Brown eyes are the most common eye colour globally. However, certain eye colours are so rare that they seem almost mythical. Eye colour is not simply a colour however; it is also an indicator of genetics, mutations, and biology. 

Mutations in Blue Eyes 

In 2008, scientists discovered that blue-eyed people possessed a mutation in a gene called OCA2. The OCA2 gene is involved in making melanin, which is the pigment found in skin, hair, and eyes. This mutation in OCA2 leads to less pigment being made in the eyes. From these discoveries, scientists concluded that the OCA2 difference is the most common cause of blue eyes. 

Origin of Blue-Eyed Ancestors

The original mutation in OCA2 arose between 6000 to 10,000 years ago in Europe. Shortly after, a mass migration occurred from the Black Sea to Northern Europe. Most blue-eyed individuals have ancestry from there. 

Less pigmentation was a useful trait due to the low-sunlight environment of Northern Europe. Individuals with lighter skin can more efficiently absorb sunlight—an important feature for making Vitamin D. After several thousand years of this natural selection, bluer eyes and lighter skin became a common feature in this part of the globe. 

The Rarest Eye Colours in the World

While blue eyes might seem rare in some regions of the world, they are quite common compared to some of the world’s rarest eye colours. 

Red & Violet

Red or violet eyes are due to a condition called oculocutaneous albinism (OCA). OCA is a genetic disorder that affects the pigmentation of the skin, hair, and eyes. Individuals with OCA often have very pale skin, light-coloured hair, and light-coloured eyes. This is known as hypopigmentation—a lack of melanin pigment. OCA is caused by mutations in genes involved in melanin production. 

OCA reduces pigmentation of the iris, which often leads to vision problems.  Most individuals with OCA have blue eyes. However, in individuals with very light blue eyes, the blood vessels inside can show through the iris, and in certain lighting conditions, the eyes can look violet or red. 

Green

Green eyes are among the rarest natural eye colours. Only about 2% of people in the world have them. Green eyes are polygenic, meaning that multiple genes are expressed to determine eye colour. The two main genes involved are OCA2 and HERC2. OCA2 controls melanin production in the iris while HERC2 regulates the OCA2 gene. 

Green eyes occur when there is a moderate amount of melanin produced in the eyes. Essentially, this means that the amount of melanin produced is more than in blue eyes but less than in brown eyes. Individuals with green eyes also have a genetic variant that produces lipochrome, which is a yellow pigment. 

Gray

Gray eyes are found in less than 1% of the global population. This eye colour is more uncommon than green, which makes it the rarest human eye colour.   

Gray eyes have even less melanin than blue eyes. There is almost no pigment in the front layer of the iris. This means that there is little melanin to absorb or reflect coloured light. It is important to note that gray eyes do not contain gray pigment. The gray colour comes from light scattering, where shorter blue light wavelengths scatter in the iris’s outer layer. Gray eyes scatter light more diffusely due to more collagen fibers in the iris. 

The diversity of eye colours is a first-hand demonstration of  how small genetic changes over millennia can produce entirely different results. A person’s eye colour carries a thousand years of genetic history.

References

Do all blue eyed people share a common ancestor?. The Tech Interactive. (n.d.). https://www.thetech.org/ask-a-geneticist/articles/2023/do-all-blue-eyed-people-share-a-common-ancestor/ 

U.S. National Library of Medicine. (n.d.). Oculocutaneous albinism: Medlineplus Genetics. MedlinePlus. https://medlineplus.gov/genetics/condition/oculocutaneous-albinism/ 

WebMD. (n.d.). How ocular albinism affects The eyes. WebMD. https://www.webmd.com/eye-health/what-is-ocular-albinism 

What is the rarest eye color in humans?. Optical Academy. (2024, March 19). https://optical-academy.com/blog/what-is-the-rarest-eye-color-in-humans/#:~:text=The%20rarest%20eye%20colors%20are,are%20considered%20the%20most%20uncommon.