the brown pigment (protoporphyrin), which is deposited only on the surface of the shell, biliverdin is present throughout the entire thickness of the shell. Therefore, green/blue eggs are colored not only on the surface but also within the shell.

The genetic basis of green/blue eggshell coloration is associated with the SLCO1B3 gene, which encodes a transport protein responsible for the transfer of biliverdin into the shell during its formation. This trait is traditionally referred to as the O allele (oocyan) and is inherited in an autosomal dominant manner. This means that the presence of a single copy of this allele is sufficient for a hen to lay green/blue eggs. Individuals with the genotype O/O or O/o produce green/blue eggs, while individuals with the genotype o/o lay eggs without the green/blue pigment, with the resulting color (white or brown) depending on other genetic factors.

At the molecular level, green/blue coloration is caused by the insertion of a retroviral element (EAV-HP) into the regulatory region of the SLCO1B3 gene. This mutation leads to active expression of the gene in the shell gland, enabling the deposition of biliverdin into the forming eggshell. Interestingly, a similar phenotype may have arisen through different regulatory mechanisms of this gene, suggesting that green/blue egg coloration may have evolved independently in different chicken lineages.

Green eggshell coloration, often referred to as “olive,” does not arise from a separate gene but from the combination of the green/blue pigment biliverdin and the brown pigment protoporphyrin. If a hen carries the O allele and also has a genetic predisposition to deposit brown pigment, the resulting eggshell color is green. The specific shade may vary depending on the amount of each pigment—from light green to dark olive.

Genetic testing can help determine the presence of variants of the SLCO1B3 gene and thus facilitate targeted breeding of chickens laying eggs of the desired color.