By Bethann Hromatka, Ph.D.
Your iris (the colored part of the eye) can tell a lot about you. Like your fingerprint, the patterns on your iris are complex and unique. Because of this, iris recognition is used in some countries for national identification systems and to automate border crossings.
Although we know that no two irises are the same, very little is understood about the genetics underlying these differences. Investigators are studying this topic to learn more about iris-related diseases and to provide clues about the brain, since iris and brain development appear to be connected. This is true on a genetic level, for instance, a mutation linked to iris loss is also associated with defects in a portion of the brain called the frontal lobe. In addition, some iris characteristics correlate with neurological diseases like Down syndrome.
A recently published genome-wide association study (GWAS) sought to elucidate the genetic factors involved in normal iris variation between people. Scientists from Australia scanned the genomes of nearly 2,700 individuals, all of European descent, and measured iris characteristics such as crypts, pigmented rings, furrow contractions, and pigmented spots on the iris.
The authors found one SNP that associated with the presence of crypts, oval-shaped areas that represent openings in the iris. This SNP is located near the SEMA3A gene, which codes for a protein that acts in both the iris and brain and directs cell movement. Interestingly, mutations in the SEMA3A gene have been previously linked to neurological disorders including schizophrenia and Alzheimer’s disease.
The arrows mark examples of crypts. A. No crypts. E. Many large crypts. The American Journal of Human Genetics 89, 334—343, August 12, 2011.
The arrow marks a pigmented ring around the pupil. A. Absence of pigmented ring. B. Yellow/green ring on a blue iris. The American Journal of Human Genetics 89, 334—343, August 12, 2011.
Another SNP was associated with more pronounced furrow contractions, which are circular and radial bands that result from the iris folding in exactly the same location each time it adapts to different light conditions. A third SNP, located near the SLC24A4 gene, was linked to pigmented rings that are visible around the pupil and distinguish different shades of green and hazel eyes. Interestingly, previous studies have also linked SLC24A4 to differences in eye color (blue versus green eyes). Since pigmented rings on blue irises can cause the eye to appear green or hazel, the authors of this study suggest that the presence or absence of pigmented rings may drive the association between and eye color.
The authors noted that older participants tended to have more furrow contractions and pigmented spots on the iris. Males had more crypts than females, but fewer pigmented spots on the iris, furrow contractions, or pigmentation rings.
It is interesting to wonder why no two irises are the same and this study suggests that some of the differences are due to our genes. These findings also add evidence to the idea that the iris and brain are tightly linked starting very early in human development. Thus, while iris patterns can be used to tell people apart, they may also truly be a window into our minds.