Eye color is one of the most striking and variable physical traits in humans. The hue of one’s eyes is determined by the amount and type of pigment in the iris. The two main pigments that determine eye color are melanin and lipochrome. Melanin comes in two forms – eumelanin which produces brown/black pigment and pheomelanin which produces red/yellow pigment. The amount of these pigments present in the iris determines the final eye color.
Genetic basis of eye color
The inheritance of eye color is complex and determined by multiple genes. The most important gene is the OCA2 gene located on chromosome 15 which encodes the P protein that is involved in the production and transport of melanin. However, there are many other genes that influence eye color to varying degrees such as SLC24A4, TYR, SLC45A2 etc. The interaction between these genes accounts for the continuum of eye colors we see in human populations.
| Gene | Chromosome | Protein | Effect on eye color |
|---|---|---|---|
| OCA2 | 15 | P protein | Melanin production and transport |
| SLC24A4 | 14 | Sodium/potassium/calcium exchanger | Melanin production |
| TYR | 11 | Tyrosinase | Converts tyrosine to melanin |
| SLC45A2 | 5 | Membrane transport protein | Melanin production and transport |
Patterns of inheritance
The inheritance of eye color does not follow simple Mendelian genetics. It is a polygenic trait controlled by multiple genes exhibiting incomplete dominance. This means each gene that controls eye color has more than two alleles. Moreover, each of these alleles may be dominant over one allele at that gene but recessive to another. This complex interactive effect between several genes and their many alleles is what produces eye colors that are not categorically brown, blue, green etc. but exist on a spectrum.
Nevertheless, some broad patterns of inheritance can be discerned:
– Brown eye color is dominant over blue as it involves the production of melanin. However, shades of brown can be inherited in a non-Mendelian manner based on variants at several gene loci.
– Blue eye color is recessive and requires individuals to inherit two non-functional copies of the OCA2 gene reducing melanin production. However, epistasis with other genes can produce intermediate eye colors.
– Green eye color arises due to interaction of multiple genes. The presence of some melanin along with lipochrome pigments leads to greenish hues. The exact genetic combination is still being studied.
– Other rarer eye colors like gray, violet, red may involve rare mutations or combinations of genes governing melanin/lipochrome levels. Their inheritance patterns are complex and unclear.
Thus, the interaction between the OCA2 gene and other eye color genes produces a spectrum of possibilities in a non-Mendelian manner. Environment, age and epigenetics further modulate these effects.
Role of the environment
Although eye color is strongly genetically determined, some environmental factors can also influence pigment production and impact final eye color:
– **Sun exposure:** Increased melanin production due to UV light exposure can lead to development of tan/brown limbal rings around the iris periphery. Prolonged exposure over years can cause overall darkening of light colored eyes.
– **Diet:** Lutein and zeaxanthin carotenoids found in green vegetables, eggs etc. may lead to increased lipochrome deposition and make eyes appear greener.
– **Age:** Iris pigment levels and eye color can change slightly over age. Melanin production may reduce over time leading to lightening of eye color in some individuals.
– **Medications:** Certain medicines like chloroquine and hormone treatments may bind to melanin and affect eye color. Some SSRI antidepressants can also cause pupillary dilations that make eyes appear darker.
However, the effect of environment is subtle, secondary to genetics and limited to minor changes in shade rather than drastically altering one’s eye color.
Conclusion
In summary, eye color inheritance is polygenic and does not follow simple Mendelian patterns. The major OCA2 gene on chromosome 15 interacting with many other eye color genes produces varying amounts of melanin pigment that determine the final eye hue. Modifiers like age, sun exposure and diet can also subtly influence eye color. Understanding the genetic basis of this striking human trait continues to be an active area of research.
References
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