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Fifty Shades of Lay: The Genetic, Environmental, and Evolutionary Story of Chicken Egg Colors

A dozen colored eggs in a carton


This article explores hen "eggspressionism" – the art of color in chicken eggs! From pure whites to vibrant blues, each hue paints a genetic coding, environmental interplay, and evolutionary adaptation picture. Suitable for those with a foundational understanding of genetics and biology, this piece aims to unravel the chromatic mysteries of these everyday marvels.

Genetic Underpinnings

The color of a chicken egg is a genetic puzzle pieced together by the interaction of multiple genes. Researchers have identified three primary pigments responsible for the varied eggshell colors: protoporphyrin, biliverdin, and biliverdin-zinc chelate​​. Protoporphyrin predominantly colors brown eggshells, while biliverdin imparts a blue hue​​.

Seven genes - CPOX, FECH, BCRP, HRG1, FLVCR, SLCO1A2, and SLCO1C1 - have been identified as significant contributors to eggshell color. These genes regulate the synthesis and deposition of the pigments, influencing the egg's color intensity and shade​​. For instance, high expression of the CPOX gene results in more protoporphyrinogen and a darker brown shell. In contrast, the FECH gene modulates this by affecting the conversion of protoporphyrinogen into heme, resulting in a lighter shell color​​.

The Evolution of Blue Eggs

The blue egg phenotype, a relatively recent evolutionary development, emerged only 200-500 years ago in South American Mapuche fowl. This coloration is an example of an autosomal dominant trait introduced by an ancient retrovirus (EAV-HP) that was inserted in the chicken genome, leading to the over-expression of the SLCO1B3 gene, which is crucial for transport of biliverdin, the primary pigment in blue eggshells​​.

Environmental Impacts

While genetics set the stage, environmental factors are the directors that can alter the outcome. Factors such as diet, housing conditions, hen age, stress levels, and disease presence can all influence eggshell coloration. For instance, specific diets rich in probiotics like Bacillus subtilis and iron-chelating carriers can intensify the color of brown eggs, whereas environmental stressors might lead to paler eggs​​.

Evolutionary Perspectives

Examining egg coloration from an evolutionary standpoint reveals a complex narrative. The diversity in eggshell colors among chickens is thought to have developed for various purposes, such as camouflage, protection from predators, and signaling individual identity. These traits have been shaped and preserved through natural selection, adapting to environmental challenges and predator-prey dynamics​​.


Studying chicken egg colors is a journey through genetics, environmental science, and evolutionary biology. Understanding these factors fulfills scientific curiosity and has practical implications for poultry breeding and agriculture. It's a testament to the intricate relationship between organisms and their environments, shaped by millions of years of evolution and relatively recent genetic events.


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