Whether you have dark or light eyes depends almost entirely on genetics. Eyes come in a wide range of colors, some more common than others. These colors include blue, gray, green, hazel, and all the shades of brown—some so dark they almost look black. The more melanin that you have in your eyes (specifically in the stroma, one of the layers in the colored part of your eye known as the iris), the darker your eyes are.
Eye color used to be thought of as a pretty simple trait. Brown-eyed parents, it was thought, could have kids with any eye color—although they usually had brown-eyed kids. And blue-eyed parents, it was believed, could only have children with blue eyes. In this overly simple scenario, the brown eye color was "dominant" over the blue eye color.
It turns out that in real life, the inheritance of eye color is a bit more complicated. More often than this simple model in which brown eyes are dominant might predict, blue-eyed parents can have brown-eyed kids. This is because more than one gene is involved in the eye color trait.
Scientists have identified four well-studied markers linked to eye color in the TYR, OCA2, and HERC2 genes, and near the SLC24A4 gene. Your pattern at these genetic markers is what determines your eye color result. Some people have markers linked only to light eye color. Some have markers tied only to dark color. And others have a combination of both light eye color markers and dark eye color markers.
The most rare eye color in people around the world is green. The most common color is brown.
It's likely that originally all humans had brown eyes. Around 6,000 to 10,000 years ago a genetic mutation popped up in the Black Sea region that likely led to blue eyes. Actually, if you have blue eyes that does not indicate your having blue eye pigment. Instead, it indicates the front part of your eye has hardly any pigment at all. blue eyes indicates the lack of pigment, not the dominance of any particular color.
It's likely that originally all humans had brown eyes. Around 6,000 to 10,000 years ago a genetic mutation popped up in the Black Sea region that likely led to blue eyes. In fact, all blue-eyed people have are believed to have a common ancestor: a European from the Black Sea region who probably lived between 6,000 and 10,000 years ago.
Blue eyes are most common in Northern Europe. A relatively high percentage (27%) of blue-eyed people in America is partially thanks to Americans with Eastern European, Irish, and British ancestry.
Today, brown is still the most widespread eye color in the world. Light brown eyes are most common in the Americas, West Asia, and Europe, while dark brown eyes are most frequently found in Africa, Southeast Asia, and East Asia.
Green eyes are most frequently found in Northern and Central Europe. Although green eyes can occur naturally in all races, about 16% of people with green-eyed people are of Germanic and Celtic ancestry. To be more precise, a staggering 86% of people from Ireland and Scotland have green eyes.
Less than 1% of the world’s population has gray eyes, which makes them one of the least common eye colors. The shades of gray eyes may vary from greenish to smokey blue to hazel-brown, which often depends on the environment, especially lighting. They are most common in Northern and Eastern Europe.