Eye color is a
polygenic
phenotypic character and is
determined by the amount and type of
pigments in the
eye's
iris.
Humans and animals have many
phenotypic
variations in eye color, as blue, brown, green and others. These
variations constitute
phenotypic
traits.
The genetics of eye color are complicated and eye color is
determined by multiple genes. Some of the eye color genes include
EYCL1 (a green/blue eye color gene located on chromosome 19), EYCL2
(a brown eye color gene) and EYCL3 (a brown/blue eye color gene
located on chromosome 15). The once-held view that blue eye color
is a simple
recessive trait has been
shown to be wrong. The genetics of eye color are so complex that
almost any parent-child combination of eye colors can occur.
In
human eyes, these variations in color
are attributed to varying ratios of
eumelanin produced by
melanocytes in the iris. The brightly colored
eyes of many bird species are largely determined by other pigments,
such as
pteridines,
purines, and
carotenoids.
Three main elements within the iris contribute to its color: the
melanin content of the iris pigment
epithelium, the melanin content within the
iris stroma, and the cellular density of
the iris stroma. In eyes of all colors, the iris pigment epithelium
contains the black pigment, eumelanin. Color variations among
different irises are typically attributed to the melanin content
within the iris stroma. The density of cells within the stroma
affects how much light is absorbed by the underlying pigment
epithelium.
OCA2 gene polymorphism, close to
proximal
5′ regulatory region, explains most
human eye-color variation.
Genetic determination of eye color
Eye colours can range from the most common colour, brown, to the
least common, green. Rare genetic mutations can even lead to
unusual eye colours: black, red, or the appearance of violet. Eye
colour is an inherited trait influenced by more than one
gene. These genes are being sought using associations
to small changes in the genes themselves and in neighboring genes.
These changes are known as
single nucleotide
polymorphisms or SNPs. The actual number of genes that
contribute to eye color is currently unknown, but there are a few
likely candidates.
A study in Rotterdam
(2009) found that it was possible to predict the
color of eyes with more than 90% accuracy for brown and blue, using
just six SNPs (from six genes).
The gene
OCA2 ( ), when in a variant form the gene causes
the pink eye color and
hypopigmentation common in human
albinism. (The name of the gene is derived from the
disorder it causes, oculocutaneous albinism type II.) Different
SNPs within
OCA2 are strongly associated with blue and
green eyes as well as variations in
freckling,
mole
counts,
hair and
skin tone. The polymorphisms may be in an
OCA2 regulatory
sequence, where they may influence the expression of the gene
product, which in turn affects pigmentation. A specific mutation
within the
HERC2 gene, a gene that regulates
OCA2
expression, is partly responsible for blue eyes. Other genes
implicated in eye color variation are:
SLC24A4 and
TYR.
Blue eyes with a brown spot, green eyes and gray eyes are caused by
an entirely different part of the genome. As Eiberg said: "The SNP
rs12913832
[of the Herc2 gene] is found to be
associated with the brown and blue eye color, but this single DNA
variation cannot explain all the brown eye color variation from
dark brown over hazel to blue eyes with brown spots."
Classification of colors

