Eye Color Genetics
Eye Color depends on pigmentation known as melanin that is produced by melanocytes (a specialized cell located in the front layers of the iris, the structure surrounding the pupil). Individuals have on average the same number of melanocytes, but the amount of melanin produced varies.
The color is determined by how much melanin is stored in the intracellular compartments known as melanosomes. Blue eyes have a minimal amount of melanin stored in a small number of melanosomes. Green eyes have a moderate amount of melanin and moderate melanosomes. Brown eyes have high amounts of melanin stored within many melanosomes.
More than 150 genes influencing eye color have been found by researchers. Two genes located nearby each other on chromosome 15 play major roles in determining eye color. The first we will discuss called OCA2, codes for P protein, and is directly involved in melansome maturation and affects the amount of melanin stored in the iris. Genetic variations in this gene increase or reduce the amount of P protein leading to a dark or light eye color respectively. HERC2 is the other main gene involved, it controls expression of OCA2 which can increase or reduce how much P protein is produced.
Eye color inheritance pattern is complex. A child’s eye color can generally be predicted by looking at the color of the parents eyes, but polymorphisms can arise and a child may have an unexpected eye color. Variations in genes are referred to as alleles. The allele for brown eyes is very dominant over the allele for blue eye, which is always recessive. This can lead to parents who happen to have the same eye color producing a child with different eye colors. For example if parents with brown eyes each pass on a pair of blue alleles to their offspring, the child would have blue eyes. But if one parents passes on a green allele, the child will have green eyes. If a brown allele is passed on, then the child will have brown eyes.
This however does not account for why parents with blue eyes can have children with brown eyes, nor does it explain grey or hazel eyes. Mutations and modifier genes can all lead to variability in eye color. Researchers are still studying exactly how these factors cause these unique variations.