Misplaced Pages

Latest revision as of 11:37, 7 December 2024

Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky. Neither blue nor green pigments are present in the human iris or vitreous humour. This is an example of structural color, which depends on the lighting conditions, especially for lighter-colored eyes.

The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids. Humans and other animals have many phenotypic variations in eye color.

The genetics and inheritance of eye color in humans is complicated. As of 2010, as many as 16 genes have been associated with eye color inheritance. Some of the eye-color genes include OCA2 and HERC2. The earlier belief that blue eye color is a recessive trait has been shown to be incorrect, and the genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.

Genetic determination

Eye color is an inherited trait determined by multiple genes. These genes are sought by studying small changes in the genes themselves and in neighboring genes, called single-nucleotide polymorphisms or SNPs. The total number of genes that contribute to eye color is unknown, but there are a few likely candidates. A study in Rotterdam (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs.

In humans, eye color is a highly sexually dimorphic trait. Several studies have shown that men are more likely to have blue eyes than women, while women are more likely to have darker eye colors (green and brown eyes) than men. Sex is therefore a major factor in the expression of eye color genotypes. One study suggested that women's higher levels of the sex hormone estrogen may explain why women tend to have darker eyes than men.

People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe. Through the analysis of ancient DNA, a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa.

There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are OCA2 and HERC2, and both are localized in chromosome 15.

The gene OCA2 (OMIM: 203200), when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are SLC24A4 and TYR. A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, allowing the explanation of about 50% of eye color variation.

Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome.

Changes in eye color

A 1997 study of White Americans found that eye color may be subject to change in infancy, and from adolescence to adulthood. 17% of children experienced a change of eye color by adulthood. Of those children, 50% of developed lighter eyes as they got older. The other 50% developed darker eyes.

Generally, children with hazel and light brown eyes tended to experience a lightening of their eye color by adulthood. Children with green eyes often experienced a darkening of their eye color. It was also found that 11% of the children's mothers experienced an eye color change during the same period, with most developing lighter eyes, relative to their original color at the time of their child's birth.

Eye color range

Brown

Almost all mammals have brown or darkly-pigmented irises. In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it. Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.

In many parts of the world, it is nearly the only iris color present. Brown eyes are common in Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, West Africa and the Americas. Light or medium-pigmented brown eyes can also be commonly found in Europe, among the Americas, and parts of Central Asia, West Asia, South Asia, and East Africa. Light brown eyes bordering amber and hazel coloration are more common in Europe, but can also be observed in East Asia, Southeast Asia, North Africa and East Africa.

Amber

Amber eyes are a solid color with a strong yellowish/golden or russet/coppery tint, which may be due to a yellow pigment called lipochrome (also found in green eyes). Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have many other colors, including green, brown, and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid gold hue. Even though amber is similar to gold, some people have russet- or copper-colored amber eyes that are mistaken for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines. The bright yellow eyes of the great horned owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons, and fish have amber eyes, whereas in humans this color occurs less frequently. Amber is the third-rarest natural eye color after green and gray, occurring in 5% of the world's population. People with amber-colored eyes are found in Europe, and in fewer numbers in the Middle East, North Africa, and South America.

Hazel

Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why hazel eyes can be mistaken as amber, and why amber is often counted as hazel in studies, and vice versa. The combination can sometimes produce a multicolored iris, i.e. an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa) when observed in sunlight.

Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell.

Around 18% of the US population and 5% of the world population have hazel eyes. 55.2% of Spanish subjects in a series of 221 photographs were judged to have hazel eyes. Hazel eyes are found in Europe, most commonly in the Netherlands and the United Kingdom, and have also been observed to be very common among the Low Saxon-speaking populations of northern Germany.

Green

This section may have misleading content. Please help clarify the content. (August 2023)

Green eyes probably result from the interaction of multiple allelic variants of OCA2 and other genes. They may have been present in southern Siberia during the Bronze Age.

Green eyes are most common in Northern, Western, and Central Europe. Around 8–10% of men and 18–21% of women in Iceland and 6% of men and 17% of women in the Netherlands have green eyes. Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry, occurring in about 16% of people with those backgrounds.

The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin), and 2) a blue shade created by the Rayleigh scattering of reflected light. Green eyes contain the yellowish pigment lipochrome.

Blue

There is no intrinsically blue pigmentation either in the iris or in the vitreous body. Rather, blue eyes result from structural color in combination with certain concentrations of non-blue pigments. The iris pigment epithelium is brownish black due to the presence of melanin. Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma. This is the same scattering that accounts for the blue appearance of the sky. The result is a "Tyndall blue" structural color that varies with external lighting conditions.

Blue eyes are a highly sexually dimorphic eye color. Studies from various populations in Europe have shown that men are substantially more likely to have blue eyes than women.

The inheritance pattern followed by blue eyes was previously assumed to be a Mendelian recessive trait, though this has been shown to be incorrect. Eye color inheritance is now recognized as a polygenic trait, meaning that it is controlled by the interactions of several genes.

