A Review of Egg Color in Chickens
Tim Adkerson
Introduction
The different egg shell colors produced by chickens are normally
associated with a specific chicken breed. A particular breed should only
produce one basic egg shell color. White leghorns are famous for their large
white eggs, a Rhode Island Red lays a brown egg, an Ameraucana lays a
blue egg but no specific breed produces a green egg shell. Within each
breed, birds can lay different shades of the basic color. For example, brown
egg layers can produce a light brown (tinted) to a chocolate brown egg. The
basic color of an egg; brown, white, blue or green is determined by the genes
a bird carries. Normally, hen’s with white ear lobes lay a white egg and hens
with red ear lobes lay a brown egg. The association between earlobe color
and egg color has been bred into today’s various chicken breeds. The truth is
egg color is not genetically linked to ear lobe color. There are exceptions to
this association, for example, Holland’s have red ear lobes but produce a
white egg and Ameraucana have red ear lobes and produce a blue egg
(Standard of Perfection, 1998). The author has produced pullets with white
ear lobes that lay a brown egg. These exceptions clearly demonstrate that
humans have bred fowl so that white ear lobes are associated with white
shell color and red ear lobes are associated with brown shell color. As was
stated earlier, genetics will determine the basic color of an egg shell and that
color can be expressed in different hues. The following paragraphs will deal
with the pigments responsible for egg shell color and the genetics of each
color and explain why eggs shells have different color shades.
Pigments and Egg Color
Some chicken eggs are white; others chicken eggs are brown or blue
or even green. When the shell-less egg enters the uterus (shell gland), the
epithelial cells that line the uterus begin to add, over the next 15 hours or so,
a calcium carbonate shell to make a complete egg (Van Krey,1990). Once
the cells have finished adding a shell to the egg, the epithelial cells lining the
uterus add the cuticle and pigments to the shell. In the case of white eggs,
pigments are not added to the shell or added to the cuticle. The color of an
egg shell is determined by the lack of a pigment (white) or the type of
pigment that is added to the shell and or to the egg’s cuticle. The cuticle (a
bacterial barrier) is a liquid, protein and fat laden coating that is deposited on
the egg’s surface just before the egg is laid (Peebles ,1998; Van Krey,1990).
This coating appears as a moist layer on the surface of the freshly deposited
egg. It takes a few minutes for the cuticle to dry and become a solid covering.
Brown pigments, mostly protoporphyrin-IX, are added to the egg during the
90 minutes before oviposition; when the egg leaves the hen’s body (Butcher
and Miles, 1995). The protoporphyrin-IX pigment is synthesized in the
epithelial cells that line the uterus. Inside the epithelial cells, a heme protein
that was once part of a red blood cell is converted to protoporphyrin-IX
(Wand et al., 2009). Some brown shelled eggs will have pigments added to
the palisade or outer shell layer with the majority of the pigments being
added to the cuticle of the egg (Lang and Wells, 1987; Butcher and Miles,
1995). If the pigment is added to the surface (cuticle) of an egg, then with a
little scrubbing the pigments can be removed exposing the white or tinted
shell surface. Pigments added to the palisade layer (outside layer) of the
shell cannot be rubbed off because the pigments are a part of the shell
(Punnett and Bailey, 1920; Steggerda and Hollander, 1944).
Blue eggs are not uncommon and are produced by many different
species of birds (Harrison, 1985). Only a few chicken breeds found in
America lay a blue egg; notably the Araucana and the Ameraucana
(Standard of Perfection, 1998). There are undoubtedly other chicken breeds
that lay a blue egg; for example the auto-sexing Cream Legbar. There is very
little difference between a blue egg shell, a white egg shell or a brown egg
shell; the main difference being the color. Blue eggs shells are blue because
of a pigment called biliverdin-IX. This pigment is synthesized inside the
uterus epithelial cells from proteins that come from degenerating red blood
cells. Red blood cells can only be operational a certain length of time and
then they must be recycled by the bird’s body. Birds can convert a red blood
cell protein called heme into biliverdin-IX. As the shell is deposited, the
cells also add the blue pigment producing a shell that is blue from the inside
of the shell to the outside of the shell. The shell’s inside is just as blue as the
shell’s outside (Zhao, 2006; Kennedy and Vevers, 1973; Punnett and Bailey,
1920).
