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10:19am on Tuesday, 24th October, 2006:

98% Human


The Independent had a piece today about gorillas, the close relatives who share 98% of our DNA (although the article itself says that refers to chimps).

I really don't like this appeal to the non-scientist's understanding of genetic variation and simple statistics.

First of all, it depends on how you define "DNA". You can do it all the way from the molecular level (how much carbon, hydrogen and oxygen there is in a strand) all the way up to the chromosome level (of which, incidentally, chimpanzees have 24 to the 23 of humans). Most comparisons seem to use base pairs, which is what gives the 98% figure. "There's only a 2% difference" sounds much closer than "there's only 80 million nucleotides difference".

That 98% isn't evenly distributed, though — it's concentrated in three chromosomes. This could be important (think what percentage of a book is in its ink, then compare two books with minor differences in the amount of ink and decide for yourself what this says about their actual similarity). Oh, and male humans are different from male chimps to a greater degree than female humans are different from female chimps, because the Y chromosome is of a different size between chimps and humans.

What particularly annoys me about this 98% thing is that it's never accompanied by any other figure by way of comparison. How much of our DNA do we share with, say, a dog? Or with a horse? Or with a fish? Or with an insect? It's hard to tell for things that we haven't yet fully DNA-sequenced, but cytochrome C amino acid chains correlate well with DNA differences so we can use those as an estimate. It's not exact — there are 104 chains, and we share all 104 with chimps — but it's close. It turns out that we share 91 with dogs, 87 with horses, 73 with tuna and 68 with moths. Then again, we share 70 with maize (horses share only 66 with maize), which rather puts things into perspective. Those cornflakes you have for breakfast are around 70% human...

Even if you insist on using the usual method of comparing base pairs between species (take a strand of DNA from one species and split it into two threads, then swap one of the threads for a thread from the other species and see what matches up), you get some strange results. Using this measure, there's more genetic variation between two gorillas in the same troupe than there is between any two human beings on the planet selected at random. So humans are more human than gorillas are gorilla?

The point of The Independent's article is conservation, of which I'm in favour. Telling me that chimps are 98% human doesn't make me any more or less in favour, though.

Oh well, I guess some readers react well to being patronised, or they wouldn't have done it.

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Copyright © 2006 Richard Bartle (richard@mud.co.uk).