In 2008, a team of researchers from the University of Copenhagen located a single mutation that causes the phenomenon of blue eyes. The research was published in the Journal of Human Genetics. The same DNA sequence of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor. The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in the northwestern part of the Black Sea region in Europe sometime between 6,000 and 10,000 years ago, during the Neolithic period. However, more recent ancient DNA research has identified human remains much older than the Neolithic period which possess the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele responsible for blue eyes dates back to the migration of modern humans out of Africa roughly 50,000 years ago, and entered Europe from western Asia.

Eiberg and colleagues suggested in a study published in Human Genetics that a mutation in the 86th intron of the HERC2 gene, which is hypothesized to interact with the OCA2 gene promoter, reduced expression of OCA2 with subsequent reduction in melanin production.

It has been proposed that blue eyes may have been adaptive to shorter day lengths at higher latitudes, as blue eyes increase intraocular light scattering, which suppresses melatonin release from the pineal gland, perhaps reducing psychological depression (which is linked to the short day length of higher latitudes).

Blue eyes are predominant in northern and eastern Europe, particularly around the Baltic Sea. Blue eyes are also found in Southern Europe, Central Asia, South Asia, North Africa, and West Asia. Approximately 8% to 10% of the global population have blue eyes. A 2002 study found the prevalence of blue eye color among the white population in the United States to be 33.8% for those born from 1936 through 1951, compared with 57.4% for those born from 1899 through 1905. As of 2006, one out of every six Americans, or 16.6% of the total US population, has blue eyes, including 22.3% of whites. The incidence of blue eyes continues to decline among American children. Of Slovenes, 56% have blue/green eyes. In a series of 221 photographs of Spanish subjects, 16.3% of the subjects were determined to have blue-gray eyes.

Gray

Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in appearance between gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma, so that the light that is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when the sky is cloudy. Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.

Gray eyes can also be found among the Algerian Shawia people of the Aurès Mountains in Northwest Africa, in the Middle East/West Asia, Central Asia, and South Asia. In the Iliad, the Greek goddess Athene is said to have gray eyes (γλαυκῶπις). Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

Gray is the second-rarest natural eye color after green, with 3% of the world's population having it.

Special cases

Two different colors

As a result of heterochromia iridum, it is also possible to have two different eye colors. This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world's population.

Red and violet

The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin, allowing the blood vessels to show through. In addition, flash photography can sometimes cause a "red-eye effect", in which the very bright light from a flash reflects off the retina, which is abundantly vascular, causing the pupil to appear red in the photograph.

Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism. Eyes that appear red or violet under certain conditions due to albinism occur in less than 1 percent of the world's population.

Medical implications

The most important role of melanin in the iris is to protect the eyes from the sun's harmful rays. People with lighter eye colors, such as blue or green, have lessened protection from the sun, and so need greater protection from the sun's rays than those with darker eye colors.

Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a uveitis, and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.

Wilson's disease

Wilson's disease involves a mutation of the gene coding for the enzyme ATPase 7B, which prevents copper within the liver from entering the Golgi apparatus in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of Kayser–Fleischer rings, which are dark rings that encircle the periphery of the iris.

Coloration of the sclera

Eye color outside of the iris may also be symptomatic of disease. Yellowing of the sclera (the "whites of the eyes") is associated with jaundice, and may be symptomatic of liver diseases such as cirrhosis or hepatitis. A blue coloration of the sclera may also be symptomatic of disease.

Aniridia

Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.

Ocular albinism and eye color

Normally, there is a thick layer of melanin on the back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism, the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation. The ocular albino also lacks normal amounts of melanin in the retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.

Heterochromia

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources in this section. Unsourced material may be challenged and removed. (November 2012) (Learn how and when to remove this message)

Heterochromia (heterochromia iridum or heterochromia iridis) is an eye condition in which one iris is a different color from the other (complete heterochromia), or where a part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury. This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetic, such as chimerism, Horner's syndrome and Waardenburg syndrome.

A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if the DNA difference happens to be in an eye-color gene.

There are many other possible reasons for having two different-colored eyes. For example, the film actor Lee Van Cleef was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses. David Bowie, on the other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated.

Another hypothesis about heterochromia is that it can result from a viral infection in utero affecting the development of one eye, possibly through some sort of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition.

A common cause in females with heterochromia is X-inactivation, which can result in a number of heterochromatic traits, such as calico cats. Trauma and certain medications, such as some prostaglandin analogues, can also cause increased pigmentation in one eye. On occasion, a difference in eye color is caused by blood staining the iris after injury.

Limbal ring

The limbal ring is also a feature of the iris contributing to eye color, appearing as a darkened, occasionally black region encircling the iris resulting from a manifestation of the optical properties of the corneal limbus. Limbal rings are not present in all individuals, and their thickness and prominence may correlate with health or youthfulness, and contributes to facial attractiveness.