Green eggs are green because a bird produces a blue shell that is
covered with brown pigments. If the surface of green egg is rubbed, the
brown pigments can be removed exposing a blue shell. Green egg shell
producers generate both types of pigments; biliverdin-IX which is a part of
the shell and protoporphyrin-IX that covers the egg’s exterior surface. The
hue of a green egg is dependent upon the amount of brown pigment that is
added to the surface of the egg. Bluish green eggs are coved by very little
brown pigment while dark green or olive eggs have a heavier covering of
brown pigment (Punnett and Bailey, 1920).
Brown Egg Shell
The genetics of brown egg shell color has not been studied enough to
be completely understood. Especially the hereditary aspects of the very dark
brown egg producers like the Marans, Welsummer, Barnevelder and
Penedesenca. The following information will give the breeder a basic
understanding concerning the inheritance of egg shell color.
All brown egg layers and white egg layers carry two autosomal
recessive alleles called wild type oocyan (o+/o+). These alleles are located at
the oocyan locus or blue egg shell locus. Birds that are o+/o+ are
programmed to produce a white egg shell but there are also other genes that
can add brown pigment to the egg shell producing a brown egg. The other
allele located at the oocyan locus is the blue egg shell allele (O). The blue
egg shell locus determines if a bird has the potential to produce a white egg
shell or a blue egg shell. Birds that lay a blue egg are heterozygous oocyan
O/o+ or are homozygous oocyan O/O and do not carry genes for brown shell
color. Poultry that produce a blue egg shell must carry at least one dominant
blue egg shell or oocyan allele (O) (Bartlett et al. 1996). Other genes in the
bird determine if a bird will produce a brown egg shell or a green egg shell.
See Table 1.1. for a summary of egg shell color genes.
Two early investigators of egg shell color were Punnett and Bailey
(1920); they performed crosses of white egg layers and brown egg layers in
an attempt to come to some understanding about egg shell color. One of the
first crosses performed by Punnett and Bailey was a Black Langshan ♂ x
Light Brown Leghorn♀; the Langshan is a brown egg layer (not a dark
brown shell) and the Leghorn is a white egg layer. This cross produced F1
black hens that laid tinted eggs; there was very little variation between the
egg shell colors. The researchers also performed a sibling (F1 x F1) cross
which produced some interesting F2 results. The results of the F2 offspring
egg color are approximately the following: 26% white, 8% slightly tinted,
37% tinted, 21% dark tinted and 8% were almost as dark as the Langshan’s
brown eggs. To further complicate things, within the percentages there were
lighter to darker shades. For example, within the dark tinted group there
were different dark hues. In another cross of a Gold Penciled Hamburg ♂ x
Black Langshan ♀, the F1 produced eggs that were a darker tint than the
Black Langshan ♂ x Light Brown Leghorn♀ F1. The results of the Hamburg
♂ x Black Langshan ♀ F2 offspring egg color ranged from a white shell to
the Langshan’s brown egg shell color. The results of the F2 offspring egg
color are approximately the following: 6% white, 10% slightly tinted, 12%
tinted, 50 % dark tinted and 22% were almost as dark as the Langshan’s
brown eggs. Punnett and Bailey believed their work to be preliminary in
nature but does illustrate the following: brown shell color is due to a
dominant gene that is epistatic to the recessive white shell gene (o+). This is
evident because the F1 offspring from both crosses laid an egg that was
covered with a brown pigment. All three of the breeds used in the research
were o+/o+; the difference between the brown egg layer and the white egg
layers is the presence of an autosomal dominant gene for brown egg shell
color. The allele to the brown egg shell gene is a recessive gene that does not
add brown color to the egg shell.