The perception of color depends upon
various factors.
These are the same eyes; however, depending on the light and
surrounding hues, the eye color can appear quite different.
Iris color can provide a large amount of information about an
individual and a classification of various colors may be useful in
documenting pathological changes or determining how a person may
respond to various ocular pharmaceuticals. Various classification
systems have ranged from a basic
light or
dark
description to detailed gradings employing photographic standards
for comparison. Others have attempted to set objective standards of
color comparison.
As the
perception of color depends on
viewing conditions (e.g., the amount and kind of illumination, as
well as the hue of the surrounding environment), so is the
perception of eye color.
Eye colors range from the darkest shades of brown to the lightest
shades of blue. To meet the need for standardized classification,
at once simple yet detailed enough for research purposes, Seddon et
al. developed a graded system based on the predominant iris color
and the amount of brown or yellow pigment present. There are three
pigment colors that determine, depending on their proportion, the
outward appearance of the iris: brown, yellow, and blue. Green
irises, for example, have blue and some yellow. Brown irises
contain mostly brown. Eye color in animals other than
Homo sapiens are differently regulated. For
example, instead of blue as in humans,
autosomal recessive eye
color in the skunk
Corucia zebrata
is black, and the autosomal
dominant color is yellow-green.
Changes in eye color throughout life
In European populations, children are most commonly born with
unpigmented (blue) eyes. As the child develops, melanocytes, cells
found within the iris of human eyes (as well as skin and hair
follicles) slowly begin to produce melanin. Because melanocyte
cells continually produce pigment, eye color, in theory, can be
changed.
Changes (lightening or darkening) of eye colors during puberty,
early childhood, pregnancy, and sometimes after serious trauma
(like
heterochromia), do represent
cause for plausible argument to state that some eyes can or do
change, based on chemical reactions and hormonal changes within the
body. Most eye changes happen when the infant is around one year
old, although it can happen up to 3 years of age.
Studies on Caucasian twins, both fraternal and identical, have
shown that eye color over time can be subject to change, and major
demelanization of the iris may also be genetically determined. Most
eye color changes have been observed or reported in the Caucasian
population with
hazel eyes.
Eye color chart (Martin-Schultz scale)
Carleton Coon created this chart by
the Martin-Schultz scale often used in
physical anthropology.
- Light eyesEyes light and light mixed are 16-12 in Martin scale.
- Light: Grey, blue, green.
- Light-mixed:
- Very light-mixed (blue with grey or green or green with
grey)
- Light-mixed (light or very light-mixed with small admixture of
brown pigment)
- Mixed eyes
- Mixed: 12-6 in Martin scale. Mixture of light eyes (blue, grey
or green) with brown pigment when light and brown pigment are the
same level.
- Dark eyes
- Dark-mixed: 6-4 in Martin scale. Brown with small admixture of
light pigment.
- Dark: 4-1 in Martin scale. Brown (light brown and dark brown)
and very dark brown (almost black).
Amber

Human amber eyes displaying the yellow
pigments.

Amber iris
Amber eyes are of a solid color and have a strong yellowish/golden
and russet/coppery tint. This might be due to the deposition of the
yellow pigment called
lipochrome
in the iris (which is also found in green and violet eyes). Amber
eyes should not be confused with hazel eyes; although hazel eyes
may contain specks of amber or gold, they usually tend to comprise
many other colors, including green, brown and orange. Also, hazel
eyes may appear to shift in color and consist of flecks and
ripples; while amber eyes are of a solid gold hue.
The eyes of some pigeons contain yellow fluorescing pigments known
as
pteridines. The bright yellow eyes of
the
Great Horned Owl are thought to
be due to the presence of the pteridine pigment
xanthopterin within certain
chromatophores (called
xanthophores)
located in the iris stroma. In humans, yellowish specks or patches
are thought to be due to the pigment
lipofuscin, also known as
lipochrome.
Blue

