The first analysis of what may be humanity鈥檚 greatest achievement, our entire DNA sequence, reveals we have far fewer genes than expected.
While past estimates varied up to 140,000, two international scientific teams have now shown the true number is between 27,000 and 40,000. The finding deals a heavy blow to genetic determinism, the idea that many aspects of a person鈥檚 life are controlled by their genes.
鈥淭he small number of genes is significant [because it means] we鈥檙e not hard-wired,鈥� says Craig Venter, head of one team at the private company Celera.
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鈥淧eople think genes are an absolute cause of traits. But the notion that the genome is the blueprint for humanity is a very bad metaphor,鈥� he says. If you think we鈥檙e hard-wired and deterministic, there should indeed be a lot more genes.鈥�
Francis Collins at the US National Institutes of Health, and a leader of the publicly-funded Human Genome Project, says: 鈥淲e might imagine we鈥檙e nothing more than robots responding to invisible signals from deep in our DNA. But it won鈥檛 take away the notion of free will or of spirituality, or who God is or what love is. We must not imagine we are machines.鈥�
The low number of genes means humans have as few as 300 more genes than a mouse and only twice the genes of the fruit fly. 鈥淎 lot of people will find that philosophically disturbing,鈥� says theoretical biologist Jean-Michel Claverie of France鈥檚 national research centre in Marseille.
鈥淟andscape of human biology鈥�
Sequencing the human genome has been a colossal effort. The first public glimpse of our genetic panorama fills more than 100 pages in the latest issues of Nature and Science and is accompanied by over three dozen related papers.
鈥淚t鈥檚 the first time we鈥檝e stood back to look at the landscape of our own human biology,鈥� says Collins.
Hidden amongst the three billion letters of DNA code were our genes. These are the gold nuggets in this new territory because they code for the proteins that carry out most of the work in cells.
Cataloguing the birthplaces of proteins will be a giant step towards solving the mysteries of human biology. Proteins are also the target of nearly all drugs, so our protein inventory is a much sought-after prize in medicine.
Size doesn鈥檛 count
The low number of genes discovered raises intriguing issues. One is that there is no genetic basis for race. All humans share 99.99 per cent of their genetic information and there is more variation within racial groups than between them.
Another is how have we become so much more complex than other creatures, whilst having relatively few extra genes. Richard Myers of Stanford University says it is not the number of our genes, but how we use them: 鈥淎 fine sports car and a junker may have the same number of pieces. The difference is the quality of parts and the sophistication with which we put them together.鈥�
For example, genes usually come in segments. By 鈥渟plicing鈥� out some segments, or using one segment rather than another, a single gene can yield many different proteins. The same gene can be used to make one protein in, say, muscle and another in the brain. Up to 60 per cent of our genes may be so-called 鈥渟plice variants鈥�.
At a meeting last year in Birmingham, England, Craig Venter of Celera even speculated that all higher vertebrates might have roughly the same genes. The differences between us and, say, dogs might simply be down to when genes are switched on and off in embryos, he said.
Excess baggage
The low number of genes is not the only intriguing finding in the new genome data. 杏吧原创s have also discovered that our ancestors appear to have inherited 223 genes directly from bacteria, many of which appear to now play a crucial role. The gene transfer must have occurred when wayward bacterial DNA integrated into the sperm or egg of a distant vertebrate forebear.
Equally striking is our vast collection of 鈥渏unk鈥� DNA. Genes turn out to fill just 1.5 per cent of our chromosomes while repetitive copies of 鈥渏umping genes鈥�, or transposable elements, claim about half our DNA real estate. Other species seem to clear away this debris more easily. Only three per cent of the fruit fly鈥檚 genome consists of such sequences.
Transposable elements may seem like junk, but they may have helped our evolution. Most are now inactive, but when they first arrived they were able to hop from place to place in our genome. This helped rearrange chromosomes, reshuffling old genes and creating new ones.
A full commentary on the newly-published human genome data will feature in the 17 February issue of New 杏吧原创 magazine.