Impact on vision

Although people with lighter eye color are generally more sensitive to light because they have less pigment in the iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity. However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times. People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting a golf ball or throwing baseballs. In another study, people with darker eyes performed better at hitting racquetballs. There are also other studies that challenge these findings, and more study is needed to verify these results.

Classification of color

Iris color can provide a large amount of information about a person, and a classification of colors may be useful in documenting pathological changes or determining how a person may respond to ocular pharmaceuticals. Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison. Others have attempted to set objective standards of color comparison.

The Martin–Schultz scale, developed from the Martin scale, is one standard color scale used in physical anthropology to establish the eye color of an individual. It was created by the anthropologists Rudolf Martin and Bruno K Schultz in the first half of the 20th century. The scale consists of 20 colors ranging from light blue to dark brown-black, corresponding to natural eye colors caused by the amount of melanin in the iris:

Normal eye colors range from the darkest shades of brown to the lightest tints of blue. To meet the need for standardized classification, at once simple yet detailed enough for research purposes, Seddon et al. developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present. There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris, along with structural color. Green irises, for example, have some yellow and the blue structural color. Brown irises contain more or less melanin. Some eyes have a dark ring around the iris, called a limbal ring.

Eye color in non-human animals is regulated differently. For example, instead of blue as in humans, autosomal recessive eye color in the skink species Corucia zebrata is black, and the autosomal dominant color is yellow-green.

As the perception of color depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.

