Can blue eyes and green eyes make brown eyes?

The actual number of genes that contribute to eye color is currently 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. 14 There is evidence that as much 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.

11 A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. 15 Other genes implicated in eye color variation are: SLC24A416 and TYR. Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome.

Iris color can provide a large amount of information about a person, and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals. 17 Various classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison. 17 Others have attempted to set objective standards of color comparison.

Eye colors range from the darkest shades of brown to the lightest tints of blue. 12 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.

19 There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris, along with structural color. Green irides, for example, have blue and some yellow. Brown irides contain mostly brown.

Eye color in animals other than Homo sapiens is differently regulated. 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.

Most babies who have European ancestry have light-colored eyes before the age of one. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed.

Most eye changes happen when the infant is around one year old, although it can happen up to three years of age. 22 Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages.

An iris that appears golden contains some melanin even at this early age and is likely to turn green or brown as the infant ages. Changes (lightening or darkening) of eye colors during puberty, early childhood, pregnancy, and sometimes after serious trauma (like heterochromia) do represent cause for plausible argument to state that some eyes can or do change, based on chemical reactions and hormonal changes within the body. Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined.

Most eye-color changes have been observed or reported in the Caucasian population with hazel and amber eyes. Carleton Coon created this chart by the Martin–Schultz scale often used in physical anthropology. Eyes light and light mixed are 16–12 in Martin scale.

15: Blue, Gray only. 16: Blue exclusive; 16-1a: pure light blue. 14–12 in Martin scale.

12–6 in Martin scale. Mixture of light eyes (blue, gray or green) with brown pigment when light and brown pigment are the same level. 6–4 in Martin scale.

Brown with small admixture of light pigment. 4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown (almost black).

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This may be due to the deposition of the yellow pigment called lipochrome in the iris (which is also found in green eyes). 2425 Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise 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 considered to be like gold, some people have russet or copper colored amber eyes that many people mistake 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. 26 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. 27 In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.

28 Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common color, whereas in humans this color occurs less frequently. There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin.

29 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. 4 This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky.

35 The result is a "Tyndall blue" structural color that varies with external lighting conditions. In humans, the inheritance pattern followed by blue eyes is considered similar to that of a recessive trait (in general, eye color inheritance is considered a polygenic trait, meaning that it is controlled by the interactions of several genes, not just one). 13 In 2008, new research suggested that people with blue eyes have a single common ancestor.

Scientists tracked down a genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Eiberg. 30 Eiberg and colleagues showed 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.

31 The authors concluded that the mutation may have arisen in a single individual probably living in the northwestern part of the Black Sea region (around modern Romania) 6,000–10,000 years ago during the Neolithic revolution.