Eye color

from Wikipedia, the free encyclopedia
Color gradations of the eye

The color of the eyes in humans and most vertebrates is determined by pigments in the stroma of the iris ( lat. Iris ). It is the result of complex processes in which several genes are involved.

gallery

  • amber eye

  • green eye

  • green-brown eye

  • gray eye

  • black eye

  • light brown eye

  • light blue eye

  • blue-green eye with central heterochromia

  • Blue eye with both central iris heterochromia and sectoral iris heterochromia

Degree of pigmentation

The basic tone in people of European descent is mostly blue, here the stroma of the iris is largely unpigmented and the coloration of the blue and gray eye color is due to different scattering in the stroma (structural colors). Green eye color is the result of a combination of little pigment and structural color (blue). The blue structural color is particularly evident in newborns with light skin color , who initially usually have blue-looking eyes , as the dye melanin , which is responsible for the coloration of the pigments, is hardly present in them. Babies from Asia , Africa and Latin America are usually born with brown eyes, depending on their parents' genes. Whether a newborn baby keeps the blue or non-brown eye color is decided in humans, depending on the genetic position, at the end of the first year of life and can even change in the course of puberty . Even domestic cats usually come with light blue eyes to the world and develop later Eye color after about three months. Various point cat breeds ( Siamese cats , Holy Burma ) and a few breeds of dogs (e.g. huskies ) also have blue eyes as adults.

Due to the storage of the brown-coloring melanin in the iris own layer, a characteristic eye color is formed which, depending on the amount of pigment, ranges from gray, yellow, green to brown, and with a correspondingly high amount of melanin even to black. In humans, this mostly correlates with skin and hair color . Light-skinned and blonde people are more likely to have blue eyes, while dark-skinned people with dark hair usually have a brown iris color. It should also be said that the skin tone is only weakly causally influenced by the alleles (gene variants) for eye and hair color, apart from those alleles for blue eyes and red hair , which cause the skin tone to be lightened.

The so-called iris heterochromia , in which a person has different eye colors, is very rare, the central heterochromia of the eyes, in which the iris has a different-colored edge around the pupil, is more common. It can also be observed that the color of the eyes is related to the amount of sunlight. People in the Arctic usually have blue eyes because there is hardly any direct sunlight there. Brown coloring of the iris is typical for many mammals.

Eye lightening from albinism and leucism

Complete albinism: a
cat who has red eyes from complete albinism

If the pigment in the posterior epithelium of the iris and in the choroid is also missing , the blood vessels of the fundus appear through and the iris appears red. Such red eyes occur occasionally in albinism and leucism , but weaker forms of albinism, leucism and various piebald genes can also lighten darker eye colors to blue or light brown. These iris lightening are possible in birds as well as in mammals and reptiles.

Leucism:
In this white cat, one of the two eyes has been lightened to blue by leucism, while the other is a normal color.

It is often wrongly assumed that one can distinguish animals with albinism from those with leucism on the basis of the color of their eyes, recognizing albinos by their red, leucistic animals by their blue eye color. Indeed, there are many examples of red-eyed animals with albinism and blue-eyed leucistic animals with completely white fur. Although in albinism the melanin formation disorder always affects the whole animal and in leucism some melanocytes often find the shorter way to the eye, there are nevertheless blue-eyed albinos, such as horses of the color Cremello . There are also leucistic animals with red or pink eyes such as mice with mutations of the Mitf gene.

Occasionally, condensation of the connective tissue in the iris stroma can lead to the formation of white spots. Such a black eye is more common in spotted dog breeds ( Dalmatian , Great Dane ) and horses. It is not a disease. These white spots can cover the whole iris, so that the whole eye appears white (glass eye).

Inheritance

At least three different genes are involved in the inheritance of eye color , the exact functions of which are not yet fully understood. Which eye color the child will have depends on the combination of genes that are passed on to the child from the two parents.

The majority of light-skinned newborns initially have blue-appearing eyes.

The 1-gene model published by GC Davenport and CB Davenport in 1907 is the easiest way to explain the inheritance of eye color, but does not adequately describe the complex reality of inheritance, as we know today. According to this model, there is only one gene that determines the eye color (brown or blue). This gene is diploid. H. so in two copies. In the Davenport model, there are two levels ( alleles ) for each copy , one for blue eyes (i.e. no ability to produce melanin ) and one allele for brown eyes (i.e. ability to produce melanin). For example, if the father has two brown eye color alleles and the mother has two blue eye color alleles, the child will receive one brown and one blue allele.

In the Davenport model, the color of the eyes that humans later have depends on which allele is dominant and which is recessive . The rule is that brown eyes are dominant, while blue eyes are recessive. That is, in the example above, the child would have brown eyes. In a further generation, however, both genes can be passed on equally. If the above child becomes the father whose wife also has brown eyes (with one blue and one brown allele), then the probability is 25% that a blue-eyed child will result who has two blue genes and cannot pass on brown genes. There is a 75% chance that a brown-eyed child will develop. Overall, the probability that at least one of the two alleles is blue is 75%.

The Davenport model described is now considered outdated. In reality, the inheritance of the eye colors brown / green / blue / gray is controlled by more than one gene. The function of the genes bey2 (abbreviation for brown eye 2 ) and gey ( green eye ) is considered to be guaranteed. For the gene bey2 there are alleles for brown and blue eyes, for the gene gey there are alleles for green and blue eyes. The following order of dominance applies: From top to bottom, dominance decreases and recessivity increases.