See also

• Hair color
• Human skin color
• Iridology
• Xanthophore

References

1. Wielgus AR, Sarna T (2005). "Melanin in human irides of different color and age of donors". Pigment Cell Res. 18 (6): 454–64. doi:10.1111/j.1600-0749.2005.00268.x. ISSN 0893-5785. PMID 16280011.
2. Prota G, Hu DN, Vincensi MR, McCormick SA, Napolitano A (1998). "Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors". Exp. Eye Res. 67 (3): 293–9. doi:10.1006/exer.1998.0518. PMID 9778410.
3. ^ Fox, Denis Llewellyn (1979). Biochromy: Natural Coloration of Living Things. University of California Press. ISBN 978-0-520-03699-4.
4. ^ Huiqiong Wang; Lin, S.; Xiaopei Liu; Sing Bing Kang (2005). "Separating reflections in human iris images for illumination estimation". Tenth IEEE International Conference on Computer Vision (ICCV'05) Volume 1. pp. 1691–1698 Vol. 2. CiteSeerX 10.1.1.87.418. doi:10.1109/ICCV.2005.215. ISBN 978-0-7695-2334-7. S2CID 2215768.
5. Sturm R.A. & Larsson M., Genetics of human iris colour and patterns, Pigment Cell Melanoma Res, 22:544-562, 2009.
6. ^ Mason, Clyde W. (1924). "Blue Eyes". Journal of Physical Chemistry. 28 (5): 498–501. doi:10.1021/j150239a007.
7. Oliphant LW (1987). "Pteridines and purines as major pigments of the avian iris". Pigment Cell Res. 1 (2): 129–31. doi:10.1111/j.1600-0749.1987.tb00401.x. PMID 3507666.
8. Morris, PJ. "Phenotypes and Genotypes for human eye colors." Athro Limited website. Retrieved 10 May 2006.
9. ^ Kayser, Manfred; Liu, Fan; Janssens, A. Cecile J.W.; Rivadeneira, Fernando; Lao, Oscar; Van Duijn, Kate; Vermeulen, Mark; Arp, Pascal; et al. (2008). "Three genome-wide association studies and a linkage analysis identify HERC2 as a human iris color gene". Am. J. Hum. Genet. 82 (2): 411–23. doi:10.1016/j.ajhg.2007.10.003. PMC 2427174. PMID 18252221.
10. ^ White, Désirée; Rabago-Smith, Montserrat (14 October 2010). "Genotype–phenotype associations and human eye color". Journal of Human Genetics. 56 (1): 5–7. doi:10.1038/jhg.2010.126. PMID 20944644.
11. No Single Gene For Eye Color, Researchers Prove. Sciencedaily.com (22 February 2007). Retrieved on 2011-12-23.
12. "Eye color definition – Medical Dictionary definitions of popular medical terms easily defined on MedTerms". Medterms.com. 29 October 2003. Archived from the original on 5 June 2011. Retrieved 19 October 2011.
13. ^ Duffy, David L.; Montgomery, Grant W.; Chen, Wei; Zhao, Zhen Zhen; Le, Lien; James, Michael R.; Hayward, Nicholas K.; Martin, Nicholas G.; Sturm, Richard A. (2007). "A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation". Am. J. Hum. Genet. 80 (2): 241–52. doi:10.1086/510885. PMC 1785344. PMID 17236130.
14. ^ Sturm RA, Frudakis TN (2004). "Eye colour: portals into pigmentation genes and ancestry" (PDF). Trends Genet. 20 (8): 327–32. doi:10.1016/j.tig.2004.06.010. PMID 15262401. Archived from the original (PDF) on 9 September 2006.
15. ^ Grant MD, Lauderdale DS (2002). "Cohort effects in a genetically determined trait: eye colour among US whites". Ann. Hum. Biol. 29 (6): 657–66. doi:10.1080/03014460210157394. PMID 12573082. S2CID 25364754.
16. "DNA test for eye colour could help fight crime", New Scientist 14 March 2009.
17. Liu, Fan; Van Duijn, Kate; Vingerling, Johannes R.; Hofman, Albert; Uitterlinden, André G.; Janssens, A. Cecile J.W.; Kayser, Manfred (2009). "Eye color and the prediction of complex phenotypes from genotypes". Current Biology. 19 (5): R192–R193. Bibcode:2009CBio...19.R192L. doi:10.1016/j.cub.2009.01.027. PMID 19278628.
18. ^ Martinez-Cadenas, Conrado; Peña-Chilet, Maria; Ibarrola-Villava, Maider; Ribas, Gloria (2013). "Gender is a major factor explaining discrepancies in eye colour prediction based on HERC2/OCA2 genotype and the IrisPlex model". Forensic Science International: Genetics. 7 (4). Elsevier BV: 453–460. doi:10.1016/j.fsigen.2013.03.007. hdl:10234/86709. ISSN 1872-4973. PMID 23601698. "This effect is what may explain the fact that there seem to be comparatively higher frequencies of blue-eyed males than blue-eyed females in populations of European origin such as Iceland , Holland , Australia or Poland , as well as in this study (see Fig. 2)."
19. Pilli, E.; Berti, A. (2021). Forensic DNA Analysis: Technological Development and Innovative Applications. Apple Academic Press. p. 207. ISBN 978-1-000-06454-4. Retrieved 22 May 2024. "Several research groups have demonstrated that females have a darker eye color than males, given the same SNP profile (Martinez-Cadenas et al., 2013; Pietroni et al., 2014; Pospiech et al., 2016)."
