In the first article of the Genetics Mini-Series, I discussed the genes responsible for egg shell color, the compounds which are involved in producing those colors, and how those colors behave during the egg laying cycle. In the second article on egg shell color, I discussed basic autosomal dominance and breeding for blue egg color. This week, I will briefly review how to obtain other egg colors.
Genetics of Egg Shell Color Part 3: Breeding For Other Egg Colors
Genetics Mini-Series Article #3
The most simplified, and thus not-entirely-accurate, explanation of egg color breeding appears in the chart above. Blue egg shells (O) are dominant over white egg shells (o). The brown outer coating is governed by at least 13 genes, and so the generation resulting from a cross will lay eggs with an intermediate level of darkness. Additionally, 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.1, 2 I discussed this and the basics of egg color in the first article of the Genetics Mini-Series.
White Egg Shells (o, wild type)
White egg shells have no brown pigment over the surface or blue pigment within the shell. However, sometimes even white-egg-layers can lay a slightly brown or tinted egg after a long period of non-laying because of a build-up of the brown pigment, protoporphyrin which is derived from hemoglobin or blood, within their reproductive tract.
Pink, Plum, Purple, and Gray
Current research acknowledges only two base shell colors: white and blue. However, pink eggs are common among Easter Eggers, purple or lavender eggs sometimes result from barnyard crosses, and Croad Langshans lay plum colored eggs. While it may be that a pink pigment can permeate the shell (see this forum discussion), pink coloring usually results from the bloom or cuticle which is a protein-like mucous coating applied by the hen’s reproductive tract just before the egg is laid which protects the egg from bacteria.3, 4 (This is why it is best not to wash clean eggs; washing removes the protective cuticle.)
This cuticle, when applied over a white egg shell, creates a pink-colored egg. Just as thicker layers of protoporphyrin result in a darker brown egg, thicker layers of bloom result in a darker plum-colored egg as in that of the Croad Langshan (a breed which was actually used in the development of the Marans.) Gray appears to be the result of a thick layer of bloom over a blue or green egg, as you can see in the photo on the third page of forum thread above.
Green and Olive
Many people love the dark green eggs. Some breeders are crossing dark brown egg layers and blue egg layers to create what is termed “olive eggers.” These mixed-breed chickens carry blue egg genes, often accompanied by pea combs, and the genetics for thick, dark brown coating. This cross is most successfully accomplished with Ameraucana blue layers from a line with the emphasis on egg color more than conformation and a Marans rooster from a dark-egg laying line. The initial cross is usually not as dark as breeders would like, so you often see the F1 (offspring of the initial cross, first filial generation) pullets being bred back to their Marans father to darken the eggs of the F2 generation. As was discussed in the previous GMS post, if the blue egg layer is heterozygous (Oo), then some offspring will not inherit the blue-egg laying gene needed under the brown to make green. Selecting offspring with pea combs is a good way to increase the chances of selecting those with the blue-egg gene.
Some breeders are trying to make true breeding olive eggers which will have a consistent shape, look, and egg color from generation to generation. Some lines are barred with beards such as these from Froggy Bog Farm. This can be accomplished by crossing Cuckoo Marans with a solid colored Ameraucana, usually black to support the barring. If you use Cuckoo Marans hens with a black Ameraucana rooster, your first generation would be sex-linkedand all of the male offspring would be barred and have a white dot on their heads as chicks. (I’ll explain sex-linkage in the next post.) If you use black Ameraucana hens with a Cuckoo Marans rooster then it would be easy to breed back the daughters to their father to further darken the eggs. The F1 generation would not be sex-linked; all of the chicks would be barred and have a white dot on their heads. Other breeders are using wheaten Ameraucanas crossed with wheaten Marans or wheaten Penedescencas to create wheaten olive eggers such as these at White Mountains Ranch. However, these layers relatively dark eggs are of a medium brown tone.
These past three weeks have been a basic primer in genetics focusing on egg color. In the upcoming weeks, I will address sex-linkage, shank color, and explain a bit about DNA and chromosomes.