A blue iris
In 2008, new research revealed that people with blue eyes have a
single common ancestor. Scientists tracked down a genetic mutation
that leads to blue eyes. The mutation occurred between 6,000 and
10,000 years ago; researchers state that before then, blue eyes did
not exist.
"Originally, we all had brown eyes", said
Hans Eiberg from the Department of Cellular and Molecular Medicine
at the University of
Copenhagen
. The mutation affected the
OCA2 gene, which is involved in the production of
melanin, the pigment that gives color to our hair, eyes and skin.
Eiberg stated, "A genetic mutation affecting the OCA2 gene in our
chromosomes resulted in the creation of a 'switch,' which literally
'turned off' the ability to produce brown eyes:
The genetic switch is located in the gene adjacent to
OCA2 and rather than completely turning off the gene, the switch
limits its action, which reduces the production of melanin in the
iris.
In effect, the turned-down switch diluted brown eyes to
blue.
If the OCA2 gene had been completely shut down, our
hair, eyes and skin would be melanin-less, a condition known as
albinism.
Blue eyes contain low amounts of melanin within the iris stroma;
longer wavelengths of light tend to be absorbed by the underlying
iris pigment epithelium, and shorter wavelengths are reflected and
undergo
Rayleigh scattering. The
type of melanin present is eumelanin. The inheritance pattern
followed by blue eyes is considered similar to that of a
recessive trait (in general, eye color inheritance
is considered a
polygenic trait,
meaning that it is controlled by the interactions of several genes,
not just one). Eiberg and colleagues showed in a study published in
Human Genetics that a mutation in the 86th
intron of the
HERC2 gene, which
is hypothesized to interact with the OCA2 gene
promoter, reduced expression of OCA2 with
subsequent reduction in melanin production.
The authors concluded
that the mutation may have arisen in a single individual in the
Near East or around the Black Sea
region 6,000-10,000 years ago during the Neolithic revolution.
Blue eyes
are most common in Northern Europe
and Central Europe and to a lesser
degree in Southern Europe, North America and southern Central Asia; Afghanistan
is a notable example. They're also found in
parts of North Africa, West Asia, and South
Asia, in particular the northern areas of India
and Pakistan
.It can
rarely occur as far south as Sri Lanka
. Moreover, blue/green/gray eyes can be found
within the population of
East Asia,
especially among Chinese, even Han Chinese,
[73673], allegedly
due to the constant wars and raids between Chinese Empire and
Indo-European Turk nomads from Eurasia Steppe . Those conflicts
could result in tens of thousands of people, mainly women and
children, being captured and brought back to the victor's society.
There are few scientific studies of light-color eyed Han
Chinese.
A 2002 study found the prevalence of blue eye color among
Caucasians in the United States to be 33.8 percent for those born
from 1936 through 1951 compared with 57.4 percent for those born
from 1899 through 1905. Blue eyes have become increasingly rare
among American children with only 1 out of every 6 – 16.6 percent
which is 49.8 million out of 300 million (22.4% of white
Americans) of the total United States population having blue eyes.
The plunge in the past few decades has taken place at a remarkable
rate. A century ago, 80 percent of people married within their
ethnic group. Blue eyes were routinely passed down, especially
among people of Western and
Northern
European ancestry. In the 1930s, eugenicists used the
disappearance of blue eyes as a rallying cry to support immigration
restrictions. They went so far as to map the parts of the country
with the highest and lowest percentage of blue-eyed people.
The outer surface of the iris of a blue eyed person is actually
clear, lacking the outer layer of pigmentation that is found in
brown eyes. Their color is caused by the inner layer of
pigmentation and the semi-opaque fibrous tissues which lay between
the two layers.
Brown

Brown iris

Light brown iris
Brown eyes are dominant in humans and, in many parts of the world
it is nearly the only iris color present.
It is less common in
countries around the Baltic
Sea
and in Scandinavia. Dark pigment of brown eyes
are most common in
East Asia ,
Africa and the Americas. In humans brown eyes contain
large amounts of melanin within the iris stroma, which serves to
absorb light at both shorter and longer wavelengths. Brown eyes are
the most common eye color, with over half of the world's population
having them.
Gray

A steel blue-gray iris

Gray iris under magnification,
exhibiting small amounts of yellow.
Gray eyes have less melanin than blue eyes, even though they are
considered a darker shade of blue (like blue-green).
Gray eyes are most
common in European Russia
, Finland
and the
Baltic States . It can also be found
to a lesser extent in parts of India
.
Under magnification, gray eyes exhibit small amounts of yellow and
brown color in the iris. Ultimately there are at least two things
that could determine gray eye color. The first is the amount of
melanin made and the second is the density of the proteins in the
stroma.
A gray iris may indicate the presence of a
uveitis. However, other visual signs make a uveitis
obvious. Gray iris color, as well as blue, are at increased risk of
uveal melanoma.
Visually, gray eyes often tend to appear to change between the
shades of blue, green and gray; this is because gray eyes are
extremely light, as mentioned before. The color change for gray
eyes is usually influenced by the lighting and the colors in the
surroundings (such as clothes, makeup, etc.).
Green

A green iris

Olive green eyes
Green eyes are the product of low to moderate amounts of melanin
and probably represent the interaction of multiple variants within
the
OCA2 and in other genes. Green eyes are
most common in Northern and Central Europe. They can also be found
in parts of
South Asia,
West Asia, and
North
Africa.
In Iceland
, 89% of
women and 87% of men have either blue or green eye
color. A study of Icelander and Dutch adults found green
eyes to be much more prevalent in women than in men. Among
European Americans, green eyes are most
common among those of
Celtic and
Germanic ancestry, about 16%.
Hazel

