Those who employ “the onion test” should recall that the test — as originally formulated by geneticist T. Ryan Gregory — asks for a “universal function” for non-coding DNA. Is this a biologically reasonable question to ask? No. As Jonathan Wells writes, in The Myth of Junk DNA (pp. 85-86):
The “onion test,” according to Gregory, “is a simply reality check for anyone who thinks they have come up with a universal function for non-coding DNA. Whatever your proposed function, ask yourself this question: Can I explain why an onion needs about five times more non-coding DNA for this function than a human?” 
Gregory directs his challenge to “anyone who thinks they have come up with a universal function for non-coding DNA.” Yet there probably is no such person. As we have seen, scientists know of many functions for non-protein-coding DNA. Nobody claims that there is “a universal function” that applies both to mammals and to onions. Based on the evidence, scientists have proposed that non-protein-coding intronic DNA helps to regulate alternative splicing in brain cells, and that non-protein-coding repetitive DNA plays a role in placental development. Why should those scientists justify their proposals by referring to onions, which have neither brains nor placentas?
See also: Thoughts on the “C-Value Enigma”, the “Onion Test” and “Junk DNA”
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