Most people are aware that the color of a person's eyes are largely determined by the color of their parents' eyes. Simply put, genetics has an influence on melanin, the substance that determines eye color. Although this may seem simple, eye color inheritance is a polygenetic (multiple gene) system which is really somewhat complicated.
Until very recently scientists believed that the color of a person's eyes was determined by one single gene. Only in the past couple of years has it been discovered that, in fact, at least eight genes are involved in human eye color. Although it is true that one very important gene known as OCA2 controls almost three quarters of the blue to brown eye color spectrum, there are also other genes that in rare cases can actually overrule OCA2.
The iris is a muscle that controls how much light is able to come into the eye. The amount of light reflected off the iris determines the color of the eye. OCA2 is involved in this process since it manufactures the substance called P-protein that forms melanin. High levels of melanin result in brown eyes while low levels result in blue eyes. Green or hazel eyes are the result of moderate levels of melanin. In extreme cases where there is a mutation of the OCA2 gene, P-protein is inhibited and a disorder called Albinism results in very little pigmentation. This absence of pigmentation has an outcome of very light eyes that may even appear pink.
Most babies are born with blue eyes, although the shades of blue may differ slightly. This is because melanin production can increase greatly in the first year of life. Although eye color has already been determined at birth by the gene code, it may take up to three years before the color stops changing.
Most people remember learning about dominant and recessive genes in high school biology and how the dominant allele determines the eye color. Brown color was always thought to be dominant over blue. However it turns out that the concept may not be quite as simple as was previously thought. In addition to OCA2 there are a number of other genes that can have an effect on the melanin. Recent findings have also shown that combinations of certain genes can increase the melanin levels thereby affecting the eye color. This accounts for two blue eyed-people having a child with green or brown eyes--something that was in previous years more difficult to explain. It should be noted that these gene combinations can also have the opposite effect by reducing the melanin production.
In 1907 Charles and Gertrude Davenport developed an important model for the role that genes play in eye color – a model that has been accepted for the last one hundred years. Although we still don't fully understand everything there is to know about genetics and eye color, recent findings are helping scientists learn more and more about this fascinating subject.