Apparently this is a question that has been known to keep some biologists awake at night. (Can't say I'm one of them; my insomnia is caused by other, equally pressing issues LOL)
Anyway, ERV has written a lovely post looking at this: apparently it's all to do with metabolic pathways and endogenous retroviruses. Go over there & soak up the goodness :-)
(And sleep well.)
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Last modified: Wed Nov 30 17:23:39 2011
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URL: http://sci.waikato.ac.nz/bioblog/2011/05/why-are-some-chickens-white.shtml
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ERV's article is now listed as one of the nominees for the Quark Prizes, so you can vote on it if you like.