The eye color is a polygenic phenotypic character determined by two distinct factors: iris pigmentation and light scattering frequency dependence by cloudy media on iris stroma.
In humans, iris pigmentation 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 content of melanin in the iris stroma (located on the front of the iris), and the cellular density of the stroma. The appearance of blue and green, as well as brown eyes, results from the deployment of Tyndall's light on the stroma, a phenomenon similar to those responsible for the bluish sky called Rayleigh scattering. Blue or green pigments never exist in human iris or ocular fluid. Eye color is an example of structural color and varies depending on lighting conditions, especially for brighter colored eyes.
The brightly colored eyes of many bird species are produced from the presence of other pigments, such as pteridine, purine, and carotenoids. Humans and other animals have many phenotypic variations in eye color. The color genetics of the eye is complicated, and the color is determined by many genes. So far, as many as 15 genes have been associated with eye color inheritance. Some eye-color genes include OCA2 and HERC2 . The previous belief that blue eye color is a simple recessive trait has proven to be untrue. Eye color genetics is so complex that almost any parent-child color combinations can occur. However, the OCA2 gene polymorphism, approaching the proximal 5? regulatory area, explaining most of the color variations of the human eye.
Video Eye color
Penentuan genetik
Eye color is an inherited trait affected by more than one gene. These genes are sought using associations for small changes in the gene itself and in neighboring genes. This change is known as single nucleotide polymorphism or SNP. The actual number of genes that contribute to eye color is currently unknown, but there are several possible candidates. A study in Rotterdam (2009) found that it is possible to predict eye color with an accuracy of more than 90% for chocolate and blue using only six SNPs. There is evidence that as many as 16 different genes can be responsible for eye color in humans; however, the two main genes associated with eye color variations are OCA2 and HERC2 , and both are localized in Chromosome 15.
The OCA2 gene (OMIM: 203200), when in varying forms, causes red eye color and general hypopigmentation in human albinism. (The gene's name comes from the resulting deformity, type II oculocutaneous albinism.) Different SNPs within OCA2 are strongly associated with blue and green eyes and variations in freckling, number of moles, hair and skin color. Polymorphisms may be in the order of the OCA2 rules, in which they can affect the expression of the gene product, which in turn affects pigmentation. A specific mutation in the HERC2 gene, the gene that regulates the expression OCA2 , is partly responsible for the blue eyes. Other genes involved in eye color variations are SLC24A4 and TYR. A 2010 study of eye color variations into hue and saturation values ​​using high resolution full-eye digital photos found three new loci for a total of ten genes, and now about 50% of eye color variations can be explained.
Blue eyes with brown spots, green eyes, and gray eyes are caused by completely different parts of the genome.
Maps Eye color
Ancient DNA and eye colors in Europe
Ancient DNA and eye color in EuropePeople of European descent show the largest variations of eye color of the population worldwide. Recent advances in ancient DNA technology have revealed some history of eye color in Europe. All the European Mesolithic hunter-gatherers still investigated so far have shown genetic markers for brightly colored eyes, in the case of western and central European hunter-gatherers combined with dark skin tones. The subsequent addition to the European gene pool, the early Neolithic peasants of Anatolia and the Yamnaya Copper Age/Bronze Age pastoralists (presumably Proto-Indo-European populations) from the northern regions of the Black Sea appear to have a much higher incidence of dark-colored alleles, and alleles which gives brighter skin, than the native European population.
Color classification
Iris color can provide a large amount of information about a person, and color classification may be useful in documenting pathological changes or determining how one can respond to ocular medicines. The classification system has ranged from basic light or dark descriptions to detailed gradations using photographic standards for comparison. Others have attempted to set an objective standard of color comparison.
Eye color ranges from the darkest brown shades to the brightest blue color. To meet the need for standard classification, as well as simple but detailed enough for research purposes, Seddon et al. developing a multilevel system based on the dominant iris color and the number of existing brown or yellow pigments. There are three colors of the pigment that determine, depending on their proportions, the outer appearance of the iris, along with the structural color. The green slice, for example, is blue and partly yellow. Brown slices mostly contain chocolate. Some eyes have dark circles around the iris, called limbal rings.
Eye color in non-human animals is arranged differently. For example, instead of blue as in humans, the color of autosomal recessive eyes in the skink species of Corucia zebrata is black, and the autosomal dominant color is yellow-green.
Because the perception of color depends on the viewing conditions (eg, the amount and type of lighting, as well as the color of the surrounding environment), so does the perception of eye color.
