The Genetics of Eye Color:
An in-depth look into the role genetics play in eye color
The color of eyes is an interesting topic. A brown-eyed child with two blue-eyed parents may be seen as of doubtful parentage in the past! However genetics are not as simple as blue + blue = blue as we shall see.
Caucasian children are usually born with blue eyes which are unpigmented. As the child develops, the cells begin to produce melanin which eventually decides 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. It is generally known that genes responsible for blue eyes are recessive and genes responsible for brown eyes are dominant, so these are all factors to add to the mix.
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.
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.
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.
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.
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