ite a feat。
most of the time our dna replicates with dutiful accuracy； but just occasionally—aboutone time in a million—a letter gets into the wrong place。 this is known as a single nucleotidepolymorphism； or snp； familiarly known to biochemists as a “snip。” generally these snipsare buried in stretches of noncoding dna and have no detectable consequence for the body。
but occasionally they make a difference。 they might leave you predisposed to some disease；but equally they might confer some slight advantage—more protective pigmentation； forinstance； or increased production of red blood cells for someone living at altitude。 over time；these slight modifications accumulate in both individuals and in populations； contributing tothe distinctiveness of both。
the balance between accuracy and errors in replication is a fine one。 too many errors andthe organism can’t function； but too few and it sacrifices adaptability。 a similar balance mustexist between stability in an organism and innovation。 an increase in red blood cells can helpa person or group living at high elevations to move and breathe more easily because more redcells can carry more oxygen。 but additional red cells also thicken the blood。 add too many；and “it’s like pumping oil；” in the words of temple university anthropologist charles weitz。
that’s hard on the heart。 thus those designed to live at high altitude get increased breathingefficiency； but pay for it with higher…risk hearts。 by such means does darwinian naturalselection look after us。 it also helps to explain why we are all so similar。 evolution simplywon’t let you bee too different—not without being a new species anyway。
the 0。1 percent difference between your genes and mine is accounted for by our snips。
now if you pared your dna with a third person’s； there would also be 99。9 percentcorrespondence； but the snips would； for the most part； be in different places。 add more people to the parison and you will get yet more snips in yet more places。 for every one ofyour 3。2 billion bases； somewhere on the planet there will be a person； or group of persons；with different coding in that position。 so not only is it wrong to refer to “the” human genome；but in a sense we don’t even have “a” human genome。 we have six billion of them。 we are all99。9 percent the same； but equally； in the words of the biochemist david cox； “you could sayall humans share nothing； and that would be correct； too。”
but we have still to explain why so little of that dna has any discernible purpose。 it startsto get a little unnerving； but it does really seem that the purpose of life is to perpetuate dna。
the 97 percent of our dna monly called junk is largely made up of clumps of lettersthat； in ridley’s words； “exist for the pure and simple reason that they are good at gettingthemselves duplicated。”
2most of your dna； in other words； is not devoted to you but toitself： you are a machine for reproducing it； not it for you。 life； you will recall； just wants tobe； and dna is what makes it so。
even when dna includes instructions for making genes—when it codes for them； asscientists put it—it is not necessarily with the smooth functioning of the organism in mind。
one of the monest genes we have is for a protein called reverse transcriptase； which hasno known beneficial function in human beings at all。 the one thing itdoes do is make itpossible for retroviruses； such as the aids virus； to slip unnoticed into the human system。
in other words； our bodies devote considerable energies to producing a protein that doesnothing that is beneficial and sometimes clobbers us。 our bodies have no choice but to do sobecause the genes order it。 we are vessels for their whims。 altogether； almost half of humangenes—the largest proportion yet found in any organism—don’t do anything at all； as far aswe can tell； except reproduce themselves。
all organisms are in some sense slaves to their genes。 that’s why salmon and spiders andother types of creatures more or less beyond counting are prepared to die in the process ofmating。 the desire to breed； to disperse one’s genes； is the most powerful impulse in nature。
as sherwin b。 nuland has put it： “empires fall； ids explode； great symphonies are written；and behind all of it is a single instinct that demands satisfaction。” from an evolutionary pointof view； sex is really just a reward mechanism to encourage us to pass on our genetic material。
scientists had only barely absorbed the surprising news that most of our dna doesn’t doanything when even more unexpected findings began to turn up。 first in germany and then inswitzerland researchers performed some rather bizarre experiments that produced curiouslyunbizarre outes。 in one they took the gene that controlled the development of a mouse’seye and inserted it into the larva of a fruit fly。 the thought was that it might producesomething interestingly grotesque。 in fact； the mouse…eye gene not only made a viable eye inthe fruit fly； it made a fly’s eye。 here were two creatures that hadn’t shared a monancestor for 500 million years； yet could swap genetic material as if they were sisters。
the story was the same wherever researchers looked。 they found that they could inserthuman dna into certain cells of flies； and the flies would accept it as if it were their own。
2junk dna does have a use。 it is the portion employed in dna fingerprinting。 its practicality for this purposewas discovered accidentally by alec jeffreys； a scientist at the university of leicester in england。 in 1986jeffreys was studying dna sequences for genetic markers associated with heritable diseases when he wasapproached by the police and asked if he could help connect a suspect to two murders。 he realized his techniqueought to work perfectly for solving criminal cases…and so it proved。 a young baker with the improbable name ofcolin pitchfork was sentenced to two life terms in prison for the murders。
over 60 percent of human genes； it turns out； are fundamentally the same as those found infruit flies。 at least 90 percent correlate at some level to those found in mice。 （we even havethe same genes for making a tail； if only they would switch on。） in field after field；researchers found that whatever organism they were working on—whether nematode wormsor human beings—they were often studying essentially the same genes。 life； it appeared； wasdrawn up from a single set of blueprints。
further probings revealed the existence of a clutch of master control genes； each directingthe development of a section of the body； which were dubbed homeotic （from a greek wordmeaning “similar”） or hox genes。 hox genes answered the long…bewildering question of howbillions of embryonic cells； all arising from a single fertilized egg and carrying identicaldna； know where to go and what to do—that this one should bee a liver cell； this one astretchy neuron； this one a bubble of blood； this one part of the shimmer on a beating wing。 itis the hox genes that instruct them； and they do it for all organisms in much the same way。
interestingly； the amount of genetic material and how it is organized doesn’t necessarily； oreven generally； reflect the level of sophistication of the creature that contains it。 we haveforty…six chromosomes； but some ferns have more than six hundred。 the lungfish； one of theleast evolved of all plex animals； has forty times as much dna as we have。 even themon newt is more genetically splendorous than we are； by a factor of five。
clearly it is not the number of genes you have； but what you do with them。 this is a verygood thing because the number of genes in humans has taken a big hit lately。 until recently itwas thought that humans had at least 100；000 genes； possibly a good many more； but thatnumber was drastically reduced by the first results of the human genome project； whichsuggested a figure more like 35；000 or 40；000 genes—about the same number as are found ingrass。 that came as both a surprise and a disappointment。
it won’t have escaped your attention that genes have been monly implicated in anynumber of human frailties。 exultant scientists have at various times declared themselves tohave found the genes responsible for obesity； schizophrenia； homosexuality； criminality；violence； alcoholism； even shoplifting and homelessness。 perhaps the apogee （or nadir） of thisfaith in biodeterminism was a study published in the journal science in 1980 contending thatwomen are genetically inferior at mathematics。 in fact； we now know； almost nothing aboutyou is so acmodatingly simple。
this is clearly a pity in one important sense； for if you had individual genes that determinedheight or propensity to diabetes or to baldness or any other distinguishing trait； then it wouldbe easy—paratively easy anyway—to isolate and tinker with them。 unfortunately； thirty…five thousand genes functioning independently is not nearly enough to produce the kind ofphysical plexity that makes a satisfactory human being。 genes clearly therefore mustcooperate。 a few disorders—hemophilia； parkinson’s disease； huntington’s disease； andcystic fibrosis； for example—are caused by lone dysfunctional genes； but as a rule disruptivegenes are weeded out by natu