Does eye colour in humans usually have a genetic cause, an environmental cause or both?

Genetic

Explanation:  Eye color is directly related to the amount and quality of melanin in the front layers of the iris. People with brown eyes have a large amount of melanin in the iris, while people with blue eyes have much less of this pigment. A particular region on chromosome 15 plays a major role in eye color.

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amber has allowed scientists to study dna that has been preserved over millions of years.