Understanding egg color genetics

DarJones

Songster
Jan 24, 2021
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There is a ton of discussion about egg color genetics on this forum. It is about 80% right, but that leaves a LOT of errors being stated. Here are some points for discussion.

All chicken eggs start out as white. This is based on the color of the calcium based eggshell. White egg color is a complete biopath where special cells secrete the eggshell to surround the egg yolk and egg white. It is important to understand that the base color of eggs is ALWAYS white. There are at least 3 variants of white eggs. The brightest white is usually found in Leghorns and other breeds with similar genetic background. There is a washed out white that I have seen most in BB Red Games. From this, take away that all chicken eggs start out white.

There are two variants of the blue egg gene on chromosome one. The South American variant of the blue egg gene is common in Ameraucanas, easter eggers, and other breeds common in the U.S. The Asian variant is almost identical to the SA blue egg gene, but is offset by one step on the chromosome. Look up the asian breeds that carry this version of blue eggs. There is quite a bit of detail online about production of blue egg color. Both blue egg variants are dominant over white. You can think of blue egg as an on/off switch. If one copy of the blue egg gene is present, then all eggs will be blue. Two copies may get slightly more intense blue color, but is otherwise indistinguishable. If there are no blue genes, then the base color white will be expressed.

Brown eggs are caused by porphyrin which is deposited as a coating on top of the white or blue egg. There is a complete porphyrin biopath which basically means that there are multiple genes involved in production of porphyrin. There are probably many more than the 13 genes that are discussed elsewhere. At least one (probably 2 or more) porphyrin gene is on the sex chromosomes (W/Z). This means that reciprocal crosses between two different breeds using for example a male Silver Laced Wyandotte on a blue egg laying breed will give different results than crossing a blue egg rooster to Silver Laced Wyandotte females. Now here is where it gets sticky. All chickens have the porphyrin biopath! Think about this for a minute. How is it possible to get white eggs? If key genes are interrupted in the porphyrin biopath, it can be disabled which causes the underlying egg shell color, whether white or blue, to be expressed. There are duration genes in the poprhyrin biopath. You can suss this out by studying the results from Bailey back in the 1920's. The intensity of the brown color is determined by how many poprhyrin secreting cells are in the duct the egg passes through. I was able to find evidence that at least 3 genes are involved in duration of porphyrin production. There is also at least 1 gene that results in over-production of porphryin which is found in Marans. At least one variant that turns off the porphyrin biopath is from a chromosome translocation. Chickens with the translocation produce significantly fewer and smaller eggs. The translocation is probably present in a percentage of Cream Legbars which explains why it is so hard to improve egg production in this breed.

What about green eggs? Green is from expression of the porphryin biopath on top of the blue egg gene on top of the white eggshell base color. Varying levels of green can be produced based on modifier genes in the porphyrin biopath. It is not possible to get green eggs from an F1 cross involving Leghorns with their intense white eggshells crossing to a brown/blue/green egg layer. The intense white appears to be a single gene that is highly expressed in all offspring resulting in very little porphyrin being deposited on the eggshell. The result is a blue egg with a very slight tan tint. If these tinted eggs are dipped in water and wiped dry with a paper towel, the tinting becomes highly visible. Please note that in further generations (F2, F3, etc) it is possible to increase the tint level or to select against it back in the direction of sky blue eggs.

What happens when Silver Laced Wyandottes are crossed to Blue Egg laying Brown Leghorns from UARK? The segregating genes cause a plethora of feather colors with varying mixes of Silver/Gold which is sex linked, varying feather colors of white, black, red, brown, and even yellow showed up in the F3. Iridescence in the feathers was highly variable. Egg colors that segregated out from the mix ranged from pure white like Leghorns, sky blue, and every imaginable variation of brown tinted eggs from almost imperceptible tan to rich brown eggs typical of Wyandottes. I am now up to the F6 generation of selection with 2 backcrosses to Silver laced Wyandottes. I still have several brown hens that look visually like Golden Wyandottes. I keep them because a cross to a Silver rooster produces Silver offspring. Most of my hens are physically colored like Silver Laced Wyandottes with the Columbian, Pattern, and Melanotic genes fully expressed. I am still working to eliminate the salt and pepper effect of the partridge gene that is in Brown Leghorns. At this time, I still have 3 hens laying brown eggs. The rest are laying varying shades of blue eggs. I have about 30 young birds of varying ages with a few due to start laying within the month and others that are nearly 3 months old. I am putting 50 to 60 eggs per week in the incubator. The number of eggs should increase as spring laying peak hits over the next few weeks. I will try to hatch at least 500 eggs this year to have an abundance for selection. With a bit of luck, I will wind up with about 50 Silver Laced Wyandotte colored birds that lay varying shades of blue eggs.

