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I keep hearing this claim that a lab "rat has 99% of the DNA of a human?" Is that so? What exactly does this mean? One source for this claim seems to CNN which only says,

"About 99 percent of genes in humans have counterparts in the mouse," said Eric Lander, Director of the Whitehead Institute Center for Genomic Research in Cambridge, Massachusetts. "Eighty percent have identical, one-to-one counterparts."

It further says,

Mice and humans each have about 30,000 genes, yet only 300 are unique to either organism. Both even have genes for a tail, even though it's not "switched on" in humans.

This article says we only share 96% with chimps!

Another implication of the finding is that we are more different to our closest living relative, the chimpanzee, than previously assumed from earlier studies. Instead of being 99 per cent similar, we are more likely to be about 96 per cent similar.

While not 99% as the original claim, here is a source along the same lines at 97.5% of their (working) DNA of humans.

Mice and men share about 97.5 per cent of their working DNA, just one per cent less than chimps and humans. The new estimate is based on the comparison of mouse chromosome 16 with human DNA.

In this video about Michio Kaku you can see the claim repeated too by Judith Campisi, PhD, Professor at the Buck Institute for Age Research

"Think of this: mice and humans, we're 97% genetically identical. Mice live three years; Humans live a hundred years. And, somewhere in that 3% of genes are regulators that determine the pace at which these two pretty similar organisms age." - Judith Campisi

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"99% of genes" is not the same as "99% of DNA". –  Oliver_C Dec 3 '12 at 10:41
    
Please list a source for this claim – I’ve never heard it. The sources you list are very different claims. The only thing I’ve heard is that chimpanzees share 97% of our genome (an easy to find claim which did at some point reflect the mainstream scientific view but no longer does). –  Konrad Rudolph Dec 6 '12 at 23:32
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Related question here. (re @Oliver_C's comment) –  Benjol Dec 7 '12 at 12:44
    
@Benjol are you suggesting women are rats? –  Evan Carroll Dec 11 '12 at 18:06
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1 Answer

We share a large amount of our DNA in common with all mammals through our shared evolutionary past. But I think you have confused two different ideas. Sharing 99% of genes is not the same as sharing 99% of DNA. While we may have nearly the same number and type of genes (i.e. rat genes have counterparts in the human genome) it does not mean that these genes are completely identical. They are comparing the number and function of the genes only.They are NOT comparing the actual DNA sequences.

In the article linked above, it states:

Genes only make up about 3% of our genome. Yes, you read that correctly. The rest of our genome is called non-coding or junk DNA.

For a more up-to-date explanation of junk DNA see ‘Junk DNA’ concept debunked by new analysis of human genome in the Washington Post.

So it is not correct to confuse the ideas of number/function of genes with actual similarity of DNA.

In the article Scientists Compare Rat Genome With Human, Mouse on Genome.gov you can find some good insight into your question:

In their Nature paper, the researchers reported that, at approximately 2.75 billion base pairs, the rat genome is smaller than the human genome, which is 2.9 billion base pairs, and slightly larger than mouse genome, which is 2.6 billion base pairs. However, they also found that the rat genome contains about the same number of genes as the human and mouse genomes. Furthermore, almost all human genes known to be associated with diseases have counterparts in the rat genome and appear highly conserved through mammalian evolution, confirming that the rat is an excellent model for many areas of medical research.

Note that it says "genes associated with disease" and that these are "highly conserved". This should not be taken to mean the genes are identical. It clearly implies that there were changes but that those changes have not been significant enough that medical experimentation is invalidated.

So the answer to your question is no, we do not share 99% of our DNA with rats. But yes we do share a large number of genes.

http://rgd.mcw.edu/

http://en.wikipedia.org/wiki/Rat_genome_database

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+1, just wanted to point out that although journalists like to talk about junk DNA it is becoming more and more clear that it is not junk at all! –  nico Dec 3 '12 at 15:36
    
Yes, I was kind of disappointed in the amount of talk about "junk DNA" in in article from 2011. But I didn't think that had a lot of bearing on the question so I didn't really know if it might cloud my answer. –  Robert Kaucher Dec 3 '12 at 15:42
    
interesting... can you link somewhere that explains about this junk DNA not being junk after all? –  Waleed Hamra Dec 6 '12 at 14:26
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@Waleed Hamra - I have added a link with more info regarding what we now know about "junk DNA". –  Robert Kaucher Dec 6 '12 at 18:34
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The claims by ENCODE about the overthrow of Junk DNA have met with some criticism by other biologists as being hyped and misleading. –  Oddthinking Dec 7 '12 at 1:03
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