The eye colour in humans is a captivating feature that varies widely across individuals and populations. It is influenced by complex genetic interactions, with shades ranging from brown to blue, green, hazel, and even rare variations like grey or violet. The science of genetics of eye colour provides insights into why we inherit certain eye colours and why there are different coloured eyes in humans, a condition known as heterochromia. Let’s go ahead!
How Is Eye Colour Determined?
The genetics of eye colour is a fascinating interplay of multiple genes, with the most significant being the OCA2 and HERC2 genes located on chromosome 15. These genes influence the amount and type of melanin (pigment) in the iris, ultimately determining the eye’s colour.
Melanin Levels
- High melanin levels result in brown eyes, the most common eye colour globally.
- Moderate melanin levels lead to hazel or green eyes.
- Low melanin levels produce blue or grey eyes.
The distribution and density of melanin in the iris layers also contribute to the eye’s final appearance, leading to subtle variations even among individuals with the same basic eye colour.
The Role of Inheritance in Eye Colour
Eye colour and genetics are closely linked, with inheritance following a polygenic model. While early theories suggested a simple dominant-recessive relationship (e.g., brown dominant over blue), modern genetics has revealed a more complex picture involving multiple genes.
- Eye colour is inherited from both parents, with each parent contributing one set of genes.
- Variations in the OCA2 and HERC2 genes, along with other minor genes, influence the likelihood of specific eye colours appearing in offspring.
For example:
- Two blue-eyed parents are likely to have blue-eyed children, but the presence of certain genetic variants may still allow for other colours.
- Brown-eyed parents are more likely to have brown-eyed children, but lighter eye colours may appear if both parents carry recessive genes.
Genetics and Eye Colour Chart
A genetics and eye colour chart can help visualise potential outcomes based on parental eye colours. However, predicting eye colour is not always straightforward due to the involvement of multiple genes and random genetic variations. The chart shown below can help you understand better.
| Parental Eye Colour | Child’s Possible Eye Colour |
| Both Brown | Brown, Hazel, Green |
| Brown + Blue | Brown, Blue, Hazel |
| Both Blue | Blue |
| Brown + Green | Brown, Green, Hazel |
| Both Green | Green, Hazel, Blue |
Different Coloured Eyes in Humans
Some individuals have different coloured eyes, a condition known as heterochromia. This occurs when there is an uneven distribution of melanin between the two irises or within a single iris.
Types of Heterochromia
- Complete Heterochromia: One eye is an entirely different colour from the other (e.g., one blue eye and one brown eye).
- Sectoral Heterochromia: A single iris contains two distinct colours.
- Central Heterochromia: The iris has a different colour around the pupil compared to its outer edge.
Causes of Heterochromia
- Genetics: Often inherited and benign.
- Trauma or Disease: Conditions like Horner’s syndrome or Fuchs’ heterochromic iridocyclitis may cause heterochromia later in life.
The Science Behind Rare Eye Colours
Certain eye colours are less common due to specific genetic variations:
- Green Eyes: Among the rarest colours, green eyes result from a combination of moderate melanin levels and the scattering of light within the iris.
- Grey Eyes: Similar to blue eyes but with less visible melanin and a unique structural arrangement of the iris.
- Amber Eyes: Caused by a higher concentration of pheomelanin, resulting in a golden or coppery hue.
- Red or Violet Eyes: Extremely rare and typically associated with conditions like albinism, where melanin is almost absent.
Eye Colour Variations and Inherited Conditions
Colour Blindness Inheritance
Colour blindness, or the inability to distinguish certain colours, is a genetic condition most commonly linked to the X chromosome. However, this has nothing to do with the eye colour in humans, nor does it affect eye colour in any way. The conditions for inheriting colour blindness are as follows:
- Since males have only one X chromosome, they are more likely to inherit colour blindness.
- Females can be carriers and may only develop the condition if both X chromosomes carry the gene.
Oculocutaneous Albinism (OCA)
This inherited condition reduces melanin production, leading to pale eye colours (blue or pinkish hues) and sensitivity to light.
The Fascination Behind Eye Colour in Humans
The genetics of eye colour are a testament to the intricate processes that shape our unique traits. From the inheritance patterns of eye colour to rare conditions, understanding this aspect of genetics enhances our appreciation for human diversity.
At Centre for Sight, we strive to provide insights into these fascinating topics while delivering expert care for all your eye health needs. Whether you’re curious about the science behind eye colour in humans or need assistance managing inherited eye conditions, our specialists are here to help.
Take the first step toward healthier eyes Book an Appointment with Centre for Sight in India!
Frequently Asked Questions
Yes, a child’s eye colour can change as melanin production increases during the first few years of life. Babies often have lighter eyes at birth that darken as they grow.
The seven common eye colours are brown, blue, green, hazel, grey, amber, and red (seen in individuals with albinism).
Green is the rarest natural eye colour, occurring in only about 2% of the population. Amber and grey eyes are also considered rare.
To identify your eye colour, observe your eyes in natural light and note their predominant hue. Use a mirror and focus on whether they appear brown, blue, green, or a mix of colours (e.g., hazel).
