A person’s eye color depends on how much of a pigment called melanin is stored in the front layers of the iris, the structure surrounding the pupil. Specialized cells called melanocytes produce the melanin, which is stored in intracellular compartment called melanosomes. People have roughly the same number of melanocytes, but the amount of melanin within melanosomes and the number of melanosomes within melanocytes both vary.
Eye color ranges depending on how much melanin is stored in these compartments. In people with blue eyes a minimal amount of melanin is found within a small number of melanosomes. People with green eyes have a moderate amount of melanin and moderate number of melanosomes, while people with brown eyes have high amount of melanin stored within many melanosomes.
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Genes involved in determining eye Color
The amount of melanin stored is determined by genes that are involved in the production, transport and storage of melanin.
To date, researchers have discovered more than 150 genes that influence eye color, a number of which have been discovered through studies of genetic disorders. Others have been identified during genomic studies of mice and fish.
Some genes play a major role in determining eye color, while others only have a small contribution.
One region of chromosome 15 contains two genes located near to each other that play major roles in determining eye color. One gene, called OCA2, codes for a protein called P protein, which is involved in melanosome maturation and affects the amount and quality of melanin stored in the iris. A number of genetic variations (polymorphisms) in this gene reduce how much P protein is produced and result in a lighter eye color.
The other main gene involved is called HERC2. Intron 86 on this gene controls the expression of OCA2, activating it or deactivating it as required. At least one polymorphism in this intron reduces the expression and activity of OCA2,which reduces how much P protein is produced.
A number of other genes play smaller roles in eye color. The roles of the genes ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1 are thought to combine with those of OCA2 and HERC2.
Eye color inheritance pattern
Due to the number of genes involved in eye color, the inheritance pattern is complex. Although a child’s eye color can generally be predicted by looking at the color of the parents’ eyes, the polymorphisms that can arise mean a child may well have an unexpected eye color.
A child’s eye color depends on the pairing of genes passed on from each parent, which is thought to involve at least three gene pairs. The two main gene pairs geneticists have focused on are EYCL1 (also called the gey gene) and EYCL3 (also called the bey2 gene).
The different variants of genes are referred to as alleles. The gey gene has one allele that gives rise to green eyes and one allele that gives rise to blue eyes. The bey2 gene has one allele for brown eyes and one for blue eyes. The allele for brown eyes is the most dominant allele and is always dominant over the other two alleles and the allele for green eyes is always dominant over the allele for blue eyes, which is always recessive. This means parents who happen to have the same eye color can still produce a different eye color in their child.
For example, if two parents with brown eyes each passed on a pair of blue alleles to their offspring, then the child would be born with blue eyes. However, if one of the parents passed on a green allele, then the child would have green eyes and if a brown allele was present, then the child would have brown eyes irrespective of what the other three alleles were.
However, this does not explain why two parents with blue eyes can have a child with brown eyes. It also does not explain how grey or hazel eyes arise. This is where modifier genes, other genes associated with eye color and mutations all come into picture, as they can all lead to variability in eye color. Scientists are still studying exactly how these other factors cause such variations.
Genetic conditions that affect the eyes
Several genetic conditions affect the eyes, with two examples being ocular albinism and oculocutaneous albinism.
In the case of ocular albinism, severely reduced pigmentation of the iris results in very light-colored eyes and vision problems. Oculocutaneous albinism also affects pigmentation of the iris, but the problem involves the skin and hair as well. People born with this condition tend to have very fair skin, white or almost white hair in addition to having very light-colored irises.
Both conditions are caused by mutations in genes that contribute to melanin production and storage.
The presence of genetic variants can also lead to a condition called heterochromia, where an affected individual has eyes that are different colors to each other.