This iris shows a mixture of brown,
green and amber colors.
Hazel eyes are due to a combination of
Rayleigh scattering and a moderate
amount of melanin in the iris' anterior border layer. Hazel eyes
often appear to shift in color from a light brown to a dark
golden-green. A number of studies using three-point scales have
assigned
hazel to be the medium-color between the lightest
shade of blue and darkest shade of brown. Hazel mostly consists of
Brown and Green. The dominant color in the eye can either be green
or light brown/gold. This can sometimes produce a multicolored
iris, i.e., an eye that is light brown/amber near the pupil and
charcoal or dark green on the outer part of the iris (and vice
versa) when observed in sunlight. Hazel is commonly found in
Europe, the
Middle
East,
Americas,
Central Asia, and parts of
South Asia.
Definitions of the eye color
hazel vary: it is sometimes
considered to be synonymous with light-brown or gold, as in the
color of a hazelnut shell. In North America,
hazel is
often used to describe eyes that appear to change color.
Red

"Red" albino eyes.
The eyes of people with
albinism may appear
red under certain lighting conditions owing to the extremely low
quantities of
melanin, allowing the blood
vessels to show through.
Medical implications
Those with lighter iris color have been found to have a higher
prevalence of
age-related macular
degeneration (ARMD) than those with darker iris color; lighter
eye color is also associated with an increased risk of ARMD
progression. An increased risk of
uveal
melanoma has been found in those with blue, green or gray iris
color. Additionally, an August 2000 study suggests that people with
dark brown eyes are at increased risk of developing cataracts, and
therefore should protect their eyes from direct exposure to
sunlight.
Eye color may also be symptomatic of disease. Aside from the iris,
yellowing of the
whites of the eyes is
associated with
jaundice and symptomatic of
liver disease, including
cirrhosis,
hepatitis and
malaria.
Aside from the above factors, yellowing of the whites of the eyes
in people with darker pigmented skin is often due to melonin being
present in the whites of the eyes. However, any sudden changes in
the color of the whites of the eyes should be addressed by a
medical professional.
Anomalous conditions
Aniridia
Aniridia is a congenital condition
characterized by an extremely underdeveloped iris which appears
absent on superficial examination.
Ocular albinism and eye color
Normally, there is a thick layer of melanin on the back of the
iris. Even people with the lightest blue eyes, with no melanin on
the front of the iris at all, have dark brown coloration on the
back of it, to prevent light from scattering around inside the eye.
In those with milder forms of
albinism, the
color of the irises is typically blue, but can vary from blue to
brown. In severe forms of albinism, there is no pigment on the back
of the iris, and light from inside the eye can pass through the
iris to the front. In these cases, the only color seen is the red
from the hemoglobin of the blood in the capillaries of the iris.
Such albinos have pink eyes, as do albino rabbits, mice, or any
other animal with total lack of melanin.
Transillumination defects can almost
always be observed during an
eye
examination due to lack of iridial pigmentation. The ocular
albino also lacks normal amounts of melanin in the retina as well,
which allows more light than normal to reflect off the retina and
out of the eye. Because of this, the
pupillary reflex is much brighter in
the albino, and this can increase the
red
eye effect in photographs.
Heterochromia
Heterochromia (also known as a
heterochromia iridis or
heterochromia iridium) is an ocular condition in which one
iris is a different color from the other iris (complete
heterochromia), or where the part of one iris is a different color
from the remainder (partial heterochromia or sectoral
heterochromia). It is a result of the relative excess or lack of
pigment within an iris or part of an iris,
which may be
inherited or
acquired by
disease or
injury. This uncommon condition usually results due
to uneven
melanin content. A number of
causes are responsible, including genetics such as
chimerism,
Horners Syndrome and
Waardenburg syndrome. A common cause in
females with heterochromia is
X-inactivation, which can result in a number
of heterochromatic traits, such as
calico
cats. Trauma and certain medications, such as some
prostaglandin analogues can also
cause increased or decreased pigmentation in one eye. On occasion,
the condition of having two different colored eyes is caused by
blood staining the iris after sustaining injury.
See also
References
External links