20. Hernando, Barbara; Ibarrola-Villava, Maider; Fernandez, Lara P.; Peña-Chilet, Maria; Llorca-Cardeñosa, Marta; Oltra, Sara S.; Alonso, Santos; Boyano, Maria Dolores; Martinez-Cadenas, Conrado; Ribas, Gloria (2016). "Sex-specific genetic effects associated with pigmentation, sensitivity to sunlight, and melanoma in a population of Spanish origin". Biology of Sex Differences. 7 (1): 17. doi:10.1186/s13293-016-0070-1. hdl:10810/32358. ISSN 2042-6410. PMC 4797181. PMID 26998216. The results of this study suggest that there are indeed sex-specific genetic effects in human pigmentation, with larger effects for darker pigmentation in females compared to males. A plausible cause might be the differentially expressed melanogenic genes in females due to higher oestrogen levels. These sex-specific genetic effects would help explain the presence of darker eye and skin pigmentation in females, as well as the well-known higher melanoma risk displayed by males.
21. ^ Hanel, Andrea; Carlberg, Carsten (September 2020). "Skin colour and vitamin D: An update". Experimental Dermatology. 29 (9): 864–875. doi:10.1111/exd.14142. ISSN 0906-6705. PMID 32621306. S2CID 220335539. "The genetic history of today's European populations is based on continuous migrations over the past 40 000 years. Homo sapiens arrived in Europe from Near East some 42 000 years ago. Like in their African origin, these humans had dark skin but due to variations of their OCA2 gene (causing iris depigmentation) many of them had blue eyes (Figure 2, left)." Figure 2: "Phenotype information was retrieved from supplementary files of the according publications or additionally assessed following the instructions of HirisPlex-S, which is a forensic DNA phenotyping tool based on an array of different marker SNPs providing additional support to the SNPs of the genes SLC24A5, SLC45A2 and OCA2."
22. ^ Sulem, Patrick; Gudbjartsson, Daniel F; Stacey, Simon N; Helgason, Agnar; Rafnar, Thorunn; Magnusson, Kristinn P; Manolescu, Andrei; Karason, Ari; et al. (2007). "Genetic determinants of hair, eye and skin pigmentation in Europeans". Nat. Genet. 39 (12): 1443–52. doi:10.1038/ng.2007.13. PMID 17952075. S2CID 19313549.
23. Liu, Fan; Wollstein, Andreas; Hysi, Pirro G.; Ankra-Badu, Georgina A.; Spector, Timothy D.; Park, Daniel; Zhu, Gu; Larsson, Mats; Duffy, David L.; Montgomery, Grant W.; MacKey, David A.; Walsh, Susan; Lao, Oscar; Hofman, Albert; Rivadeneira, Fernando; Vingerling, Johannes R.; Uitterlinden, André G.; Martin, Nicholas G.; Hammond, Christopher J.; Kayser, Manfred (2010). "Digital Quantification of Human Eye Color Highlights Genetic Association of Three New Loci". PLOS Genetics. 6 (5): e1000934. doi:10.1371/journal.pgen.1000934. PMC 2865509. PMID 20463881.
24. Bito, LZ (1997). "Eye Color Changes Past Early Childhood". Archives of Ophthalmology. 115 (5): 659–63. doi:10.1001/archopht.1997.01100150661017. PMID 9152135.
25. ^ Bito 1997, p. 660.
26. Bito 1997, p. 661-662.
27. Brenda J Bradley, Anja Pedersen, Nicholas I Mundy: Blue eyes in lemurs and humans: same phenotype, different genetic mechanism Am J Phys Anthropol. 2009
28. ^ Debrowski, Adam. "Which Eye Colors Are the Rarest?". All About Vision. Retrieved 4 February 2021.
29. ^ Fox, Denis Llewellyn (1979). Biochromy: Natural Coloration of Living Things. University of California Press. p. 9. ISBN 978-0-520-03699-4.
30. Online Mendelian Inheritance in Man (OMIM): SKIN/HAIR/EYE PIGMENTATION, VARIATION IN, 1; SHEP1 - 227220
31. Howard Hughes Medical Institute: Ask A Scientist Archived 1 September 2010 at the Wayback Machine. Hhmi.org. Retrieved on 23 December 2011.
32. Larry Bickford Eye Color Archived 23 October 2010 at the Wayback Machine. Eyecarecontacts.com. Retrieved on 23 December 2011.
33. Oliphant LW (1987). "Observations on the pigmentation of the pigeon iris". Pigment Cell Res. 1 (3): 202–8. doi:10.1111/j.1600-0749.1987.tb00414.x. PMID 3508278.
34. Oliphant LW (1981). "Crystalline pteridines in the stromal pigment cells of the iris of the great horned owl". Cell Tissue Res. 217 (2): 387–95. doi:10.1007/BF00233588. PMID 7237534. S2CID 8061493.
35. ^ Lefohn A, Budge B, Shirley P, Caruso R, Reinhard E (2003). "An Ocularist's Approach to Human Iris Synthesis". IEEE Comput. Graph. Appl. 23 (6): 70–5. doi:10.1109/MCG.2003.1242384. S2CID 537404.
36. "Amber eyes". All About Vision. Retrieved 9 May 2021.
37. Ruiz-Linares, Andrés; Adhikari, Kaustubh; Acuña-Alonzo, Victor; Quinto-Sanchez, Mirsha; Jaramillo, Claudia; Arias, William; Fuentes, Macarena; Pizarro, María; Everardo, Paola; de Avila, Francisco; Gómez-Valdés, Jorge; León-Mimila, Paola; Hunemeier, Tábita; Ramallo, Virginia; Silva de Cerqueira, Caio C. (25 September 2014). "Admixture in Latin America: Geographic Structure, Phenotypic Diversity and Self-Perception of Ancestry Based on 7,342 Individuals". PLOS Genetics. 10 (9): e1004572. Bibcode:2014PLOSG..10.4572R. doi:10.1371/journal.pgen.1004572. ISSN 1553-7390. PMC 4177621. PMID 25254375.
