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Science & Nature SpecialThe humbling of Homo sapiens

If genes of mice and men are the same, how do the species differ?

14 June 2003

12:00 AM

14 June 2003

12:00 AM

Scientists are not interested in facts. What they like is ignorance. They mine it, eat it, attack it – choose the metaphor you prefer – and in the process they keep discovering more ignorance. Every answer leads to a set of new questions. The past few years have seen a once-in-an-aeon explosion of new knowledge about the human body and mind, as a consequence of our becoming the first creature in four billion years to read our own genetic recipe. Even more, they have seen an explosion of newly discovered ignorance.

Most people now know the humiliating news from the Human Genome Project that we have the same number of genes as a mouse. There is no special set of 50,000 genes for making human brains, as was being seriously mooted just a decade ago. The news keeps getting more deflating, because even the recent estimate of 30,000 human genes looks like an overestimate. The current betting is for fewer than 25,000, which is barely twice as many as a fruit-fly, a mere 6,000 more than a microscopic worm, 2,000 fewer than a mustard weed and 15,000 fewer than a rice plant. Dethronement on this scale has not happened since Copernicus took us out of the centre of the solar system.

Consolingly, the first few genes we have looked at are revealing some profound insights. The ASPM gene seems to control brain size; the FOXP2 gene is apparently a key to learning language; the AVPR1A gene may partly explain the ability to fall in love; the APOE gene can help to predict Alzheimer’s; the CREB gene is part of the mechanism of memory; the DAF2 gene controls aging (at least in worms); the MAOA gene affects our response to child abuse; the KAL1 gene affects penis size, libido and sense of smell; the SRY gene makes males male; and so on. How many more secrets lie in the other 24,993 genes? What a mother lode of ignorance!

It gets worse. Not only do we have the same number of genes as a mouse, but to all intents and purposes we have the same set. Even flies share many genes with us. This is by far the biggest scientific shock of the past decade, that animals have so many genes in common. The genes that lay out the body plan, or allow learning and memory in the brain, are the same in a fruit-fly as in a person, and were inherited with few alterations from a distant common ancestor called the roundish flat-worm which died out about 600 million years ago. Nobody predicted this discovery.

How can we be different from mice if we share the same genes? It seems as if the answer lies in some of the other sections of the genome, the parts that control the turning on and off of genes, sometimes known as promoters. In animals, though not in bacteria or plants, evolution consists more of subtle spelling changes in these promoters than in changes in genes themselves. These changes alter the pattern of use of genes in time and space. In the same way, two novels are different not because they use different vocabularies, but because they use roughly the same lexicon of words in a different order. A mouse differs from a human being in that it switches on its genes at different times and different places.


In principle, it could now be possible to identify the total set of all animal genes. Call it the Cambridge Thesaurus of Animal Genes, or CTAG. Each animal uses a subset of these genes, in the same way that a novelist uses a subset of words in the OED. Most are universal genes, just as there are many universal words that all novelists use (‘the’, ‘a’, ‘is’, ‘love’). Many are common genes that most animals will use (as novelists will probably use words such as ‘childhood’, ‘marriage’, ‘death’); and some are rare genes that only a few animals will use (‘finnegan’, ‘muggle’). Coincidentally, I would expect the CTAG to have about the same number of entries as the more abridged editions of the OED – some 100,000. With the mouse, fly, worm and human genomes already read and the zebra fish, puffer fish, chicken, rat, chimp, dog, cat, cow, kangaroo, toad and octopus genomes on the way, compiling the CTAG may soon be feasible.

The analogy of books should also reassure those who find genetics threatening. Taking a gene from one species and adding it to another is a bit like taking a word from a foreign language and adding it to an English dictionary. If it is a word that English does not have but could usefully employ (schadenfreude, for instance), then it can be an improvement, and it carries few risks. Genes, like words, are all just combinations of existing letters. Adding one gene to the 40,000 genes in a rice plant is more momentous than adding three words to the 120,000 in the pocket Oxford English Dictionary, but not much more so.

The word ‘carotenoid’ is not in my Pocket Oxford English Dictionary (1969 edition). It means a protein that is the precursor of vitamin A, a crucial ingredient of vision. The genes for making carotenoids are lacking in human beings, which is why we must eat vitamin A or go blind. The gene is also lacking in rice grains, so a person who subsists largely on rice may go blind. Approximately 500,000 children in the developing world suffer this exact fate every year, and determined efforts by aid agencies to get vitamin supplements or green vegetables to these people have so far failed.

Along comes Ingo Potrykus of Switzerland with a simple solution. Why not genetically engineer a rice plant so that it has the genes to make carotenoids in its grains? So he took the necessary genes from a daffodil and put them into rice. In effect, he added the word ‘carotenoid’ to the rice plant’s book. He soon had a form of rice that was identical in every respect, except that eating just 200 grams of it a day gave you a daily sufficiency of vitamin A. (Further refinements, including the addition of vitamin E’s precursor, have since made the rice even more health-giving.)

He then carefully negotiated away all the patents he had infringed, so that he could give the new ‘golden rice’ away free to peasants who could plant it in their fields. Since the rice would be self-fertile, there was nothing to stop the peasants he gave it to growing their own seed and giving it away to their neighbours. When he had finished in 2000, it was possible for the first time to envisage a realistic, practical and cheap way to prevent 500,000 children going blind every year.

Yet he found himself opposed and vilified by the so-called environmental movement, which was then enjoying a boom in donations from rich people disturbed by the half-truths, scare stories and wild predictions that they had been told about genetic engineering of plants. In vain did Potrykus make clear that his ‘golden rice’ would not enrich him, neither would it enrich any multinational company; it would damage no ecosystem, hurt no human being, invigorate no weed and assist no landlord; it would benefit the poor; it was not an American invention. It answered every one of the environmentalists’ excuses for their lucrative opposition to GM foods.

Still the Greens opposed it. More than two years have now passed and no governments have approved golden rice for use, frightened of the Green backlash if they do. Potrykus said to me recently, in his mild way, ‘Should I start to show pictures of blind children in the talks I give?’ ‘They would,’ I replied.

Matt Ridley is the author of Genome and Nature Via Nurture (Fourth Estate).


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