It is the author’s suggestion that the gene for brown egg shell color is
incompletely dominant. There may be another incompletely dominant gene
that adds brown pigment to the egg shell (Wei et al., 1992). This second
gene would be a modifier of the brown egg shell gene. It is also the author’s
suggestion that the incompletely dominant nature of these two genes may
explain why there are numerous brown shades in the F2 brown egg layers in
both experiments. This would agree with Punnett’s and Bailey’s (1920)
suggestion that there may be one or two modifiers of the brown egg shell
gene. Another question arises as to why the Black Langshan ♂ x Light
Brown Leghorn♀ F1 cross segregated so many white egg shell layers. The
answer is that the Leghorn also carried a recessive autosomal gene that
inhibited the production of brown pigments (Wei et al., 1992). This
recessive gene turned off the production of brown shell pigments in many of
the white egg laying Black Langshan ♂ x Light Brown Leghorn♀ F2
offspring. This would explain why 26% of the Black Langshan ♂ x Light
Brown Leghorn ♀ F2 females produced a white egg. In the case of the Gold
Penciled Hamburg, this bird only carried the recessive alleles to the brown
egg shell gene and the modifier(s); the Hamburg did not carry any genes that
would add brown pigments to the egg shell therefore the egg shell was white.
It may also be noted that the F1 and F2 of the Gold Penciled Hamburg ♂ x
Black Langshan ♀ parents as a whole laid darker eggs than the F1 and F2 of
the Black Langshan ♂ x Light Brown Leghorn ♀ parents; it may be possible
that the Leghorns also carried another modifier that caused the eggs to be a
lighter color. This modifier may be the dominant autosomal gene that
Punnett (1933) described in his research with the Chilean blue egg laying
hen.
Research by Wei et al., (1992) was conducted in order to study the
genetics of tinted eggshell colors in two breeds of chickens laying white
shelled eggs; a White Leghorn line and an Ancona line. Reciprocal crosses
were made between the White Leghorn line and the Ancona line. The
F1 birds were intercrossed and F1 females were backcrossed to each of the
original lines. Distribution comparisons indicated that two major
autosomal loci affected the trait in these lines. The amount of pigment
deposition was controlled by one gene having incomplete dominance; the
other gene was homozygous recessive and completely inhibited pigment
deposition.
In summary, all chickens are recessive wild type oocyan o+/o+ or
heterozygous oocyan O/o+ or homozygous oocyan O/O. Brown and white
shell producers are o+/o+ while blue or green egg shell producers are O/o+ or
O/O (Bartlett et al. 1996; Punnett, 1933). Birds that produce white shelled
eggs may or may not carry an autosomal recessive brown pigment inhibiting
gene that is responsible for a white egg shell. Some birds may only carry
wild type recessive alleles for white shell color at a brown egg shell locus
and therefore produce a white egg shell; the birds do not carry any dominant
brown shell alleles henceforth the shell is white. Birds that carry the
recessive brown pigment inhibitor may also carry other genes that, in the
absence of the recessive brown pigment inhibitor, would normally add
brown pigment or modify the amount of brown pigment added to the egg
shell. The recessive brown pigment inhibitor gene prevents the eggs from
having a brown shell even if a bird carries dominant genes for brown shell
color. There is one documented autosomal incompletely dominant gene that
adds brown pigment to the egg shell. This gene may be modified by one or
more other genes (Wei et al., 1992; Punnett and Bailey, 1922). Punnett
(1933) has also suggested that there is a dominant autosomal gene that will
inhibit brown egg shell pigmentation. It would seem that there are at least 3
genes, one incompletely dominant and two dominant, that add brown
pigment to a shell, one locus (oocyan) that can determine if a egg is white or
blue, one recessive gene that inhibits brown egg shell pigments and one
dominant gene that inhibits brown egg shell color. More research needs to be
performed in order to make a firm case for some of the genes mentioned in
the preceding sentence.
Table 1.1. Suggested Possible Egg Shell Phenotypes and Their Genotypes
Shell
Phenotype
Oocyan
Alleles
Dark
Brown
Brown
Tinted
Blue
o+/o+
3
Blue
White
White
Green
Dark Green
o+/o+
o+/o+
O/O or O/o+
O/O or O/o+
o+/o+
o+/o+
O/O or O/o+
O/O or O/o+
Dominant
Brown Egg
Shell Alleles
Recessive
Brown Shell
Inhibitor
Dominant
Brown Shell
Inhibitor
Yes
No
No
Yes
Yes
2
No
Yes
Yes
2
No
Yes
Yes
No
No
No
No
Yes
Yes
Yes
Yes
Yes or No
No
No
Yes or No
1
Yes or No
Yes or No
Yes
No
1. Hens that produce the darkest brown eggs are homozygous for the brown egg shell
gene and homozygous for two modifiers of the brown egg shell gene.
