What causes the differences in physical characteristics like hair color among people?

It has to do with pheomelanin and eumelanin

Explanation:

According to explorebiotech.com, "low concentrations of the eumelanin pigment will yield blonde hair, while high concentrations result in darker brown hair. People with natural red hair have much higher pheomelanin levels in their hair than people with more common hair colors like brown, blonde, and black." (Here's the link https://explorebiotech.com/science-behind-hair-colors/)

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