Changes in eye color
Most of the newborns who have European descent have brightly colored eyes. As the child develops, melanocytes (cells found within the iris of the human eye, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continue to produce pigments, theoretically the color of the eye can be changed. Mature eye colors are usually formed between 3 and 6 months, although this can be later. Observing the baby's iris from the side using only transmitted light without reflection from behind the iris, it is possible to detect the presence or absence of low levels of melanin. A blue-shaped iris under this observational method is more likely to remain blue during infancy. A golden iris contains some melanin even at this early age and tends to change from blue to green or brown during infancy.
The change (enlightenment or darkening) of eye color during childhood, puberty, pregnancy, and sometimes after serious trauma (such as heterochromia) is indeed the cause for a plausible argument that states that some eyes can or do change, based on chemical reactions and hormonal changes in the body.
Studies on Twin Caucasus, both brotherhood and identical, have shown that eye color over time may change, and the major demelanisation of the iris can also be determined genetically. Most eye color changes have been observed or reported in the Caucasian population with brown and yellow eyes.
Eye color chart (Martin scale)
Carleton Coon graphs with the original Martin scale. Numbering is reversed on the scale below in Martin-Schultz's (later) scale, which is still used in physical anthropology.
Light and light-blends of eyes (16-12 in Martin scale)
Pure light (16-15 in Martin scale)
- 16: pure bright blue
- 15: gray
The mixes (14-12 in the Martin scale)
- 14: Very bright (blue with grey or green or greyish green)
- 13-12: Light mixed (light or very bright mixed with a mixture of small brown color)
Mixed eye (Scale 11-7 in Martin scale) Mixed light (blue, gray or green) eyes with brown when bright and brown appear on the same level.
Mixed dark and dark eyes (6-1 in Martin scale)
- Dark color: 6-5 in the Martin scale. Chocolate with a small light mixture
- Dark: 4-1 on the Martin scale. Chocolate (brown and dark brown) and very dark chocolate (almost black)
Amber
Yellow eyes have a solid color and have a strong yellow/gold and pink/copper color. This may be due to the deposition of a yellow pigment called lipoxrome in the iris (which is also found in green eyes). Amber's eyes should not be confused with brown eyes; Although brown eyes may contain yellow or gold spots, they usually tend to contain many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of spots and ripples, while the yellow eye is a solid gold color. Although amber is considered to be gold, some people have reddish brown or copper-colored amber eyes that many people are mistaken for hazel, although hazel tends to be duller and contains a green color with red/gold spots, as mentioned above. Amber's eyes can also contain a very light gray gold color.
The eyes of several pigeons contain a yellow fluorescence pigment known as pteridine. The bright yellow eyes of the large horned owl are thought to be due to the presence of xanthopterin pteridine pigments in certain chromatophores (called xanthophores) located on the iris stroma. In humans, patches or yellowish spots are thought to be due to lipofuscin pigment, also known as lipoxrome. Many animals such as canine teeth, domestic cats, owls, eagles, pigeons and fish have yellow eyes as a common color, whereas in humans this color is less common.
Blue
There is no blue pigmentation either in the iris or in ocular fluid. Dissection revealed that the iris pigment epithelium is blackish brown due to melanin. Unlike brown eyes, blue eyes have low melanin concentrations in the iris stroma, located in front of dark epithelium. Longer wavelengths of light tend to be absorbed by dark epithelials, while shorter wavelengths are reflected and experience Rayleigh scattering in stroma cloudy medium. This is the same frequency dependence of the scattering that causes the appearance of the sky blue. The result is a "Tyndall Blue" structural color that varies with external lighting conditions.
In humans, the pattern of inheritance followed by blue eyes is thought to be similar to the recessive nature (in general, the inheritance of the eye color is considered a polygenic property, meaning that it is controlled by the interaction of several genes, not just one). In 2008, new research tracked a single genetic mutation leading to blue eyes. "At first, we all had brown eyes," Eiberg said. Eiberg and colleagues suggest in a study published in Human Genetics that mutations in the 86th intron of the HERC2 gene, hypothesized to interact with OCA2 > gene promoter, reduces expression OCA2 with the next decrease in melanin production. The authors suggest that mutations may appear in the northwestern part of the Black Sea region, but added that "it is difficult to calculate the age of mutations."
Blue eyes are common 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. In West Asia, the proportion of Israelis comes from Ashkenazi, of which the nature is relatively high (a study conducted in 1911 found that 53.7% of Ukrainian Jews were blue-eyed).
The same sequence of DNA in the OCA2 gene region among blue-eyed people suggests they may have the same single ancestor.
