Man or Mouse?

Have we over-estimated the importance of genomes (and by implication the Human Genome Project)? Is epigenesis a more fruitful field for understanding human disease and behavior? Is the rush to patent genes equivalent to seeking patents for each word in the dictionary?

Matt Ridley in an article titled “Humans have no more genes than mice, but don’t feel small” points out that the more we find out about genomes, the more humiliating the news they bring us. He says, “the human genome turns out to be profoundly ordinary. We have known for decades that human beings have one fewer chromosome than chimpanzees, which should have been ample warning. We have known for years that grasshoppers have three times as much DNA per cell as we do, deep sea shrimps ten times, salamanders 20 times and African lungfish a staggering 40 times. But we still kidded ourselves until just the last few years that human beings would prove to have more genes, arranged in a more sophisticated way, than most other creatures. How else to explain our exquisite brains?”

He adds, “We have 25,000 genes (or recipes for protein molecules) which is the same as a mouse, just 6,000 more than a microscopic nematode worm and 15,000 fewer than a rice plant.” Our sophistication is not related to the number of genes, but rather how those genes are put together, like a recipe in a cookbook. The same ingredients can make mush or cordon bleu dishes.

Almost daily new genome maps of organisms are being published and much of the understanding of what genes do will come from comparing the genomes of different species. Ridley goes on to say, “But comparing the genomes with each other is beginning to unveil some fascinating insights. There is, for example, an intriguing difference between animals and plants, to wit that plants tend to have more genes. This seems to touch upon a fundamentally different approach to innovation employed by the two kingdoms during evolution. When plants need a new trait (or rather, when natural selection imposes an advantage on a plant that has accidentally acquired a new trait), it happens by duplication and divergence: a duplicated version of an old gene evolves into the new one. That is how biologists thought all evolution happened. But animals seem to do it differently. They add a new switch, or “promoter sequence,” to the front of an old gene, thus enabling the body to switch it on in a different place or at a different time: the same gene gets an additional job, in effect. The switch, too, is made of DNA text”.

Not only do we have the same number of genes as a mouse but they are generally the same genes and only the switches are different. Mapping the human genome was only like opening a book and looking at how many different words were used. The author the children’s Dr. Seuss books once famously used only 55 words to write a book. The challenge now is to glean the meaning of the combinations of genes and how they are switched on and off, between and within species.

Matt Ridley is the author of “Nature via Nurture” (Harper Perennial).

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