The Genetics of Eye Color are More Complex than We Thought

The pigmentation of the iris, which covers the tiny black hole in the middle of the eye called the pupil, helps regulate how much light can reach the eye and determines a person’s eye color. The hue of the iris varies from very light blue to very dark brown. Variations in a person’s chromosomes decide eye color. The majority of genes linked to eye color are involved in forming, transporting, or preserving melanin, a pigment. The quantity and consistency of melanin in the front layers of the iris are closely related to eye color. The iris of people with brown eyes contains a lot of melanin, while the iris of people with blue eyes contains a lot less.

In the most extensive genetic study of its kind to date, a multinational team of researchers led by King’s and Erasmus University Medical Center, Rotterdam, discovered 50 new genes for eye color. The genetic analysis of nearly 195,000 individuals from Europe and Asia was used in the research. These studies will aid in the comprehension of eye disorders involving eye pigment levels, such as pigmentary glaucoma and ocular albinism. The researchers discovered that Asians with various shades of brown skin color are genetically identical to Europeans with eye color varying from dark brown to light blue.

This research builds on previous studies, which found a dozen genes related to eye color and suggested that there may be even more. Previously, scientists believed that skin color was determined solely by one or two chromosomes, with brown eyes predominating over blue eyes. The amount of melanin contained in these compartments determines the color of the eyes. In individuals with blue eyes, a certain percentage of melanosomes contain a small amount of melanin. Green-eyed people have a small amount of melanin and a moderate number of melanosomes. In contrast, brown-eyed people have a large amount of melanin contained inside a large number of melanosomes.

Such genes play a significant role in deciding eye color, although others play a minor role. Two genes that play significant roles in deciding eye color are found near each other on chromosome 15 in one area. One gene, OCA2, produces the P protein, which influences the volume and content of melanin in the iris and is involved in melanosome maturation. Several genetic variants such as polymorphisms in this gene decrease P protein release, resulting in lighter eyes. HERC2 is the other major gene involved. This gene’s intron 86 regulates OCA2 expression, activating or deactivating it as required. At least one polymorphism in this intron inhibits OCA2 expression and function, lowering P protein production.

The large number of involved genes complicates the inherited eye color process. While the color of a child’s eyes can typically be predicted by looking at the color of his or her parents’ eyes, the occurring polymorphisms suggest that a child’s eye color can be unpredictable. The pairing of genes passed on by each parent determines a child’s eye color, which is believed to include at least three gene pairs. EYCL1, also known as the gey gene, and EYCL3 are the two primary gene pairs geneticists have concentrated on, also known as the BEY2 gene.

Alleles are the various variations of a gene. Green eyes are caused by one variant of the gey gene, whereas another allele causes blue eyes. One variant of the BEY2 gene is responsible for brown eyes, and the other is responsible for blue eyes. The allele for brown eyes is the most prevalent and is always dominant over the other two alleles. In contrast, the allele for green eyes is always prevalent over the recessive allele for blue eyes. This means that parents with the same eye color can still have a child with different eye color. For instance, if two brown-eyed parents each pass on a pair of blue alleles to their infant, they will be born with blue eyes. If one of the parents passed down a green allele, the infant would have green eyes, and if a brown allele was passed down, the child would have brown eyes, regardless of the other three alleles.

Although there is a wide range of skin color, shades other than brown are only seen in people of European ancestry. Brown-eyed people are popular in Africa and Asia. While individuals with lighter eyes are more open to light because their iris has less pigment to shield them from the sun, there is little to no evidence that eye color has a significant effect on vision qualities like visual acuity.