By Alex Hepp
Have you ever taken a look into the mirror at your own eyes or made eye contact with a friend and noticed a particular pattern or an especially striking eye color?
The genetic basis underlying inheritance of eye color has been the subject of much debate. It has been deemed too complex for mendelian dominant-recessive gene models, which refers to an inheritance pattern that follows the laws of segregation and independent assortment where a gene inherited from either parent segregates into gametes, or reproductive sex cells at an equal frequency. However, new molecular techniques in recent years have allowed genome-wide association studies in which several genes were identified that were significantly associated with eye color.
The data points to single nucleotide polymorphisms (SNPs), which is variation in a single base pair in a DNA sequence, within pigmentation related genes that were associated with changes in blue-brown eye color variation, most being located within the 15th chromosome.
In a 2004 study led by researchers examining eye color in 502 twin families, it was estimated that one interval on chromosome 15 containing the OCA2 locus could account for 74% of variance of eye color (Zhu et al., 2004). This suggests that the major genetic determinant for blue-brown eye colors is located within this region. More recent studies expanded upon this, using fine association mapping of eye color SNPs in a region upstream the OCA2 and neighboring the HERC2 gene. Results indicated that a SNP in the intron, a nucleotide sequence within a gene that does not code for proteins and instead interrupts the sequence of genes, 86 of the HERC2 gene predicted eye color significantly better than OCA2. Additional studies indicated that a region within the HERC2 gene was associated with blue eye color.
A Bit of Anatomy
The physical expression of one’s eye color is determined by several biological, chemical, and genetic factors. The iris is the colorful structure of the eye composed of connective tissue and muscle with a central opening called the pupil. Its function is to regulate the amount of light entering the eye which, in conjunction with the lens and the retina, provides us with a sense of vision.
The human iris consists of five cell layers; the anterior border layer, stroma, the sphincter and dilator muscle fibers, and the posterior pigment epithelium. The two most important layers for iris coloration are the anterior layer and the underlying stroma. These layers contain several cell types and fibers that play a role in determining eye color. Melanocytes are one of those cell types that have the ability to produce and distribute melanin, which plays a significant role in pigmentation. For example, in the brown iris there is an abundance of melanocytes and melanin in the anterior border layer and stroma whereas in the blue iris these layers contain very little melanin. Additionally, collagen fibers play a role in the color expressed in the iris. In irises with a limited amount of melanin, different shades of grey, green, and hazel are created, determined by the thickness and density of the iris itself and by the accumulation of white collagen fibers and tissue loss in the anterior border layer and stroma.
Iris Patterns & Heritability
The human iris has several different types of color classifications and patterns. While iris color typically exists in a continuum from light blue to a deep brown-black, a three-point scale of color is used for classification. The three major classes of eye color in this scale are blue, green-hazel, and brown; all with and without the peripupillary ring. The patterns found within the iris include Fuchs’ Crypts, nevi, Wolfflin nodules, contraction furrows, and Brushfield spots (Figure 1). These patterns are formed as a result of different structures and deposits present in the tissue itself.
The majority of the adult population displays iris nevi, which appear on the surface of the anterior border layer of the eye when melanocytes increase their melanin production. It’s hypothesized that the molecular changes that are associated with melanocytes and melanin development in iris tissue are similar to that of the skin (Sturm & Larsson, 2009). These mechanisms include upregulation of the cell-cell adhesion molecules and downregulation of molecules thought to prevent dissociation of epithelial cells, thus resulting in a darker phenotype.
Several studies also indicated that the importance of genetic heritability on iris characteristics is substantial. In one study involving twin samples, the heritability for Fuchs’ crypts, nevi, Wolfflin nodules, and contraction furrows were 66%, 58%, 78% and 78% respectively. Earlier studies of the same sample found that 9 of 30 iris characteristics were highly heritable. There were no sex differences in heritability in these studies. Broadly, these results indicate that iris characteristics in populations are moderately to highly heritable.
Iris tissue and the complex patterns it forms are also used as biomarkers for a variety of purposes. It can be used as a reliable means of personal identification in order to login to your phone or secure bank account. Clinically, tissue markers in the iris are often associated with eye diseases such as ocular melanoma and glaucoma as well as neurological diseases such as down syndrome. Pupillary reflex or lack thereof can also be used to diagnose potential brain stem or cranial nerve damage (Sturm & Larsson, 2009).
So if you’re looking for someone to blame for your eye color and pattern, give your parents a call and unleash the rage. Or if you’re in a good mood, pick up a mirror and stare yourself directly in the eye to determine what type of eye patterns you display.
Sturm, R. A., & Larsson, M. (2009). Genetics of human iris colour and patterns. Pigment Cell & Melanoma Research, 22(5), 544–562. https://doi.org/10.1111/j.1755-148X.2009.00606.x
Zhu, G., Evans, D. M., Duffy, D. L., Montgomery, G. W., Medland, S. E., Gillespie, N. A., Ewen, K. R., Jewell, M., Liew, Y. W., Hayward, N. K., Sturm, R. A., Trent, J. M., & Martin, N. G. (2004). A genome scan for eye color in 502 twin families: most variation is due to a QTL on chromosome 15q. Twin Research: The Official Journal of the International Society for Twin Studies, 7(2), 197–210. https://doi.org/10.1375/136905204323016186