Eye color is one of the most anticipated aspects of a baby's physiology, as parents are often anxious to see what color their child's eyes will be. Will it look like their eyes, their spouses, or one of their parent's eyes? Although it is not yet possible to predict exact the color of a human's eyes before birth, we do know what causes the development of eye color, and how it develops:
What Causes Eye Color?
The colorful part of your eye is the iris – a muscular focus lens that expands and contracts to control the amount of light that enters the eye. The color seen in the iris is caused by the presence of melanin pigments, which also cause a person's skin color and hair color. The more melanin contained in the iris, the darker the color will be. So, brown eyes have the most melanin, grey and blue eyes have the least amount of melanin, and green or hazel eyes contain a moderate amount of melanin. The color of a person's eyes (or how much pigment is produced in the iris, and in which patterns) is entirely determined by the genetic makeup inherited from their parents and grandparents.
The Genetics of Eye Color
Scientists are continually studying the genes that contribute to the development of eye color, and so far three gene pairs have been attributed to the formation and development of iris color. To understand how these genes determine eye color, it is important to have a very basic understanding of genetics:
Human beings have 46 chromosomes (strands of DNA) inside the nucleus of every single cell in our body. These chromosomes are attached in 23 pairs, with each pair consisting of one chromosome from the mother and one from the father. Similarly, attached to these chromosomes are pairs of genes – the most basic units of human heredity that affect nearly every trait expressed by the body. Attached to the loci of the genes are pairs of alleles, which affect the way a gene is expressed. One alleles will always be expressed over another, and the one that is most likely to be expressed is called “dominant,” while the weaker is called “recessive.”
The reason why it is sometimes difficult to say for certain what color a person's eyes will be, is because it is determined by three gene pairs (with 2 alleles on each), and parts of each gene can come from the mother, father, or any of the grandparents. In addition, we only fully understand how two of the gene pairs work. However, the basics of eye color genetics apply to everyone:
The three gene pairs that are known to affect eye color are the bey 1 and bey 2 genes on chromosome 15, and the gey gene on chromosome 19. The bey 1 gene on chromosome 15 contains the central brown allele, which is considered the most dominant (meaning it will usually cause a person's eyes to be brown if present), hence brown eyes being the most common. The bey 2 gene on chromosome 15 contains a blue and brown alleles, and the gey gene on chromosome 19 contains a green allele and blue allele.
Allele's that cause brown eyes to develop are dominant over alleles that cause green and blue eyes, while green alleles are more dominant than blue (the most recessive). Thus, in order for a person to have blue eyes, both genes in a pair (all 4 alleles) must be genes that cause blue eyes. That is not to say that both parents must have blue eyes, as a brown-eyed parent could pass on a recessive blue gene that they inherited from their parent or grandparent but did not manifest. Typically, a green-eyed person has a green allele on chromosome 19 and at least 2 of the other 3 alleles are blue. Although it is very possible for a child to have eye's that differ in color from both of their parents, in most cases if both parents have brown eyes, there is a high probability that the baby will also have brown eyes.
The Development of Eye Color
How much a person's eye color develops after birth depends on the race of their parents. Children of Asian, Hispanic, or African descent are usually (but not always) born with brown eyes, and this is the color their eyes will remain for the rest of their life.
Because melanin pigments are responsible for eye coloring, and Caucasians produce the least melanin, it is not uncommon for Caucasian or partially Caucasian babies to be born with eyes that have no melanin (appearing grayish-blue). As the child's body continues to develop and the genes responsible for laying down the melanin pigments in varying thickness and patterns begin to influence cells in their body, most of their true eye color will begin to appear during the first 3 years of their life, with the vast majority developing their final eye color between the ages of 6-12 months.
Long term studies have shown that approximately 10-15% individuals with light brown, blue, gray, green, or hazel eyes may continue to see very subtle changes in the color of their eyes even into early adulthood. The exact reason for this is unknown, but it is speculated that dietary or environmental changes may cause some people to produce more melanin in their eyes, thereby lightening or changing the color very slightly. If the amount of melanin pigments produced in each eye varies, it is possible for a person to develop two different eye colors (heterochromia).