DNA studies in ancient human remains confirm that bright skin, hair and eyes were present at least tens of thousands of years ago in the Neanderthals, who lived in Eurasia for 500,000 years. In 2016, the earliest pigmented and blue-eyed light remains from Homo Sapiens were discovered in 7,700 years of Mesolithic hunter-gatherers from Motala, Sweden.
A 2002 study found that the prevalence of blue-eye color among white population in the United States to 33.8% for those born from 1936 to 1951 compared with 57.4 percent for those born from 1899 to 1905. In 2006, one in every six people, or 16.6% of the total population, and 22.3% whites, have blue eyes. Blue eyes continue to be less common among American children.
Blue eyes are rarely found in mammals; one example is a newly discovered marsupial, blue-eyed cuscus ( Spilocuscus wilsoni ). This trait is now known only from single primates other than humans - lemurs Sclater ( Eulemur flavifrons ) from Madagascar. While some cats and dogs have blue eyes, this is usually due to other mutations associated with deafness. But in cats alone, there are four identified gene mutations that produce blue eyes, some of which are related to congenital neurological disorders. Mutations found in Siamese cats are associated with strabismus (eye cross). Mutation is found in a dense, blue-eyed white cat (in which the color of the fur is caused by a gene for "epistatic white") is associated with deafness. However, there is the same genotype, but different genotypes, blue-eyed white cats (in which the color of the fur is caused by genes for white patches) where feather color is not strongly associated with deafness. In blue-eyed Ojos Azules proliferate, there may be other neurological defects. The blue-eyed non-white cat of the unknown genotype also occurs randomly in the cat population.
Brown
In humans, brown eyes result from a relatively high concentration of melanin in the iris stroma, which causes shorter and longer wavelength light to be absorbed.
Dark brown eyes are dominant in humans and in many parts of the world, almost the only irises available. The dark brown eye pigment is common in Europe, Southern Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa, America, etc. As well as parts of Eastern Europe and Southern Europe. The majority of people in the world as a whole have brown eyes to dark brown eyes.
Light brown or medium-pigmented eyes can also be found in Southern Europe, among America, and parts of Central Asia (Middle East and South Asia).
Gray
Like blue eyes, gray eyes have dark epitelium on the back of the iris and a relatively clear stroma on the front. One possible explanation for the difference in the appearance of gray and blue eyes is that the gray eyes have larger collagen deposits in the stroma, so the light reflected from the epithelium undergoes Noodle scattering (which does not depend on the strong frequency) of scattering Rayleigh (where shorter wavelengths of light are scattered over). It will be analogous to the sky's color change, from the blue provided by Rayleigh's ray from the sunlight by small gas molecules when the sky is bright, to the gray caused by the scattering of large water droplets when the sky is cloudy. Or, it has been suggested that gray and blue eyes may differ in melanin concentrations in front of the stroma.
Gray eyes are most common in Northern and Eastern Europe. Gray eyes can also be found among the Algerian Shawia people from the AurÃÆ'¨s Mountains in Northwest Africa, in the Middle East, Central Asia, and South Asia. Under enlargement, gray eyes show a small amount of yellow and brown in the iris.
Green
Like blue eyes, the green eye color does not just come from iris pigmentation. The green color is caused by a combination of: 1) yellow or light brown pigment in iris stroma (which has a low or medium melanin concentration) with: 2) the blue color created by the scattering of Rayleigh rays from the reflected light. The green eye contains yellowish pigment lipochrome.
Green eyes may result from interactions of some variants within OCA2 and other genes. They were present in southern Siberia during the Bronze Age.
They are most common in Northern Europe, the West and the Middle. In Ireland and Scotland, 14% of people have brown eyes and 86% have blue or green eyes. In Iceland, 89% of women and 87% of men have blue or green eyes. A study of Icelandic and Dutch adults found green eyes much more common in women than in men. Among European Americans, green eyes are the most common among Celtic and Germanic ancestors, about 16%. 37.2% of Italy from Verona and 56% of Slovenia have blue/green eyes.
Green eyes are common in Tabby cats as well as Chinchilla Longhair and their short-line equivalents are well-known for their dark-colored sea-black eyes.
Hazel
Brown eyes are caused by a combination of Rayleigh scattering and a certain amount of melanin in the anterior layer of the iris border. Chocolate eyes often appear to shift in color from brown to green. Although hazel is predominantly brown and green, the dominant color in the eye may be brown/gold or green. This is how many people think hazel eyes turn yellow and vice versa. This can sometimes produce colorful irises, ie yellow/yellow eyes near the pupils and charcoal or dark green on the outside of the iris (or vice versa) when observed in the sun.