  1. brown
  2. green
  3. blue
  4. Gray

Each of these genes is diploid. In this way, as in the simplified Davenport model described above, recessive genes can be passed on to the child generation, even if these genes do not determine the phenotype ( i.e. the external appearance) in the parent generation .

One assumes the effect of other genes that influence the different shades of the colors and control the expression (i.e. the reading out) of the other genes involved, since the genes bey2 and gey outlined above do not explain all inheritance cases either . The case of brown-eyed children in parents with blue or green eyes beyond the genes described above is explained (a) by mutations in the male germ line, (b) by the effect of other (possibly regulating) genes and (c) by the complexity of the Melanin production. This process consists of a chain of chemical reactions. To clarify the scheme, imagine the process of melanin production as a result of the conversion of substance A to B and B to melanin, greatly simplified. If the first process (conversion from A to B) does not work for genetic reasons in one parent, but the subsequent reaction B to melanin can take place and A can be converted into B in the other parent, but B cannot be converted into melanin due to the genes present then no parent has brown eyes (since no parent can complete the chain of reactions). However, a child can inherit the ability to convert A to B from one parent and the ability to convert B to melanin from the other parent, so that they have brown eyes. This explains, on the one hand, why the color of the eyes of children can differ greatly from the color of both parents, not only because of diploid chromosome sets . On the other hand, it also becomes clear that the color of the eyes of children does not necessarily have to have come from the grandparents' generation (or other generations).

distribution

Around 90 percent of all people worldwide have brown eyes, including the vast majority of people of non-European descent.

The rest is divided into blue, green and gray, with green being the rarest eye color with less than 2%. There are very few brown-eyed people in the Baltic Sea region . Estonia is the country where blue eyes are the most common (99% of the population).

According to statements made by genetic researcher Hans Eiberg from the University of Copenhagen, the gene switch for the development of blue eyes only emerged through mutation an estimated six to ten thousand years ago. This change is so specific that he suspects that all blue-eyed people today are descended from the same person. Geographically, he locates it northeast of the Black Sea .

A survey conducted in 2013 genome analysis of a good 30-year-old man ( Braña 1 ) that about 7000 years ago in the north of the Iberian Peninsula had lived and whose bones the end of 2006 in the La Braña -Arintero Cave in Cantabrian Cordillera had been discovered , the earliest genes for blue eyes have been identified (same mutation of the HERC2 gene on both chromosomes as in modern humans). The fact that genes for dark skin and hair color, which are typical for Africans, were identified.

Eye color as a biological characteristic

The color of the eyes and their connection with skin tone and hair color have a decisive influence on the appearance of a person. The color of the eyes is entered as a (largely) unchangeable body characteristic in the identity card and is usually part of a precise description of the person.

Web links

Commons : Eye Color  - Collection of images, videos, and audio files

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  1. ^ Lee Ann Remington, Denise Goodwin: Clinical Anatomy of the Visual System E-Book . Elsevier Health Sciences, July 29, 2011, ISBN 1-4557-2777-6 , pp. 46-7.
  2. ↑ Focus magazine: Do all babies have blue eyes? (June 2, 2008)
  3. ↑ Peter Frost: Why Do Europeans Have So Many Hair and Eye Colors? ( Memento from January 2, 2008 in the Internet Archive ) Yet skin color is weakly influenced by the different alleles for hair color or eye color, apart from the ones for red hair or blue eyes. Some have no effect at all on skin pigmentation.
  4. ↑ Duffy et al. 2004. Interactive effects of MC1R and OCA2 on melanoma risk phenotypes All blue-eyed R / R individuals (Note: R stands for a strong red hair allel, in contrast to r. Both are recessive.) Were in the fair / pale skin category but this decreased to 85.4% with fair / pale skin for brown / green-eyed R / R individuals, the remainder having medium skin color. This proportionate lightening in all genotypic groups when carrying both recessive blue-eyed b and red-hair R alleles indicates additive action of MC1R and BEY2 / OCA2 loci on constitutive skin color.
  5. ↑ 90% of people have brown eyes . In: Optiker Magazin . July 19, 2010. Archived from the original on November 16, 2012. Retrieved October 25, 2014.
  6. ↑ http://www.usatoday.com/news/health/2008-02-05-blue-eyes_N.htm In Estonia, 99% of people have blue eyes, Eiberg says. In Denmark 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In Germany, about 75% have blue eyes.
  7. ↑ Hans Eiberg et al. 2007. 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 .
  8. ↑ Ancestor of the blue eyes, trend color of evolution, Sueddeutsche.de - Knowledge (www.sueddeutsche.de)
  9. ↑ Vidal Encinas, Julio M. et al .: LOS HOMBRES MESOLÍTICOS DE LA BRAÑA-ARINTERO (VALDELUGUEROS, LEÓN): UN HALLAZGO FUNERARIO EXCEPCIONALEN LA VERTIENTE MERIDIONAL DE LA CORDILLERA CANTÁBRICA , No. 5, Férvedes Vilalba 2008 .: 153–164 at academia.edu, accessed on January 28, 2014 (Spanish, with English summary).
  10. ↑ Steve Connor: Revealed: First Ol 'Blue Eyes is 7,000 years old and was a caveman living in Spain on independent.co.uk on January 26, 2014, accessed on January 28, 2014.
  11. ↑ Olalde I. et al .: Derived immune and ancestral pigmentation alleles in a 7,000-year-old Mesolithic European , nature.com from January 26, 2014, accessed on January 28, 2014 (English). doi: 10.1038 / nature12960
  12. ↑ Hubert Filser: Blue eyes and dark skin among Europe's Stone Age people , süddeutsche.de from January 27, 2014, accessed on January 28, 2014.

Eye color in leucism and albinism