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Eye color and iris pattern
Human eyes are fascinating - just take a look at the many different shades of eye color and the intricate, unique iris patterns from all over the world. What we call eye color is actually the color of the iris. The amount of melanin found in your iris, in conjunction with white collagen fibers, produces different shades of grey, green and hazel. For example, as light travels through a relatively melanin-free iris, collagen fibers within the iris scatter the short blue light to the surface creating the blue appearance of the iris. In fact, iris color exists in a gradient of shades and color patterns: from the lightest shades of blue to the darkest browns and everything in between. In general, brown is the most common eye color, followed by blue, grey and then green.
In addition to color, the fibrous tissue in the iris also forms a unique pattern in each individual’s eyes. Just like one’s fingerprints, an iris pattern can be used for biometric identification. Some banks now use iris scans, instead of pin codes for identification and some countries are now using iris scans for border control purposes. For example, United Arab Emirates (UAE) has been operating an iris scan system since 2001 at all ports of entry to screen for expellees.
Genetic determination of eye color
It was originally thought that eye color was a simple Mendelian trait, meaning it was determined by a single gene, with brown being dominant and blue recessive. It is now clear that eye color is a polygenic trait, meaning it is determined by multiple genes. Among the genes that affect eye color, OCA2 and HERC2 stand out. Both are located on human chromosome 15. The OCA2 gene produces a cell membrane transporter of tyrosine, a precursor of melanin. Mutations in OCA2 result in oculocutaneous albinism, a condition associated with vision problems such as reduced sharpness and increased sensitivity to light. HERC2 regulates the OCA2 genes’ expression. In the European population, a common polymorphism in HERC2 gene is responsible for the blue eye phenotype. A person who has two copies of C allele at HERC2 rs1293832 will likely have blue eyes while hom*ozygous TT predicts likely brown eyes.
| rs12913832 | TT Allele | CC Allele | TC Allele |
|---|
| Likelihood of eye color for people of European descent | 85% chance of brown eyes;
14% chance of green eyes;
1% chance of blue eyes. | 72% chance of blue eyes;
27% chance of green eyes;
1% chance of brown eyes. | 56% chance of brown eyes;
37% chance of green eyes;
7% chance of blue eyes. |
It was later found that a set of six SNPs within OCA2-HERC2, SLC24A4, SLC45A2, TYR and IRF4 genes formed a very effective predictor of eye color, giving successful rate of 93% for brown, 91% for blue and 72% for intermediate eye color [1].
Beyond eye color
In addition to eye color, melanin/melanocytes are also responsible for our hair and skin colors. One interesting observation is that only European populations have developed a significant lightening of the pigmentation phenotypes of skin, hair and eye color traits due to genetic selection of multiple genes. If we consider the new lactase persistence allele as an adaptive change in response to availability of dairy products (see Lactose Intolerance), then what is the adaptive significance of lighter skin and eyes? This could be explained by the need for maximized utilization of low level UV light (for vitamin D absorption) in high latitude European regions (see Skin Color). Several studies have linked lighter eye color to the ability to overcome seasonal affective disorder (SAD), a major depressive illness. As Dr. Richard Sturm suggested “perhaps those with blue eyes may have been able to withstand the dark, depressing days of the Neolithic European winters better than those with brown eye color?” [2] As the field of genetics advances, more satisfying and convincing answers will develop… so stay tuned.
We invite you to share your thoughts in the comments section below.
References
1.Fan Liu et al. Eye color and the prediction of complex phenotypes from genotypes. Current Biology 19 (5): R192–R193. doi:10.1016/j.cub.2009.01.027
2.Richard A. Sturm et al. Genetics of human iris colour and patterns. Pigment Cell Melanoma Res. 22; 544–562. doi: 10.1111/j.1755-148X.2009.00606.x
FAQs
Eye color is directly related to the amount of melanin in the front layers of the iris. People with brown eyes have a large amount of melanin in the iris, while people with blue eyes have much less of this pigment. A particular region on chromosome 15 plays a major role in eye color.
How do genes determine eye Colour? ›
Although there are about 16 different genes responsible for eye color, it is mostly attributed to two adjacent genes on chromosome 15, hect domain and RCC1-like domain-containing protein 2 (HERC2) and ocular albinism (that is, oculocutaneous albinism II (OCA2)).
How your DNA determines the color of your eyes and hair? ›
DNA contains all the information needed to build your body. Did you know that your DNA determines things such as your eye color, hair color, height, and even the size of your nose? The DNA in your cells is responsible for these physical attributes as well as many others that you will soon see.
What is the genetic test for eye color? ›
Forensic DNA phenotyping works by analyzing specific genes that are known to be involved in eye color determination. By looking at variations in these genes, scientists can predict the likelihood of a person having certain eye colors.
How to identify eye color? ›
The colored part of the eye is called the iris. The iris has pigmentation that determines the eye color. Irises are classified as being one of six colors: amber, blue, brown, gray, green, hazel, or red.
How does the eye determine color? ›
Light travels into the eye to the retina located on the back of the eye. The retina is covered with millions of light sensitive cells called rods and cones. When these cells detect light, they send signals to the brain. Cone cells help detect colors.
How is eye color determined at birth? ›
We have specialized cells in our bodies called melanocytes whose job it is to go around secreting melanin. Over time, if melanocytes only secrete a little melanin, your baby will have blue eyes. If they secrete a bit more, his eyes will look green or hazel.
How does DNA code for your eye colour? ›
To have pigment, two genes need to work properly at the same time, HERC2 and OCA2, located on chromosome 15. Both affect the production of proteins that determine eye color by influencing melanin production and protein interactions.
What is the rarest eye color? ›
While the global data on eye colors is limited, red and violet eyes are likely the rarest eye colors since they only affect a small group of people with albinism. But if you exclude eye colors brought on by albinism, then green and gray are likely the rarest.
Do mutations determine eye color? ›
So far, researchers know of dozens of genes and DNA mutations that can affect eye color. So, while parents' eye colors are a major factor, they aren't the only one. And sometimes, eye colors skip generations. So, if a baby's eye color doesn't match their parents', there are plenty of reasons why that might happen.
When broken down by gender, men ranked gray, blue, and green eyes as the most attractive, while women said they were most attracted to green, hazel, and gray eyes. Despite brown eyes ranking at the bottom of our perceived attraction scale, approximately 79% of the world's population sports melanin-rich brown eyes.
What are the two true eye colors? ›
Technically, eye color is on a spectrum from very dark brown to very light grey or blue. Unlike other animals which produce many kinds of pigment (and thus have strikingly different colors), the only major pigment in human eyes is melanin--the same pigment that is in your skin.
How rare are grey eyes? ›
Grey eyes are indeed considered quite rare, with only an estimated 3% of the global population having them. This makes them the second rarest eye color, after green eyes which have an estimated 2% prevalence.
Who determines eye color mom or dad? ›
While a baby inherits half of their eye color genetics from one parent and half from the other parent, the way that the multiple genes interact also plays a role in determining eye color.
Can two blue-eyed parents have a brown-eyed child? ›
Myth: Two blue-eyed parents can't produce a child with brown eyes. Fact: Two blue-eyed parents can have a child with brown eyes, although it's very rare. Likewise, two brown-eyed parents can have a child with blue eyes, although this is also uncommon.
Why are green eyes so rare? ›
Green eyes are the most rare eye color in the world. Only about 2 percent of people in the world have naturally green eyes. Green eyes are a genetic mutation that results in low levels of melanin, though more melanin than in blue eyes. Green eyes don't actually have any color.
We invite you to share your thoughts in the comments section below.