I'm sure Harry heard about genetic codes from the Spencer side, with the talk about his paternity being questioned. Blondes and gingers on one side of the family, dark and light brown brown on the other.
I have cousins on my mother's side of the family that were born with blonde hair. It turned dark like the rest of us, but we had little latins in the family with blonde hair and we all loved it while it lasted. The Sussex children are still very young and their hair might turn dark in a few years.
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