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It’s definitely a genetic thing. There can
be a mutation on the X chromosome that decreases the number
of proteins that are sensitive to red and green light in the
retina. That’s why colour-blindness affects boys more
than girls: boys have only one X chromosome (XY), so any mutation
is going to show up in every “eye” cell (which
means all those cells will be less sensitive to red/green).
Girls have two X chromosomes (XX) so if one X is mutated,
the other X can cover – at least some cells will be
able to distinguish red and green light.
(And a word of
advice: if you’re looking for a gift for your little
brother, and some shyster tries to sell you a colouring book
for colour-blind kids, don’t believe them. I learned
the hard way – AFTER I had brought it home, opened it
up, and discovered the only crayon it came with was…
grey. I am so gullible sometimes…)
To
learn more about sex-linked inheritance:
Glossary terms:
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