2 The birds contain the wild type recessive alleles that prevent the production of brown
eggshell pigments therefore they do not produce eggs with brown shells. These genes are
different than the brown eggshell pigment inhibitor gene.
3 This is hypothetical and is based upon the author’s observations and Punnett (1933).
With brown eggs, genetics will determine if the egg shell is brown but
there are other factors that will establish if a shell is a dark shade of brown
or a light shade of brown. Research has identified reasons why chickens can
produce various shades of brown egg shells; most notably: age of the
chicken, stress upon the chicken, medication taken by a chicken, diet of the
chicken and disease (Butcher and Miles, 1995). There are other factors that
can cause various shades because not every hen of a specific breed will lay
the same shade of brown egg and even the shade of the eggs laid by an
individual will vary (Mills et al., 1991 and Nys et al., 1991). It is also
important to remember that different genetic lines can produce an egg shell
with different shades of brown (Solomon, 1997). This variance between
genetic lines would indicate that genetic selection can produce birds that lay
darker eggs or lighter eggs. This is apparent in production brown egg layers
which have been selected for uniform brown shelled eggs while broiler
breeder stock, which has not undergone genetic selection, produces a wide
range of brown shelled egg colors. The broiler’s egg shell color can range
from a bleached white to a dark brown (Butcher and Miles, 1995).
What does the age of a chicken have to do with the color of the egg?
When a pullet begins to lay eggs, the eggs are normally small but as the hen
ages the eggs she produces weigh more and become larger in size (Roland et
al., 1975; Fletcher et al., 1983; Sell et al., 1987). Solomon (1997) reported
that there is no evidence that suggests a variation in the amount of pigment
that is produced according to egg size. Or in other words, a chicken produces
the same amount of pigment if the egg she lays is small or large. The hen
will deposit the same amount of pigment over a larger surface area therefore
the egg shell’s color will become a lighter shade of brown. Research by
Odabaşi et al. (2007) supports the Solomon (1997) position; Odabaşi et al.
determined that: 1) during the first 10 months of the laying cycle, egg shell
pigmentation decreased and 2) an increase in egg weight produced a larger
eggshell surface area, which resulted in lighter colored eggs. Other
researchers reported similar results; the color of eggs shells from a given
flock turns paler with age of the flock (Lang and Wells, 1987; Solomon,
1997).
Stress is another factor that can alter the hue of a brown egg shell.
This color change may include eggs becoming a pale brown, a pink color or
the egg developing brown spots. The pink colored egg may be due to a
dusting of an amorphous calcium carbonate deposition (shell dusting) that
covers the egg (Hughs et al., 1986; Lang and Wells, 1987). Fear is the most
obvious stressor that the breeder can observe. Any number of things may
cause fear in a female chicken: loud noises, handling the chickens, relocation
of the chickens, introducing new chickens to a flock, harassment by
predators, and overcrowded conditions are just a few (Raynard and Savory,
1997; Butcher and Miles, 1995). When a female chicken is exposed to some
form of stress, the hen’s body reacts to the stress by releasing corticosterone
into the blood stream. The corticosterone then directly or indirectly effects
the female reproductive system. Long term exposure to corticosterone due to
stress may effect the female reproductive system differently than a single
stressful event (Shini et al., 2009; Raynard and Savory, 1997; Fahey and
Cheng, 2008). Stress can result in egg whitening and may be caused by the
premature termination of shell pigmentation; the egg shell gland (uterus)
would normally add more brown pigments to the shell but the shell gland
contracts prematurely expelling the egg (Mills et al., 1991 and Nys et al.,
1991). Social stress can also lead to delayed oviposition which may cause
shell abnormalities and a pink color in egg shells (Raynard and Savory, 1997;
Campo and Prieto, 2010; Butcher and Miles, 1995). Research has also
indicated that older chickens are better at handling stress and produce lighter
colored eggs less often than younger hens (Mills, et al., 1991).
