TLDR: Blue eyes result from having two recessive blue eye genes from both parents. The majority of blue-eyed individuals share a single H-1 haplotype (a group of genes within an organism that was inherited together from a single parent), indicating a common ancestor.

How do blue eyes work?: "A person needs only one dominant brown-eyed gene (from one parent) to be brown-eyed but needs to have two recessive blue-eyed genes (one from each parent) to be blue-eyed. Eye color is controlled not by one gene passed from parent to offspring, but by two genes working in tandem.

These genes are called OCA2 and HERC2 (represented as O, o, H and h in Image 2). The simplified explanation is that the OCA2 gene controls pigment in the stroma (the tissue and blood vessels) of the iris (the colored part of the eye around the pupil) and the HERC2 gene is needed to help turn on the OCA2 gene to cause it to produce this pigment, resulting in brown eyes. If a person has a non-functioning OCA2 gene, they will always have blue eyes, because the HERC2 gene can’t make the broken OCA2 gene work."

How do we know all blue eyed people are related?: "Homo sapiens (modern humans) emerged around 200,000 years ago in Africa, but the mutation that causes blue eyes did not appear until sometime around 10,000 years ago.

In a study conducted by Professor Hans Eiberg and a team from the University of Copenhagen, researchers examined mitochondrial DNA from 155 blue-eyed subjects from Denmark, two from Jordan, five from Turkey, and 45 brown-eyed candidates, looking at the locus (specific location or position of a gene) responsible for brown or blue eyes. The result was the discovery that more than 97% of blue-eyed people share the single H-1 haplotype (a group of genes within an organism that was inherited together from a single parent). Eiberg and his team write, “A shared haplotype among blue-eyed individuals is almost perfect and suggests the blue color phenotype is caused by a founder mutation.” This means that the vast majority of people with blue eyes share a single inherited genetic mutation, rather than each person with blue eyes possessing a unique mutation.

The study also tested seven blue-eyed Mediterranean individuals unrelated to the Danish participants as a control group. They, too, carried the H-1 haplotype. These individuals with the H-1 haplotype all inherited the same switch at the same location in their genetic coding, whereas, brown-eyed individuals have a number of variations in melanin production and DNA, with brown-eyed phenotypes being spread out between haplotypes H-5 and H-10. In short, almost all blue-eyed people came from a single ancestor, which is proven by the possession of the exact mutation at the same location in their genetic coding. "

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