The definition of hazel color varies: sometimes considered synonymous with light brown or gold, such as hazelnut.
Brown eyes occur throughout the population of Caucasids, especially in areas where blue-eyed, green and brown-eyes mix together.
Red and purple
The eyes of people with severe forms of albinism may appear red under certain lighting conditions because of the very low amount of melanin, allowing the blood vessels to show through. In addition, lightning photography can sometimes lead to "red-eye effect", where a very bright light from flash reflects the retina, which is abundantly vascular, causing the pupils to appear red in the photo. Although the blue eyes of some people like Elizabeth Taylor can appear purple at certain times, the "right" purple eye only occurs due to albinism.
Spectrum of eye color
Medical implications
Those with lighter iris colors have been found to have a higher prevalence of age-related macular degeneration (ARMD) compared with those with darker iris color; Bright eye color is also associated with an increased risk of ARMD development. A gray iris may indicate the presence of uveitis, and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. However, a study in 2000 showed that people with dark brown eyes were at an increased risk of developing cataracts and therefore had to protect their eyes from direct exposure to sunlight.
Wilson's disease
Wilson's disease involves gene mutations encoding the ATPase7B enzyme, which prevents the copper in the liver from entering the Golgi apparatus in the cell. In contrast, copper accumulates in the liver and in other tissues, including the iris. This results in the formation of the Kayser-Fleischer ring, which is a dark circle that surrounds the edges of the iris.
Coloring sclera
Eye color outside the iris can also be a symptom of the disease. The yellowing of sclera ("white eyes") is associated with jaundice, and possibly symptoms of liver disease such as cirrhosis or hepatitis. The blue sclera may also be a symptom of the disease. In general, abrupt changes in the sclera color should be handled by a medical professional.
Anomalous conditions
Aniridia
Aniridia is a congenital condition characterized by a very underdeveloped iris, which appears to be absent on superficial examination.
Albinism eye and eye color
Usually, there is a thick melanin layer on the back of the iris. Even the people with the lightest blue eyes, without melanin in front of the iris altogether, have a dark brown color on the back, to prevent the light from spreading around the eyes. In those with milder forms of albinism, iris colors are usually blue but may vary from blue to brown. In the form of severe albinism, there is no pigment in the back of the iris, and the light from within the eye can pass through the iris forward. In this case, the only visible color is red from blood hemoglobin in the iris capillaries. The albino has pink eyes, as do albino rabbits, mice, or other animals with total melanin. Transillumination defects are almost always observable during eye examination due to lack of iridial pigmentation. Ocular albino also does not have the normal amount of melanin in the retina, which allows more light than usual to bounce the retina and out of the eye. Therefore, pupil reflexes are much more prominent in albino individuals, and this can emphasize the red-eye effect on the photo.
Heterochromia
Heterochromia ( heterochromia iridum or heterochromia iridis ) is an eye condition in which one iris is a different color than another (complete heterochromia), or where part of one iris is a color which differs from the rest (partial heterochromia or sectoral heterochromia). This is the result of the relative excess or lack of pigment in the iris or part of the iris, which may be inherited or acquired by disease or injury. This unusual condition is usually caused by uneven melanin content. A number of responsible causes, including genetics, such as chimerism, Horner syndrome and Waardenburg syndrome.
A chimera can have two different colored eyes that two brothers can have - because each cell has different eye color genes. Mosaic can have two different colored eyes if DNA differences occur in eye color genes.
There are many other possible reasons for having two different colored eyes. For example, film actor Lee Van Cleef was born with one blue eye and one green eye, a trait commonly reported in his family, indicating that it is a genetic trait. This anomaly, which, according to the film producer will disturb the movie audience, is "fixed" because Van Cleef wears brown contact lenses. David Bowie, on the other hand, has a different eye-color appearance due to an injury that causes one pupil to be permanently dilated.
Another hypothesis about heterochromia is that it can result from a virus infection in utero that affects the development of one eye, perhaps through some type of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition.
A common cause in women with heterochromia is X-inactivation, which can produce a number of heterochromatic properties, such as calico cats. Trauma and certain drugs, such as some analogue prostaglandins, can also cause increased pigmentation in one eye. Occasionally, eye color differences are caused by blood staining iris after injury.
See also
- Hair color
- Limbal ring
- Iridology
- Human skin color
- Xanthophore
- List of Mendelian traits in humans
References
External links
- genetics.thetech.org
- Eye Color and Human Illness
Source of the article : Wikipedia
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