It takes most hens over 25 hours to produce an egg; with some hens
taking almost as long as 30 hours (Rose, 1997). According to Butcher and
Miles (1995), it is during the last three to four hours of the egg production
cycle that is the most important with respect to adding the brown pigment to
an egg shell. The hen stops producing the shell approximately 90 minutes
before the egg is laid; it is at this time that the brown pigments begin to be
added to the shell. If the last 90 minutes is interrupted with stress, pale
brown eggs are produced because the shell gland contracts causing the egg
to leave the fowl’s body prematurely or the brown pigment is not added to
the egg (Mills et al., 1991 and Nys et al., 1991) .
Other factors can also cause egg shell color to be paler due to stress.
Red mite infestations, excessive heat, and even changing the feed offered to
birds can cause bird to lay paler eggs (Mertens, et al. 2010). Medication can
also inhibit egg color. The administration of nicarbazin an antibiotic for
coccidiosis can cause the rapid depigmentation of hen’s eggs. In one event,
Hyline Brown egg layers were raised on liter and were fed a diet that
unknowingly contained nicarbazin. It took about three days for the eggs to
go from a brown to a white color with some eggs becoming white by the
second day. When the nicarbazin was removed from the food supply, the
color rapidly returned to the eggs; some within 24 hours (Charlton, 2005;
Hughs et al., 1991). Disease can also inhibit the color of an egg. Infectious
bronchitis along with egg drop syndrome can cause egg shells to be paler in
color (Charlton, et al., Whittow, 1999). Even the addition of certain
enzymes that digest non-starch polysaccharides (fiber) to a barley-based diet
consumed by production laying hens can cause some lightening in the color
of the brown egg layer’s egg shells (Roberts and Choct, 2006) .
Dark Brown Egg Shell
Very little research has been carried out with respect to the inheritance
of dark shell egg color. Punnett (1933) performed a few crosses using a
Welsummer male over Chilean blue egg laying hens and back crosses of a
Welsummer male with the (Welsummer X Chilean) F1. The Chilean hens
did not produce brown egg coloring; there were no brown pigments found
on the surface of the hens’ blue eggs. The Welsummer X Chilean F1 pullets
produced the following types of eggs: olive (blue + deep brown), green (blue
+ brown), brown and tinted. The Welsummer X (Welsummer x Chilean) F1
back cross offspring pullets laid olive, deep brown and brown eggs.
According to Punnett, the Chilean hens may have carried a dominant
inhibitor of brown egg shell color which was inherited by some of the F1
pullets; this would explain why some F1 pullets produced tinted eggs. This
dominant inhibitor would cause a dark brown egg layer to produce a light
brown egg. Punnett also suggested that the hens that produced the olive eggs
and the deep brown eggs were homozygous for a major autosomal dominant
gene that is responsible for brown egg shell color and also homozygous
dominant for two other modifying genes; the expression of the major gene
would be a brown egg with the modifiers causing the addition of enough
brown pigments to produce a dark brown egg (Yang et al. 2009; Butcher and
Miles, 1995). Some of the Welsummer x (Welsummer x Chilean F1)
offspring produced a brown egg. These brown egg laying backcross pullets
may have been heterozygous for one or both of the brown egg modifiers and
homozygous for the major brown pigment gene.
Another part of Punnett’s (1933) research included the cross of a
Welsummer male over Light Sussex hens which laid light brown or tinted
eggs. The F1 pullets produced dark brown eggs but not as dark as
Welsummer eggs. This would indicate the Light Sussex did not carry the
dominant brown shell pigment inhibitor. The Light Sussex did not carry the
dominant modifiers and was most likely heterozygous for the brown egg
shell gene.
Blue Egg Shell
The inheritance of the blue egg shell trait is determined by the oocyan
allele O which is located at the oocyan or blue egg shell locus. The hen must
be heterozygous oocyan O/o+ or homozygous oocyan O/O in order to lay a
blue egg (Bartlett et al. 1996). Birds that are homozygous for the recessive
alleles o+/o+ can lay a white egg or a brown egg; o+/o+ birds will lay a
brown egg if they inherit genes for brown egg shell. Observe Table 1.1. for a
summary of egg shell inheritance. The earliest work concerning the genetics
of the blue egg shell was published by Punnett (1933). In Punnett’s study,
he crossed a Gold Penciled Hamburg (white egg) with a Chilean hen (blue
egg); this cross produced both white and blue egg layers. The fact that the
cross produced both blue and white egg layers indicated the Chilean hen was
heterozygous for blue shell (O/o+) and the Hamburg was homozygous
recessive for white shell (o+/o+) (Punnett, 1920). The Chilean hen could not
have been (O/O). If a homozygous (O/O) for blue egg shell hen is crossed
with a homozygous white egg shell bird (o+/o+), all of the F1 will carry an
allele for blue egg shell and be heterozygous (O/o+); every F1 hen will lay a
blue egg. The F1 (O/o+) males may or may not pass on the blue egg shell trait
to their F2 offspring; some of the F2 offspring will inherit an O allele while
others will inherit an o+.