38. ^ Zhu, Gu; Evans, David M.; Duffy, David L.; Montgomery, Grant W.; Medland, Sarah E.; Gillespie, Nathan A.; Ewen, Kelly R.; Jewell, Mary; Liew, Yew Wah; Hayward, Nicholas K.; Sturma, Richard A.; Trenta, Jeffrey M.; Martina, Nicholas G. (2004). "A genome scan for eye color in 502 twin families: most variation is due to a QTL on chromosome 15q". Twin Res. 7 (2): 197–210. doi:10.1375/136905204323016186. PMID 15169604.
39. Albert, Daniel M; Green, W Richard; Zimbric, Michele L; Lo, Cecilia; Gangnon, Ronald E; Hope, Kirsten L; Gleiser, Joel (2003). "Iris melanocyte numbers in Asian, African American, and Caucasian irides". Transactions of the American Ophthalmological Society. 101: 217–222. PMC 1358991. PMID 14971580.
40. ^ Mitchell R, Rochtchina E, Lee A, Wang JJ, Mitchell P (2003). "Iris color and intraocular pressure: the Blue Mountains Eye Study". Am. J. Ophthalmol. 135 (3): 384–6. doi:10.1016/S0002-9394(02)01967-0. PMID 12614760.
41. Lindsey JD, Jones HL, Hewitt EG, Angert M, Weinreb RN (2001). "Induction of tyrosinase gene transcription in human iris organ cultures exposed to latanoprost". Arch. Ophthalmol. 119 (6): 853–60. doi:10.1001/archopht.119.6.853. PMID 11405836.
42. ^ Frank RN, Puklin JE, Stock C, Canter LA (2000). "Race, iris color, and age-related macular degeneration". Trans Am Ophthalmol Soc. 98: 109–15, discussion 115–7. PMC 1298217. PMID 11190014.
43. ^ Regan S, Judge HE, Gragoudas ES, Egan KM (1999). "Iris color as a prognostic factor in ocular melanoma". Arch. Ophthalmol. 117 (6): 811–4. doi:10.1001/archopht.117.6.811. PMID 10369595.
44. Hawkins TA, Stewart WC, McMillan TA, Gwynn DR (1994). "Analysis of diode, argon, and Nd: YAG peripheral iridectomy in cadaver eyes". Doc Ophthalmol. 87 (4): 367–76. doi:10.1007/BF01203345. PMID 7851220. S2CID 30893783.
45. Hammond BR, Fuld K, Snodderly DM (1996). "Iris color and macular pigment optical density". Exp. Eye Res. 62 (3): 293–7. doi:10.1006/exer.1996.0035. PMID 8690039.
46. ^ Muiños Díaz, Yerena; Saornil, Maria A.; Almaraz, Ana; Muñoz-Moreno, Mari F.; García, Ciro; Sanz, Ruperto (2009). "Iris color: validation of a new classification and distribution in a Spanish population-based sample". European Journal of Ophthalmology. 19 (4): 686–689. doi:10.1177/112067210901900427. ISSN 1120-6721. PMID 19551689. S2CID 40940828.
47. "Where are hazel eyes most common?". 26 February 2023.
48. Beddoe, John (1971). The Races of Britain. A contribution to the anthropology of Western Europe. Michigan State University. p. 39. ISBN 9780091013707. the Saxons...being very often hazel-eyed
49. Keyser, Christine; Bouakaze, Caroline; Crubézy, Eric; Nikolaev, Valery G.; Montagnon, Daniel; Reis, Tatiana; Ludes, Bertrand (2009). "Ancient DNA Provides New Insights into the History of South Siberian Kurgan People". Human Genetics. 126 (3): 395–410. doi:10.1007/s00439-009-0683-0. PMID 19449030. S2CID 21347353. Indeed, among the SNPs tested was rs12913832, a single DNA variation within a regulatory element of HERC2 gene which is associated to blue eye color in humans. This polymorphism, together with the diplotypes obtained from variations of the OCA2 locus (major contributor to the human eye color variation) showed that at least 60% of the ancient Siberian specimens under study had blue (or green) eyes.
50. Blue Eyes Versus Brown Eyes: A Primer on Eye Color Archived 17 October 2008 at the Wayback Machine. Eyedoctorguide.com. Retrieved on 23 December 2011.
51. Why Do Europeans Have So Many Hair and Eye Colors?. Cogweb.ucla.edu. Retrieved on 23 December 2011.
52. ^ Sulem, P.; Gudbjartsson, D.; et al. (2007). "Genetic determinants of hair, eye and skin pigmentation in Europeans". Nature Genetics. 39 (12): 1443–1452. doi:10.1038/ng.2007.13. PMID 17952075. S2CID 19313549. Retrieved 21 February 2022.
53. "OCA2: The Gene for Color" Archived 6 October 2016 at the Wayback Machine. allaboutgenes.weebly.com. Retrieved on 8 September 2016.
54. Menon IA, Basu PK, Persad S, Avaria M, Felix CC, Kalyanaraman B (1987). "Is there any difference in the photobiological properties of melanins isolated from human blue and brown eyes?". Br J Ophthalmol. 71 (7): 549–52. doi:10.1136/bjo.71.7.549. PMC 1041224. PMID 2820463.
55. Duffy et al. 2007
56. ^ Bryner, Jeanna (31 January 2008). "Genetic mutation makes those brown eyes blue". NBC News. Archived from the original on 1 July 2013. Retrieved 19 October 2009.
