system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does Eye color work? How does one group end up with Hazel, Blue and Green eyes while another is mostly Brown and Black?

How does inheriting these colors work?
gpt
It's more complicated than basic genetics. So, there are two genes that matter for eye color. Both genes come in two variants. Gene one is brown and blue, gene two is green and blue. Brown is dominant. In your body you have two of each gene, and each gene can come in either variant. So, if you have one brown copy of gene one, regardless of any other copy, you have brown eyes. So, it doesn't matter if the other copy of gene one is blue, and if both copies of gene two are blue, your eyes are brown. So, brown is very dominant. If you have a brown gene at all, your eyes are brown. Now, green isn't as powerful as brown, but it dominates over blue. So, if both copies of gene one are blue, then if one or both copies of gene two are green, you have green eyes. So, if you only have blue and green genes, your eyes are green. So, the only way to have blue eyes is for every copy of both genes (so, four genes total) are blue. Blue is completely recessive.

So, to answer your original question, a group will have mostly brown eyes if you have lots of brown copies of gene one in your population. Remember, the other genes don't matter if you have  a brown copy, so having lots of brown copies ensures most people have brown eyes. If you don't have the brown genes floating around, gene one is totally blue. So, gene two takes over, and you either get mixtures of green and blue eyes (like with Ireland for example) because you've got that green gene floating around, or nearly everyone has blue eyes, because blue eyed people are pretty much only going to have blue eyed children.

It's actually somewhat more complicated than that, because there are also shades of color, but that's the basic idea.