Hypothetically, a blue egg layer may produce blue eggs if they carry
brown egg shell genes. Normally the combination of blue eggshell genes and
brown egg shell genes produces a green egg shell layer. In this case, the
author suggests that if a bird is homozygous (carried two genes) for brown
egg shell pigment inhibitor genes; the brown pigments would be eliminated
leaving a blue egg shell (Wei et al., 1992, author’s observation). Punnett
(1933) also suggests the blue egg shell producing hens in his study may have
carried a dominant autosomal gene that inhibits brown egg shell pigments.
This gene would lighten the hue and not eliminate the brown pigments from
the shell.
Research indicates that both the protoporphyrin-IX and biliverdin-IX
are synthesized in the epithelial cells of the uterus and both pigments are
produced from the same precursor (Zhao et al., 2006; Wand et al., 2009).
There is a difference in how the pigment is added to the egg shell.
Concerning blue egg shells, as the shell is deposited; the epithelial cells also
add the blue pigment producing a shell that is blue from the inside of the
shell to the outside of the shell. The shell’s inside is just as blue as the shell’s
outside (Zhao, 2006; Kennedy and Vevers, 1973; Punnett and Bailey, 1920).
With brown egg shells, pigments may be added to the palisade or outer shell
layer with the majority of the pigments being added to the cuticle of the egg
(Lang and Wells, 1987; Butcher and Miles, 1995). The following are
suggestions by the author: 1) blue egg shells may be effected in the same
way as brown egg shells; producing a lighter blue egg, 2) occurrences that
effect the last 3-4 hours before the egg is laid may have little effect upon the
blue egg shell color, 3) things that are long term like applied medication,
disease or long term stress may have a greater effect upon the blue egg shell
color causing the egg shell to be a lighter blue color.
Green Egg Shell
The hereditary aspects of green egg shell are different than white egg
shell, brown egg shell and blue egg shell. Green egg shells are produced by
birds that are heterozygous (split) oocyan O/o+ or homozygous oocyan O/O
for blue egg shell and carry genes for brown shell color (Punnett, 1933;
Bartlett et al. 1996). In order for a bird to produce a green egg shell, a bird
must carry genes for brown egg shell and blue egg shell. The blue and the
brown egg shell colors blend to make a green egg shell. The depth of the
green color is dependent upon the hue of the brown cuticle or in other words
the amount of brown pigment added to the cuticle. Birds that produce an egg
with very little brown pigment in the cuticle (tinted) and also produce a blue
egg shell will have a greenish tinged blue egg shell color while birds that
produce a dark brown cuticle will produce an olive colored egg. Just as in
birds that lay a brown egg, the brown pigments can be rubbed off the green
egg shell; but with green shelled eggs, the egg shell is blue and not tinted or
white (Punnett, 1933).
If a person wants to produce green egg laying fowl, it is important that
one bird carries brown egg genes and the other carries blue egg genes.
Crossing a bird that carries the genes for a brown tinted egg with a blue egg
layer will produce F1 pullets that lay a bluish green egg. Crossing a bird that
carries the genes for a brown egg with a blue egg layer will produce F1
pullets that lay a green egg. Crossing a bird that carries the genes for a dark
brown egg with a blue egg layer will produce F1 pullets that lay a dark green
or olive egg. The preceding would be true if the blue egg layer does not
carry a dominant brown egg pigment inhibitor. If the blue egg layer carries
an inhibitor, the F1 pullets will produce a blue egg. Just as pullets can
produce a wide range of brown egg shell colors so can a wide range of green
shelled eggs be produced; greenish blue to green to olive.
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