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It's widely claimed that we share 50% of our DNA with bananas. Does this have actual basis or is it a myth?

Example claims The Mirror (UK), NHGRI

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Finally, a question covering my nominal area of expertise. To answer this question meaningfully we'll need to define some concepts but first.

Yes, sort of. The statement is factually correct for reasonable interpretations.

So, on to the terms.

I'll link to a more specific stackexchange to support the definitions

Homology.

To draw an analogy: If someone said "humans share 90% of their skeleton with birds" would it be a reasonable statement? the overall structure is the same, most bones have an equivalent that's longer, shorter, thicker, thiner. The bones might have adaptions for strength or weight but stretch and squish things a little and you'll get something that looks the same.

homology

Orthology

Orthologs are genes in different species that evolved from a common ancestor. Orthologs normally retain the same function.

Paralogy

Paralogs are genes which have been duplicated. One copy may end up doing something very different to the original but retain some things in common. enter image description here

When do 2 genes count as the same?

Lets look at an example

Histone H1

Histone is found in most species and it's highly conserved. It's not identical across species but it's mostly the same.

histone

In fact a lot of the genes which we need to live are common across many species. Genes to copy DNA, genes to repair cell walls, genes to control temperature, genes to metabolize various sugars. No matter whether you're a human or a banana plant you need a lot of the same basic machinery to live.

The genes aren't perfectly identical but since they mostly have to do the same job and you'll likely die of fail to breed without them, they tend to be highly conserved, with most of the differences at less important points in the genes.

The genes get moved around, they get flipped back to front or swap chromosomes or chromosomes get merged or split, but they're generally there somewhere.

For example, here's a mouse genome colored by what sections have homologs in the human genome.

mouse genome

It's all about how strict you are with what you count as "sharing" DNA

If you count genes sequences with a single base different as not the same, then most humans wouldn't count as very similar to other humans.

If you allow for 1, 2, 3, 4 or more mutations per 100 bases while still counting something as "the same", then you can get almost any percentage you want.

That's why I said "sort of" above.

I could link to some paper where they give some number, but that wouldn't be terribly informative. I could point to 3 more which give different numbers for the same thing because it's all about where you set the cutoff points when deciding if something counts as the same.

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    This is a really good answer - it is informative, referenced, and explains well the subject. A few people will note like it since it's not really the usual format (one or two papers referencing the subject), but I still like it. +1 from me! – T. Sar Sep 22 '16 at 14:09
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    That diagram you show of the color-coded mouse/human homologs; does that type of diagram have a name? – Jason C Sep 22 '16 at 15:08
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    Oh wait, nevermind; cinteny.cchmc.org generates them and seems to refer to them as "synteny maps". They don't have bananas in their database, sadly, but they do have rice. I made this but I'm not sure what parameters they used to generate the mouse / human one. – Jason C Sep 22 '16 at 15:15
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    Yep, that sounds about right. With very distantly related species you need to limit it to exons and accept a lot more mismatches to get a reasonable degree of synteny. Checking out their human-mouse synteny diagram/map it looks like they have slightly stricter default parameters than were used to generate the diagram I included in my post. – Murphy Sep 22 '16 at 15:57
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    Won't this answer be better phrased as "humans share 50% of genes with bananas"? From the synteny map with rice, it's pretty clear that the "50% DNA" claim is false. – March Ho Sep 25 '16 at 16:57
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No - there is no interpretation under which anyone has ever shown this to be true, and one can trivially confirm that some of the obvious candidate interpretations are false.

Per Wikipedia, the human genome is roughly 3 giga base pairs long. The banana (Musa Acuminata) genome, on the other hand, is only around one fifth of that length - 600 Mb according to ProMusa, or just 520 Mb according to the first publication of a reference genome in 2012.

Clearly, then, we cannot align slices of a single copy of the banana genome against the human genome and have them cover 50% of its length, because there aren't even enough base pairs for this to be possible!

But Murphy's answer suggests that it's reasonable to instead take the claim that we "share 50% of our DNA" with bananas to mean that 50% of humans' tens of thousands of genes have homologs in banana genes. Supposing that we accept that this is a reasonable interpretation, is it true?

No. Bioinformatician Neil Saunders tried using the Orthologous Matrix Browser to look for such orthologs, and he reports that there are only 3440 human genes with such orthologs - a result you can trivially confirm yourself by counting the unique human genes from https://omabrowser.org/cgi-bin/gateway.pl?f=PairwiseOrthologs&p1=HUMAN&p2=MUSAM&p3=OMA, as shown below in Python:

>>> import requests
>>> result_tsv = requests.get('https://omabrowser.org/cgi-bin/gateway.pl?f=PairwiseOrthologs&p1=HUMAN&p2=MUSAM&p3=OMA').text
>>> unique_human_gene_names = {line.split('\t')[0] for line in result_tsv.splitlines()}
>>> len(unique_human_gene_names)
3484

He notes that this

given that there are twenty thousand-ish human protein coding genes, equates to around “17% banana”.

But surely whoever originally made the claim must've had some definition in mind for which they knew it to be true? It would seem not; Saunders notes that the claim dates back to 2002 (specifically, to this interview with Steve Jones in episode Almost Like A Whale of The Science Show on ABC radio on 12th January 2002), a full decade before the publication of the first banana genome. In the context of the transcript, we can see that Steve is using the claim as a rhetorical device to make the point that genomically similar organisms can still be very different from each other:

... we also share about 50% of our DNA with bananas and that doesn't make us half bananas, either from the waist up or the waist down. So there are limits in what genetics can tell us about what it means to be human ...

Given that the original source of the claim is an unsubstantiated wisecrack on a radio show a decade before we even had a banana reference genome, and that bioinformaticians trying to substantiate the claim in light of that published genome have failed to find any methodology for which it comes out as true, I think it's fair to simply characterise the claim as outright false.

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    "a result you can trivially confirm" It is trivial to run your code. It is not trivial to confirmt hat your code does anything meaningful, or that the fetched data is meaningful. – Oddthinking Jun 16 at 17:00
  • @Oddthinking If you don't trust that particular code (or me, or the cited source) then just count the unique genes in some other way. All it's doing is counting the number of unique items in one column of a TSV. I agree that confirming the fetched data is meaningful is a trickier problem. – Mark Amery Jun 16 at 17:20
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    Counting the unique items in one column of a TSV is trivial (to a Python developer). But you haven't demonstrated that counting unique items in that column is a meaningful way to count genes, and doing so shouldn't dismissed with a hand-waved "trivial". – Oddthinking Jun 16 at 23:34
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    @Oddthinking That's a reasonable criticism. I will tweak the answer to provide more justification for the method. – Mark Amery Jun 17 at 9:50

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