All About Eye Color
What Color Eyes Would Your Children Have?
A really great site that calculates percentage wise the eye color of your potential children. Click the title to proceed.
The Eye Care Reports
PLEASE READ THIS: The information contained herein is not intended as a
substitute for medical advice and care from qualified, licensed health
care providers. The reader should regularly consult a physician in
matters relating to his or her health and particularly with respect to
any symptoms that may require diagnosis or medical attention. The
information is presented here to educate and inform and to guide you to
an understanding of cause, prevention as well as treatment.
All About Eye Color
Brown eyes, blues eyes, and green eyes. These are the "standard" eye
colors, but we've all seen people with even different eye colors.
There's also gray, "hazel", a mixed brown/green/blue, golden brown or
amber, even violet. Many children a born with non-descript gray eyes and
some become blue, some brown. How much do we really know about eye
color?
Eye color is determined primarily by the concentration and distribution
of melanin pigment on the iris fibers. The iris is located inside the
eye, between the cornea (front most surface) and the internal
crystalline lens. The hole in the middle is called the pupil and it
changes size to regulate the amount of light entering the eye. If the
iris contains pigment on both the front and back surface, the incident
light that reflects off the iris is brown. Sometimes there is little or
no pigment on the front surface. The light interacts with the gray iris
fibers and the iris stromal cells and reflects as blue.
The size and spacing of the fibers and stromal cells determines the
"blueness" or "greenness' of the reflected light. Although the majority
of irises have similar pigment density on the back surface (called the
iris pigment epithelium), some people have less and that allows for some
reflection from the retina in the back of the eye. The retina reflects
red from the network of blood vessels contained within its structure.
This is also what causes the red reflex in the iris pupil seen in
photographs taken with a flash. The red color interacts with the blues
and browns to create aqua and violet iris colors. There's a rare genetic
condition called albinism in which those individuals do not have pigment
on the back of the iris and their iris color appears pink. Sometimes,
the pigment from the back surface “rolls” up onto the front surface
right at the pupil border. This results in a brown ring around the
pupil, which can look quite unusual in an otherwise “blue” eye.
Many of us learned in school the so-called simple Mendelian, or "two
gene" genetic model to predict eye color. This model promoted the notion
that blue eye color is a simple recessive trait. Based on the color of
the mother and father's eyes, little Johnny or little Jill would have
blue or brown eyes. This model has repeatedly been shown to be wrong,
yet is still presented as an explanation for eye color. As we learned
more about genetics and cell chemistry, it was determined that melanin,
a brown pigment, is controlled by the brown-blue gene on chromosome 15
and the green-blue gene on chromosome 19. The new and improved genetic
model still doesn't work all that perfectly and, in fact, science has
discovered at least one more color-determining gene, another brown-type
also located on chromosome 15. Recently, science has found a brownish
yellow pigment, lipofuscin, also called lipochrome that appears in
amber, green and violet irises. And there's likely at least one other
gene, not yet located, that plays a role in iris color.
Many infants, notably Caucasians are born with "neutral" eye color. As
the eye is exposed to sunlight, the melanocytes within the iris begin to
produce melanin pigment and the eye color slowly begins to change
towards its adult coloring, reaching at least 50% of adult melanin
density by age one and complete pigmentation by age 3.
An interesting question about changes in adult eye color is often asked.
As we have discussed previously, eye color is about reflection of
ambient light from the structure of the iris. People with lightly
colored irises note that their eye color changes according to the colors
they wear. No mystery there, but the resultant apparent color of the eye
is a combination of the color reflected from one's clothing (or even eye
make up) and is not always what you might expect!
Some people have noted that when they are ill or under stress that their
eye color becomes darker or lighter. Clearly there has been a change in
the distribution and/or density of melanin or lipofuscin on the iris.
How this actually occurs is not fully understood, but we are beginning
to have some clues as to the processes. A recent medication used to
treat glaucoma, latanoprost, causes such a color change. We understand,
at least partially, the mechanism of action of the drug, a prostaglandin
analog, which means it mimics this hormone, which is normally present in
the body. It would appear, then that hormones may have effect iris
pigmentation and this might explain the change in eye color sometimes
seen in adults. It should be mentioned that there are certain specific
systemic and eye diseases that can cause a change in iris color and any
marked change of short period of time requires prompt medical attention.
So here, in 2004, what we DO know is that eye color is a polygenic trait
and that we humans have a variety of iris colors. Eye color changes in
infants as a developmental response and eye color changes in adults is
not well understood.
Here’s an interesting statistic about the distribution of eye color.
Please note that this information relates to a specific population and I
do not have the actual makeup of that group.
In a sample of 324 patients from 5 “urban clinics” in Massachusetts and
Maryland (therefore spanning many socioeconomic and racial groupings),
those investigators reported:
32% blue/grey irises
15% blue/grey/green irises with brown/yellow specks
12% green/light brown irises with minimal specks
16% brown irises with specks
25% dark brown irises
Here's a partial list of references, some of which contributed to the
information contained in this report:
http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?227240
http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?227220
http://madsci.wustl.edu/posts/archives/feb98/888162819.Ge.r.html
http://www.fi.edu/tfi/units/life/forums/anatomy/eyes.html
The eye care reports article: Copyright ©2004 by Larry Bickford. All Rights Reserved. The publication must be copied in its entirety and include the copyright notice. Any use or sale for profit is strictly prohibited.