Genes involved:
Blue on chromosome 1 which I brought in via the cross to UARK brown Leghorns
Rose comb on chromosome 7 ( an inversion causes rose comb and whacks a sperm gene in the process causing reduced sperm viability)
Straight comb on chromosome 1 is partially linked to the blue egg gene with linkage broken in Silverudds, UARK Brown Leghorns, and Cream Legbars
Inverted rose comb which is a background gene in Jerry Foley's Silver Laced Wyandottes, very useful as selecting against it produces roosters with blue genes
Melanotic which produces the intense black background color in Silver Laced Wyandottes
Pattern which produces the laced effect of black/white outlined ovals
Columbian which causes the black/white effect on hackle feathers
Red not to be confused with brown, red is present in Brown Leghorns, usually espressed on the wings
Brown different to red and is typically expressed on body feathers
Partridge is a background gene in the Brown Leghorns
White egg from the Brown Leghorn background
There are a ton of other genes I could add to the list, but they are bit players in the overall selection effort where the above genes are major players.
 
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DarJones

Songster
Jan 24, 2021
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Study it carefully. I don't know nearly enough about the genetics yet, but I am learning. The one statement in the above that I am a bit uncertain about is the partridge gene. I know what partridge looks like. What I don't know for sure is whether the gene from Brown Leghorns is actually the same as the gene commonly described as partridge. Said another way, I suspect that partridge is actually from a combination of 2 or more genes interacting. I have not investigated this in detail since my intent is to breed out the partridge gene(s) from my birds in favor of the pure patterned colors as in Silver Laced Wyandottes.

I agree that references would be highly useful. Much of what I stated can be found in publications readily available on the internet. Some of what I state is from extrapolation and from my own experience. It is possible that parts of the above are wrong. What I am confident of is:

1. The egg biopath in chickens produces white eggshells.
2. The porphyrin biopath in chickens exists in all extant chickens.
3. The blue egg gene has at least 2 variants commonly described as Asian and South American.
4. It is possible to interrupt the porphyrin biopath in several places which produces various colors of white to tinted eggs.
5. Bright blue eggs are from combining the blue egg gene with one or more genes that disable the porphyrin biopath, i.e. white combined with blue.
 

NatJ

Crossing the Road
5 Years
Mar 20, 2017
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The one statement in the above that I am a bit uncertain about is the partridge gene. I know what partridge looks like. What I don't know for sure is whether the gene from Brown Leghorns is actually the same as the gene commonly described as partridge. Said another way, I suspect that partridge is actually from a combination of 2 or more genes interacting. I have not investigated this in detail since my intent is to breed out the partridge gene(s) from my birds in favor of the pure patterned colors as in Silver Laced Wyandottes.

I turned up an old post that might help:
In the US, partridge is a pattern called penciling. There are three genes that are needed by a bird to make the partridge pattern, the eb or brown gene , which at one time was called partridge, the sex linked gold gene (s+) and the pattern gene (Pg). There are some other genes needed to make very good penciling but they are not documented. In Europe partridge means wild type . I forget if it means the same in Australia.

Take a look at a partridge rock and you will see the pattern.

Your Brown Leghorns are probably the wildtype, so e+/e+ and no pattern gene.

Have you played with the genetics calculator?
http://kippenjungle.nl/breeds/crossbreeds.html
I've found it helpful to just change the genes and watch the pictures & descriptions change, when learning about the feather-color genes.

The calculator can show the results of crosses, which some people find useful but I rarely do, because I'm already used to working Punnett squares or percents for whichever traits I care about at the time.
 

DarJones

Songster
Jan 24, 2021
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One of those really interesting things to read about is that Punnett - meaning the guy who showed how to use "Punnett" squares - worked with blue egg layers. I had a lot of fun a few years ago reading where he documented that blue egg is a dominant trait and the steps he took to develop true breeding blue egg layers.
 

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