Explain why human eyes are of different colors?

The role of genetics in eye color.

The color of your eyes is primarily a story of genes. Essentially, you inherit genetic instructions from your parents, and that determines whether you have blue, green, or brown eyes. The main culprit is a gene called OCA2, located on chromosome 15, which directly influences the production of melanin in the iris. But it doesn't act alone; other genes like HERC2 can enhance or inhibit its activity. In fact, it was previously thought that a single gene dominated all of this, but in reality, it's a complex combination of several genes that precisely determines your eye color. That’s why even with two brown-eyed parents, a child can perfectly have blue eyes.

The importance of melanin and other ocular pigments

The color of the eyes mainly depends on the amount and type of pigments in the iris, particularly a pigment called melanin. The more melanin there is, the darker the eye will be, ranging from light brown to very dark brown, almost black. Conversely, the less there is, the more the eyes will be blue, green, or gray. Melanin comes in two main forms: eumelanin, a brown-black form, and pheomelanin, which is more yellow-reddish. In blue eyes, for example, it is primarily the absence of eumelanin that allows light to pass through and reflect in a particular way, giving that typical blue color. Some lesser-known pigments can slightly vary the hue or create special effects, such as those small "golden spots" sometimes seen on green or light brown eyes.

Geographical and ethnological variations of eye colors

The color of the eyes varies greatly depending on where in the world your ancestors grew up. In Northern Europe, for example, you'll see a lot of blue and green eyes, the result of genetic adaptation and a lower presence of melanin. If you wander over to Africa, Asia, or South America, it's the brown shade that clearly dominates, as these populations have more dark pigments that provide better protection from intense sunlight. Indigenous peoples from North America or Central Asia may sometimes exhibit slightly lighter eye shades, but this is very variable, and dark colors remain predominant on a global scale.

Genetic mutations and specific cases of eye colors

Sometimes, a simple genetic mutation can cause rare or unusual eye colors. Heterochromia, for example, is when a person has two eyes of different colors, or even multiple colors in one eye. This phenomenon is often due to a genetic variation that affects the distribution of melanin. Certain rare diseases, such as Waardenburg syndrome, also lead to striking pigmentation characteristics: very light eyes or unusual colors. These mutations provide unique combinations of eye colors, sometimes truly surprising.

The impact of evolution on the diversity of eye colors.

Over generations, evolution has played a role in the diversity of our eye colors. Initially, all humans likely had brown eyes, more suited to conditions of strong sunlight (with plenty of melanin to protect against UV rays). Then, around 6,000 to 10,000 years ago, a small genetic mutation that appeared in some individuals gave rise to the first lighter eye colors, such as blue or green. In certain regions where sunlight is less intense, this trait did not pose a disadvantage: it was passed on and spread naturally. According to some researchers, rarer colors could be considered attractive, influencing partner choice and promoting their spread. These genetic changes, passed down from generation to generation, explain a significant part of the incredible diversity of eye colors we see today.