The Genetics of Eye Color



The color of eyes is an interesting topic. In years past a brown eyed child seemed like an unlikely outcome from two blue eyed parents, but as we learn more about genetics we learn that eye color isn't as simple as blue + blue = blue. Let's learn more about the genetics of eye color.

Caucasian children are usually born with unpigmented, blue eyes. As the child develops, the cells begin to produce melanin which eventually determines the final color of the eyes as well as the skin and hair. Eye colors range from brown, which is the most common, to green, the most rare, with shades of blue, amber, hazel and gray somewhere in between. Eye color is generally determined by the amount of pigment in the eye and this is decided by our genetic makeup long before we are born!!

There are several genes which influence the color of a person's eyes. As a general rule brown eyed genes are often dominant and blue eyed genes tend to be recessive. These factors of genetic dominance are added to the complex genetic equation that our bodies use to determine our eye color

Our genes are made up of two alleles. We receive one allele from our mother and a second from our father. Dominant alleles are typically shown as an uppercase letter and recessive are shown as a lowercase letter.

Let's say there was a gene that determined eye color. A B allele would confer brown eyes and a b allele would result in blue eyes. Someone with BB would have brown eyes while another person with Bb would also have brown eyes, although possibly lighter. Finally a person with bb would have blue eyes.

But, eye color isn't that simple. Multiple genes play a role in determining eye color. While the B gene we just talked about might play one role, there may be a second or even third gene involved in the process.

In a second eye color gene let's say that G confers green or hazel eyes and g results in lighter eyes. In simple terms in one gene the B allele confers brown eye color, and the recessive b allele gives blue eyes. In another gene G confers green or hazel eyes and g would confer lighter eyes. In this instance B would be dominant over all the other alleles and the eyes would be brown. If you are homozygous (with identical genes) to the B alleles, the eyes would be darker brown than if you are heterozygous (with dissimilar pairs of genes). If you are homozygous for the G allele in the absence of B, your eyes would be darker, or more hazel, than if you had just one G allele.

If one BG or a Bg allele crosses with any other BG, Bg, bG or bg allele then the result will be brown eyes, but in varying shades. BBGG would create the darkest brown eyes. bbGG would result in very green/hazel eyes. Green eyes need a bG allele to cross with a bG or a bg and the darkest green would be created with a bG bG cross. True blue eyes can only be produced from a genotype bbgg.

Returning to the first example of two blue eyed parents producing a brown eyed child, probably the parents were bbGg with a shift to the lighter side of hazel influencing the shade of blue. The child is bbGG and the presence of two G alleles confers a brown color in this instance.

To further complicate the issue, both genetic and environmental influences also affect the eye color to a degree too. Pregnancy, puberty and trauma can also see a change in the color of a person's eyes. Eye color is complicated.

Blue eyes

Blue eyes have become increasingly rare in American children in just the last few decades. As blue eyes are recessive, it needed parents of English, Irish and North European descent to pass on these traits. Increasingly, immigration has brought a wider pool of genes which are more dominant, with the resulting decline in blue eyes. Thirty years ago about 30% of American babies had blue eyes; now that statistic has changed to about 1 in 6.

Researchers believe that all blue eyed people share one common ancestor. It is believed that the mutation that caused blue eyes occurred sometime between 6,000-10,000 years ago. Isn't it strange to think that all blue eyed people are somehow related?

Brown eyes

Brown eyes are predominant in humans and in many populations it is the only iris color. More than half the world's population has brown eyes and 90% of the world has brown, hazel or amber eyes which are all variants of brown eyes. Dark brown eyes are prominent in East Asia and contain large amounts of melanin within the iris.

Although brown eyes are predominant and are the main eye color worldwide, in some parts of the world they are very rare. In Iceland 80% of the population has green or blue eyes.

Gray eyes

Gray eyes are darker than true blue eyes and have less melanin than blue eyes. Gray eyes are predominant in Russia, Finland , Estonia, Latvia and Lithuania. The gray eye color is determined by the density of proteins and the amount of melanin in the iris. Gray eyes are influenced by the light and may appear to change color in different lighting, or to reflect makeup and clothing colors.

Green eyes

Green eyes have moderate or low amounts of melanin and are often associated with red hair. Only 2% of the world has green eyes and strangely they are more prevalent in women. They are common in the Icelandic population and those with Celtic and German ancestors.

Hazel eyes

Hazel eyes are defined as being the color midway between the lightest blue and the darkest brown eyes. Hazel eyes have a large amount of melanin in the anterior border of the iris. In different lights they may appear to change from light brown to medium gold or even dark green. Sometimes they have a multicolored iris, lightest in the center of the iris and dark brown or green on the outer part.

Why are my baby's eyes two different colors?

Most babies, especially those born to Caucasian parents, are born with varying shades of blue eyes. The eyes may stay blue for as long as three years, until the melanin pigment develops in the eye (if it is going to). By the time the child is five years old their eye color will be pretty well defined. However, there are also times when babies are born with two distinctly different colored eyes. This may occur for a number of reasons including:

Local trauma either while the baby was still in the womb or shortly after birth

Faulty developmental pigment transport

A genetic disorder (benign)

Inflammation

Diffuse nevus of the iris (this is technically a freckle)

Horners syndrome, a potentially serious condition

Why do our eyes change color?

In babies, the change in eye color is a result of the development of pigmentation. For adults the change in eye color could be a warning sign of some serious medical conditions. As many as 15% of Caucasian adults have some change in their eye color as they age, typically from darker to lighter, but if an adult's eyes change color dramatically, suddenly or the change is noted in only one eye, he or she should seek immediate medical attention. It could be the result of conditions such as:

Fuchs heterochromic iridocyclitis

Horners syndrome

Pigmentary glaucoma

Throughout our lifetime, the human eye may appear to change colors, even if the effect is only slight, because of lighting, mood and to some extent, what we are wearing. The iris, which is the colored part of the eye, is a muscle which in part controls the size and shape of the pupil. The pupil is wider in times of low light and is narrower in times of bright light. When the iris expands or contracts around the pupil, the pigments of its surface are either spread out or compressed together affecting the color that we perceive. In addition, certain emotions may cause the pupils to dilate and thus change the color of our eyes. What we wear doesn't actually cause a physical change in the color of our eyes; however the perception of our eye color can be affected by the clothing, makeup or jewelry that we are wearing.

The future of eye color genetics

Researchers are working on interesting ways to use DNA to determine a person's eye color. This information could be particularly useful during criminal investigations. There may even come a time when science can tell you what color your baby's eyes will be while he or she is still in the womb.

New technologies are also being developed to change eye color, without changing genetics. One eye doctor believes that he can permanently change brown eyes to blue by using a special laser to remove the pigment from the eye.

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