What does it mean that humans share a percentage of genes with other species?

[Drakkith]

I commonly read that humans share a certain percentage of their genes with other species, with the exact amount becoming smaller for more distantly related species. What exactly does this mean? Is it talking about the actual base pairs of our DNA, or of something else? Links to further information are welcome!

 

[VoteSaxon]

Do keep in mind that genetic similarities might not equate relatedness, just as genome size doesn't equate complexity (frogs have a far larger genome than you or I.) There are an increasing number of full genomes for species, which can be compared using complex specific programs. However, I think that it is more common for species to be compared on specific, key sequences in the genome.

Check this out for more on cross-species genetic analysis:
http://m.genome.cshlp.org/content/13/1/1.full
And the wiki page for "Competitive Genomics:"
http://en.m.wikipedia.org/wiki/Comparative_genomics

 

[Pythagorean]

I commonly read that humans share a certain percentage of their genes with other species, with the exact amount becoming smaller for more distantly related species. What exactly does this mean? Is it talking about the actual base pairs of our DNA, or of something else? Links to further information are welcome!

You might imagine that right after speciation, two species are pretty similar in their DNA. But given time, each species may accumulate mutations at a different rate, so with time, comparison of genetic overlap becomes less representative of relatedness.

Also, when people make these statements, you have to be careful to note what exactly they're talking about. Some such statements exclude "junk DNA", which it turns out is not junk at all. Many of the differences between us and other apes have to do with how our genes are expressed more so than which genes are expressed and that's regulated by the so-called "junk DNA".

 

[Drakkith]

Also, when people make these statements, you have to be careful to note what exactly they're talking about. Some such statements exclude "junk DNA", which it turns out is not junk at all. Many of the differences between us and other apes have to do with how our genes are expressed more so than which genes are expressed and that's regulated by the so-called "junk DNA".

I guess that's part of my question. Sometimes it's said that a percentage of our DNA is the same, sometimes its our genes. What's the difference, if any?

 

[VoteSaxon]

I guess that's part of my question. Sometimes it's said that a percentage of our DNA is the same, sometimes its our genes. What's the difference, if any?

I'd think saying 'percentage of DNA' is ambiguous - it could be referring to sample sequences, or genes, or SNPs, or a multitude of different DNA properties/components.
On a slight tangent (don't quote me on this as I haven't checked if it is true or not,) a gene has quite a large number of nucleotides,so I would think you are far more likely to see sequences conserved between two creatures than entire genes.

 

[Pythagorean]

I guess that's part of my question. Sometimes it's said that a percentage of our DNA is the same, sometimes its our genes. What's the difference, if any?

DNA is the "stuff" that genes are made of.

 

[Ygggdrasil]

In order to figure out how closely related two species are, biologists will generally take a bunch of sequences of genes encoding protein that are shared between the species, align the sequences to figure out which amino acids in one species correspond to which amino acids in the other species, then look at the number and pattern of amino acid differences between the different species. Often, this comparison can be summarized by stating the percentage of amino acids that are identical between the two species or the percentage of amino acids that are similar (e.g. the amino acids are different but they share the same properties). Usually this comparison is done at the protein level because DNA sequences (especially non-coding DNA sequences) change fairly rapidly compared to the timescale of speciation.

 

[Drakkith]

DNA is the "stuff" that genes are made of.

Of course.

In order to figure out how closely related two species are, biologists will generally take a bunch of sequences of genes encoding protein that are shared between the species, align the sequences to figure out which amino acids in one species correspond to which amino acids in the other species, then look at the number and pattern of amino acid differences between the different species.

So there are differences in these amino acids, but they perform the same functions in both species?

 

[Ygggdrasil]

So there are differences in these amino acids, but they perform the same functions in both species?

Yes. Proteins are fairly robust to mutation. Researchers have done studies to test what fraction of mutations impair the function of a protein, and have found that ~60% of amino acid substitutions have no measurable effect on the function of the protein (http://www.nature.com/nrg/journal/v11/n8/full/nrg2808.html). This ability of proteins to tolerate mutation and drift around sequence space is thought to help underlie their ability to evolve new functions.

 

[Drakkith]

Thanks!