57. "How one ancestor helped turn our brown eyes blue". The Independent. 31 January 2008. Retrieved 21 December 2015. "Everyone with blue eyes alive today can trace their ancestry back to one person who probably lived about 10,000 years ago in the Black Sea region, a study has found."
58. Eiberg, Hans; Troelsen, Jesper; Nielsen, Mette; Mikkelsen, Annemette; Mengel-From, Jonas; Kjaer, Klaus W.; Hansen, Lars (2008). "Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression". Hum. Genet. 123 (2): 177–87. doi:10.1007/s00439-007-0460-x. PMID 18172690. S2CID 9886658.
59. Highfield, Roger (30 January 2008). "Blue eyes result of ancient genetic 'mutation'". The Daily Telegraph. London. Archived from the original on 1 November 2009. Retrieved 19 October 2011.
60. Lucock, Mark D. (25 June 2022). "The evolution of human skin pigmentation: A changing medley of vitamins, genetic variability, and <scp>UV</scp> radiation during human expansion". American Journal of Biological Anthropology. 180 (2). Wiley: 252–271. doi:10.1002/ajpa.24564. ISSN 2692-7691. PMC 10083917. PMID 36790744. This may also be a direct counter‐measure to a short winter photoperiod at high latitudes; blue eyes increase intraocular light scattering and thereby suppress melatonin release from the pineal gland. This may be an adaptive trait to reduce/prevent depression, a condition linked to short day length (Higuchi et al., 2007; Lucock et al., 2021).
61. Cavalli-Sforza, Luigi Luca; Cavalli-Sforza, Luca; Menozzi, Paolo; Piazza, Alberto (1994). The History and Geography of Human Genes. Princeton University Press. ISBN 978-0-691-08750-4.
62. "Distribution of Bodily Characters. Pigmentation, the Pilous System, and Morphology of the Soft Parts". Archived from the original on 26 July 2011.
63. Belkin, Douglas (18 October 2006). "Blue eyes are increasingly rare in America - Americas - International Herald Tribune (Published 2006)". The New York Times. ISSN 0362-4331. Retrieved 2 February 2021.
64. Belkin, Douglas (17 October 2006). "Don't it make my blue eyes brown Americans are seeing a dramatic color change". The Boston Globe. Archived from the original on 22 October 2014.
65. Kastelic, V; Pośpiech, E; Draus-Barini, J; Branicki, W; Drobnič, K (2013). "Prediction of eye color in the Slovenian population using the IrisPlex SNPs". Croat. Med. J. 54 (4): 381–6. doi:10.3325/cmj.2013.54.381. PMC 3760663. PMID 23986280.
66. ^ Lucy Southworth. "Are gray eyes the same as blue in terms of genetics?". Understanding Genetics: Human Health and the Genome. Stanford School of Medicine. Archived from the original on 27 September 2011. Retrieved 19 October 2011.
67. (in French) Provincia: bulletin trimestriel de la Société de Statistique ..., Volumes 16–17 By Société de statistique, d'histoire et d'archéologie de Marseille et de Provence p. 273 l'iris gris est celui des chaouias...
68. Iliad 1:206 http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0133%3Abook%3D1%3Acard%3D206
69. "Gray eyes". All About Vision. Retrieved 9 May 2021.
70. ^ "The World's Population By Eye Color". WorldAtlas. 6 October 2020. Retrieved 10 May 2021.
71. NOAH – What is Albinism? Archived 14 May 2012 at the Wayback Machine. Albinism.org. Retrieved on 23 December 2011.
72. Dave Johnson (16 January 2009). "HOW TO: Avoid the red eye effect". New Zealand PC World. Archived from the original on 24 February 2010. Retrieved 9 January 2010.
73. Palmer, Roxanne (25 March 2005). "Elizabeth Taylor: Beautiful Mutant". Slate. Archived from the original on 27 March 2011. Retrieved 26 March 2011.
74. Fertl, Dagmar; Rosel, Patricia (2009). "Albinism". Albinism. Encyclopedia of Marine Mammals (Second ed.). Academic Press. pp. 24–26. doi:10.1016/b978-0-12-373553-9.00006-7. ISBN 978-0-12-373553-9. Retrieved 11 October 2022.
75. White, Désirée; Rabago-Smith, Montserrat (2010). "Genotype–phenotype associations and human eye color". Journal of Human Genetics. 56 (1): 5–7. doi:10.1038/jhg.2010.126. PMID 20944644. S2CID 33808164.
76. Solano, F. (2014). "Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes". New Journal of Science. 2014. hindawi.com: 1–28. doi:10.1155/2014/498276. Eye color change and the affecting factors.
77. Nicolas, Caroline M; Robman, Luba D; Tikellis, Gabriella; Dimitrov, Peter N; Dowrick, Adam; Guymer, Robyn H; McCarty, Catherine A (2003). "Iris colour, ethnic origin and progression of age-related macular degeneration". Clin. Experiment. Ophthalmol. 31 (6): 465–9. doi:10.1046/j.1442-9071.2003.00711.x. PMID 14641151. S2CID 25878963.
