Download Eye color and iris pattern Human eyes are fascinating

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 homozygous TT predicts likely brown eyes.
rs12913832
Likelihood of eye color for people of
European descent
TT ALLELE
CC ALLELE
TC ALLELE
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
].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.