The onion test is a way of assessing the validity of an argument for a functional role for junk DNA. It relates to the paradox that would emerge if the majority of eukaryotic non-coding DNA were assumed to be functional and the difficulty of reconciling that assumption with the diversity in genome sizes among species. The term "onion test" was originally proposed informally in a blog post by T. Ryan Gregory in order to help clarify the debate about junk DNA. The term has been mentioned in newspapers and online media, scientific journal articles, and a textbook. The test is defined as: The onion test is a simple reality check for anyone who thinks they have come up with a universal function for junk 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?Onions and their relatives vary dramatically in their genome sizes, without changing their ploidy, and this gives an exceptionally valuable window on the genomic expansion junk DNA. Since the onion (Allium cepa) is a diploid organism having a haploid genome size of 15.9 Gb, it has 4.9x as much DNA as does a human genome (3.2 Gb). Other species in the genus Allium vary hugely in DNA content without changing their ploidy. Allium schoenoprasum (chives) for example has a haploid genome size of 7.5 Gb, less than half that of onions, yet Allium ursinum (wild garlic) has a haploid genome size of 30.9 Gb, nearly twice (1.94x) that of onion and over four times (4.1x) that of chives. This extreme size variation between closely related species in the genus Allium is also part of the extended onion test rationale as originally defined:Further, if you think perhaps onions are somehow special, consider that members of the genus Allium range in genome size from 7 pg to 31.5 pg. So why can A. altyncolicum make do with one fifth as much regulation, structural maintenance, protection against mutagens, or [insert preferred universal function] as A. ursinum?
C-value paradox
Some researchers argue that the onion test is related to wider issues involving the C-value paradox and is only valid if one can justify the presumption that genome size has no bearing on organismal physiology. According to Larry Moran, the onion test is not an argument for junk DNA, but an approach to assessing possible functional explanations for non-coding DNA. According to him, it asks why allium species need so much more of that proposed function than do humans, and why so much more (or less) than other closely related species of allium and does not address the variation in genome size (C-value) among organisms itself.