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- The onion test is a way of assessing the validity of an argument for a functional role for non-coding DNA, sometimes called "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? (en)
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- Twitter is not a reliable source. (en)
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- The onion test is a way of assessing the validity of an argument for a functional role for non-coding DNA, sometimes called "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: (en)
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