78. Rafnsson V, Hrafnkelsson J, Tulinius H, Sigurgeirsson B, Olafsson JH (2004). "Risk factors for malignant melanoma in an Icelandic population sample". Prev Med. 39 (2): 247–52. doi:10.1016/j.ypmed.2004.03.027. PMID 15226032.
79. Stang A, Ahrens W, Anastassiou G, Jöckel KH (2003). "Phenotypical characteristics, lifestyle, social class and uveal melanoma". Ophthalmic Epidemiol. 10 (5): 293–302. doi:10.1076/opep.10.5.293.17319. PMID 14566630. S2CID 1592701.
80. Cumming RG, Mitchell P, Lim R (2000). "Iris color and cataract: The Blue Mountains Eye Study". American Journal of Ophthalmology. 130 (2): 237–238. doi:10.1016/S0002-9394(00)00479-7. PMID 11004303.
81. McDonnell G, Esmonde T (1999). "A homesick student". Postgrad Med J. 75 (884): 375–8. doi:10.1136/pgmj.75.884.375. PMC 1741256. PMID 10435182.
82. ^ de la Maza, Maite Sainz; Tauber, Joseph; Foster, C. Stephen (2012). "Noninflammatory Diseases of the Sclera". The Sclera. pp. 277–297. doi:10.1007/978-1-4419-6502-8_8. ISBN 978-1-4419-6501-1.
83. Wheatley, TJ (2006). "Upper gastrointestinal surgery". In Kingsnorth, Andrew N; Majid, Aljafri A (eds.). Fundamentals of Surgical Practice. pp. 230–248. doi:10.1017/CBO9780511545740.013. ISBN 978-0-511-54574-0.
84. Aniridia at eMedicine
85. Ocular Manifestations of Albinism at eMedicine
86. Imesch PD, Wallow IH, Albert DM (1997). "The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation". Surv Ophthalmol. 41 (Suppl 2): S117–23. doi:10.1016/S0039-6257(97)80018-5. PMID 9154287.
87. Biography for Lee Van Cleef at IMDb
88. Hejkal TW, Camras CB (1999). "Prostaglandin analogs in the treatment of glaucoma". Seminars in Ophthalmology. 14 (3): 114–23. doi:10.3109/08820539909061464. PMID 10790575.
89. Darren Peshek; Negar Semmaknejad; Donald Hoffman; Pete Foley (2011). "Preliminary evidence that the limbal ring influences facial attractiveness" (PDF). Evolutionary Psychology. 9 (2): 137–146. doi:10.1177/147470491100900201. PMC 10519137. PMID 22947961.
90. Brown M, Sacco DF (2018). "Put a (Limbal) Ring on It: Women Perceive Men's Limbal Rings as a Health Cue in Short-Term Mating Domains". Pers Soc Psychol Bull. 44 (1): 80–91. doi:10.1177/0146167217733072. PMID 28978250.
91. ^ O'Connor, Anahad (19 January 2009). "The Claim: Eye Color Can Have an Effect on Vision". The New York Times. ISSN 0362-4331. Retrieved 5 May 2021.
92. ^ Miller, L. K.; Rowe, P. J.; Lund, J. (August 1992). "Correlation of eye color on self-paced and reactive motor performance". Perceptual and Motor Skills. 75 (1): 91–95. doi:10.2466/pms.1992.75.1.91. ISSN 0031-5125. PMID 1528697. S2CID 31398375.
93. Rowe, P. J.; Evans, P. (August 1994). "Ball color, eye color, and a reactive motor skill". Perceptual and Motor Skills. 79 (1 Pt 2): 671–674. doi:10.2466/pms.1994.79.1.671. ISSN 0031-5125. PMID 7808908. S2CID 6743916.
94. Crowe, M.; O'Connor, D. (October 2001). "Eye colour and reaction time to visual stimuli in rugby league players". Perceptual and Motor Skills. 93 (2): 455–460. doi:10.2466/pms.2001.93.2.455. ISSN 0031-5125. PMID 11769902. S2CID 22785194.
95. ^ German EJ, Hurst MA, Wood D, Gilchrist J (1998). "A novel system for the objective classification of iris colour and its correlation with response to 1% tropicamide". Ophthalmic Physiol Opt. 18 (2): 103–10. doi:10.1016/S0275-5408(97)00070-7. PMID 9692029.
96. Fan S, Dyer CR, Hubbard L. Quantification and Correction of Iris Color." Technical report 1495, University of Wisconsin–Madison, Dec 2003.
97. "Martin-Schulz Eye Color Chart". Archived from the original on 2 August 2016. Retrieved 13 January 2017.
98. Piquet-Thepot M.-M. - Bulletins et Mémoires de la Société d'anthropologie de Paris, XII° Série, tome 3 fascicule 3, pg. 207,208 - (1968)
99. Piquet-Thepot, M.-M. (26 March 1968). "Contribution à l'anthropologie des Corses: Anthropologie de la tête (suite)". Bulletins et Mémoires de la Société d'anthropologie de Paris. 3 (3): 183–218. doi:10.3406/bmsap.1968.1417.
100. Seddon, JM; CR Sahagian; RJ Glynn; RD Sperduto; ES Gragoudas (1 August 1990). "Evaluation of an iris color classification system". Investigative Ophthalmology & Visual Science. 31 (8): 1592–8. PMID 2201662. Retrieved 19 October 2011.
101. Jones, S.L.; Schnirel, B.L. (2006). "Subspecies comparison of the Genus: Corucia". Polyphemos. 4 (1): 1–25. Archived from the original on 2 February 2009.
102. Color Perception Archived 20 October 2006 at the Wayback Machine. Edromanguitars.com. Retrieved on 23 December 2011.

External links

• Ask a Geneticist: If both parents have blue eyes, how could they have a child with brown eyes?, The Tech Interactive
• Eye Color and Human Diseases

• Eye color
• Eye
• Facial features
• Color