Posted last October, Larry Moran on Junk DNA, creationism, and Ryan Gregory’s Onion test is worth a read.
Note that the Onion Test is for people who think they have a functional explanation for the vast amount of putative junk in our genome. What Ryan is suggesting is that such proposals should be able to account for the huge genome of onions as well as the huge genome of humans.
Let me give you some examples. Some people suggest that we need a big genome in order to protect our genes from mutation. If that’s true then why do onions need five times more DNA? Some people suggest that we have big genomes because we’re so complex and we need huge amounts of regulatory sequence. If so, why do onions need more?