Eye Color[]
Melanin is the pigment that determines your eye color, as well as skin and hair color. Eyes can be anywhere from deep brown to pale blue, depending on how much melanin is in the iris cells. Eye color is an example of a trait that exhibits pleiotropy, meaning multiple genes and variants of these genes interact in the determination of eye color.
Important Discoveries Regarding Eye Color[]
There is a hypothesis that 6,000-10,000 years ago, a mutation event in a population living somewhere near the Black Sea created the SNP that determines blue vs. non-blue eyes. SNPs are single nucleotide polymorphisms, or variations of single nucleotides between shared sequences of DNA. In 1994 there was a definitive association made between the P gene (now known as OCA2) and oculocutaneous albinism 2. This condition causes people to have light hair, skin, eyes and vision problems. A study conducted on Europeans in 2004 denotes that 74% of variation in eye color can be linked to the OCA2 gene. In 2008, several groups of scientists discovered that a SNP in the HERC2 gene, extremely close to the OCA2 gene is likely the only determinant of blue vs. non-vlue eye color.
Genotype vs. Phenotype[]
There are three genotypes that indicate eye color. The genotype AA indicates a high chance of brown eyes with severely decreased chances of green eyes and almost no chance of blue eyes. Genotype AG has more similar chances of having green or brown eyes but still a drastically smaller chance of having blue eyes. GG, the last genotype, displays a very high chance of blue eyes with a small chance of green eyes and nearly no chance of brown eyes. Studies have approximated that 90-99% of human eye color variation is due to genetics, indicating that the heritability of eyes color is extremely high.
The HERC2 and OCA2 Genes[]
It has been known for quite some time that eye colors such as green and hazel (due to variations of the most common eye color; brown) can sometimes be elucidated by SNP's in a gene known as OCA2. This gene encodes a protein dubbed "P protein", located in melanin-producing cells. Although scientists are unclear on the precise function of the P protein, it is integral for standard pigmentation and most likely plays a key role in the production of melanin. Although the OCA2 gene can sometimes explain variations in eye color, this is not always the case. Scientists now believe that variations in a SNP in the HERC2 gene, located near OCA2, may affect how much P protein OCA2 synthesizes.
Individuals homozygous for the A version of the SNP in HERC2, are almost always brown-eyed. Heterozygotes exhibiting an A copy and a G copy of the SNP can produce variations of green or brown eyes, these colors likely depend on other variations in OAC2 or other genes. People who are homozygous for the G version of this SNP normally have blue eyes but green eyes are also a possibility, variations of these eye colors are due to other SNPs. In the case of eye color, AA, is the dominant genotype, denoting the most common phenotype in humans; brown eyes. In a case of complete dominance, the presence of A in any genotype would lead to brown eyes, however this is not true. What is exhibited in the case of eye color, is incomplete dominance. This is used to describe a dominance situation in which homozygotes of the dominant allele display one phenotype, homozygotes of the recessive allele display a separate phenotypes, and heterozygotes display a blend of the two phenotypes. This is shown in eye color where genotype AA means brown eyes, genotype GG means blue or sometimes green eyes, and genotype AG means a mix of brown and green eyes.
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Table 1. Showing relationship between genotype and phenotypes of eye color.
Based on the sample collected from John Burke, a professor at the University of Vermont, by 23andMe, a company that sequences genomes, it was possible to explore different aspects of Professor Burke's genome. According to the data gathered from the sample he sent in, his genotype at the SNP on OCA2 that determines eye color is GG. This is the recessive genotype that most often presents as a blue or green eyed phenotype. Professor Burke is unique, as blue is one of the least common eye colors in humans.
References[]
Inc., a. (2007). 23andMe, from https://www.23andme.com/you/