Eye color is a trait that is controlled by many factors. Scientists once believed that just a single gene was responsible for it but have since discovered over 150 genes that influence the characteristic. Each of them has a different function though some have more control over pigmentation than others.
What are the Major Influences?
One of the major genes involved is BEY2 (this gene has many other names including OCA2 and EYCL3). BEY2 is believed to have two different versions, both having different effects on the maturation of the cells that produce and store melanin pigment. One of these versions, also known as an allele, leads to brown eyes while the other allele causes blue eyes. The brown eye allele is dominant over the blue eye allele. This means that, of the two versions of the gene that an individual receives from their parents, both must be the blue eye allele for them to have blue eyes. Any other combination will cause brown eyes. The gene HERC2 is located on the same chromosome as BEY2 and helps to regulate it. Together, the two sections control most blue and brown eye color inheritance.
Another gene that causes even more variation in eye color between individuals is the EYCL1 gene, once referred to as the GEY gene. This gene also has two alleles. One allele can cause blue eyes and the other can cause green eyes, with the blue eye allele being dominant. Typically, if the combination of alleles in an individual for the BEY2 gene indicates that an individual will have brown eyes, they will still have brown eyes. This is in spite of the allele combination for the EYCL1 gene.
What are the Minor Influences?
Many other genes are suspected to be involved in determining eye color. For example, the SLC45A2 gene and the TYRP1 gene are believed to take part in melanin production though their exact functions are unknown. The TYR gene is responsible for creating a product that acts as a precursor to melanin. These genes and dozens of others act along with BEY2, HERC2, and EYCL1 to produce eye colors that can vary greatly from one individual to the next.
