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byu/LesserOmentum from discussion
inaskscience
I can‘t really think of a reason that fits generally, but I have an idea. Like on what color your hair or skin has, the color of your eyes is determined by the interaction of many genes. It‘s possible that some of these genes also interact with the muscles that control the dilation on drug influence, but this won‘t fit generally. As an example: people with active gene 1, 2, 3 have brown eyes and people with active gene 4, 5, 6 have brown eyes, but only gene 3 interacts with our needed muscles, so person 1 will show your observation but person 2 won‘t, although both have brown eyes.
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