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er at once： 鈥渕e； too！鈥潯n the most literal andfundamental sense we are all family。
we are also uncannily alike。 pare your genes with any other human being鈥檚 and onaverage they will be about 99。9 percent the same。 that is what makes us a species。 the tinydifferences in that remaining 0。1 percent鈥斺�渞oughly one nucleotide base in every thousand；鈥�
to quote the british geneticist and recent nobel laureate john sulston鈥攁re what endow uswith our individuality。 much has been made in recent years of the unraveling of the human genome。 in fact； there is no such thing as 鈥渢he鈥潯uman genome。 every human genome isdifferent。 otherwise we would all be identical。 it is the endless rebinations of ourgenomes鈥攅ach nearly identical； but not quite鈥攖hat make us what we are； both as individualsand as a species。
but what exactly is this thing we call the genome？ and what； e to that； are genes？
well； start with a cell again。 inside the cell is a nucleus； and inside each nucleus are thechromosomes鈥攆orty…six little bundles of plexity； of which twenty…three e from yourmother and twenty…three from your father。 with a very few exceptions； every cell in yourbody鈥�99。999 percent of them； say鈥攃arries the same plement of chromosomes。 （theexceptions are red blood cells； some immune system cells； and egg and sperm cells； which forvarious organizational reasons don鈥檛 carry the full genetic package。） chromosomes constitutethe plete set of instructions necessary to make and maintain you and are made of longstrands of the little wonder chemical called deoxyribonucleic acid or dna鈥斺�渢he mostextraordinary molecule on earth；鈥潯s it has been called。
dna exists for just one reason鈥攖o create more dna鈥攁nd you have a lot of it inside you：
about six feet of it squeezed into almost every cell。 each length of dna prises some 3。2billion letters of coding； enough to provide 103；480；000；000possible binations； 鈥済uaranteed tobe unique against all conceivable odds；鈥潯n the words of christian de duve。 that鈥檚 a lot ofpossibility鈥攁 one followed by more than three billion zeroes。 鈥渋t would take more than fivethousand average…size books just to print that figure；鈥潯otes de duve。 look at yourself in themirror and reflect upon the fact that you are beholding ten thousand trillion cells； and thatalmost every one of them holds two yards of densely pacted dna； and you begin toappreciate just how much of this stuff you carry around with you。 if all your dna werewoven into a single fine strand； there would be enough of it to stretch from the earth to themoon and back not once or twice but again and again。 altogether； according to onecalculation； you may have as much as twenty million kilometers of dna bundled up insideyou。
your body； in short； loves to make dna and without it you couldn鈥檛 live。 yet dna is notitself alive。 no molecule is； but dna is； as it were； especially unalive。 it is 鈥渁mong the mostnonreactive； chemically inert molecules in the living world；鈥潯n the words of the geneticistrichard lewontin。 that is why it can be recovered from patches of long…dried blood or semenin murder investigations and coaxed from the bones of ancient neandertals。 it also explainswhy it took scientists so long to work out how a substance so mystifyingly low key鈥攕o； in aword； lifeless鈥攃ould be at the very heart of life itself。
as a known entity； dna has been around longer than you might think。 it was discoveredas far back as 1869 by johann friedrich miescher； a swiss scientist working at the universityof t眉bingen in germany。 while delving microscopically through the pus in surgicalbandages； miescher found a substance he didn鈥檛 recognize and called it nuclein （because itresided in the nuclei of cells）。 at the time； miescher did little more than note its existence； butnuclein clearly remained on his mind； for twenty…three years later in a letter to his uncle heraised the possibility that such molecules could be the agents behind heredity。 this was anextraordinary insight； but one so far in advance of the day鈥檚 scientific requirements that itattracted no attention at all。
for most of the next half century the mon assumption was that the material鈥攏owcalled deoxyribonucleic acid； or dna鈥攈ad at most a subsidiary role in matters of heredity。 itwas too simple。 it had just four basic ponents； called nucleotides； which was like having an alphabet of just four letters。 how could you possibly write the story of life with such arudimentary alphabet？ （the answer is that you do it in much the way that you create plexmessages with the simple dots and dashes of morse code鈥攂y bining them。） dna didn鈥檛do anything at all； as far as anyone could tell。 it just sat there in the nucleus； possibly bindingthe chromosome in some way or adding a splash of acidity on mand or fulfilling someother trivial task that no one had yet thought of。 the necessary plexity； it was thought；had to exist in proteins in the nucleus。
there were； however； two problems with dismissing dna。 first； there was so much of it：
two yards in nearly every nucleus； so clearly the cells esteemed it in some important way。 ontop of this； it kept turning up； like the suspect in a murder mystery； in experiments。 in twostudies in particular； one involving the pneumonococcus bacterium and another involvingbacteriophages （viruses that infect bacteria）； dna betrayed an importance that could only beexplained if its role were more central than prevailing thought allowed。 the evidencesuggested that dna was somehow involved in the making of proteins； a process vital to life；yet it was also clear that proteins were being made outside the nucleus； well away from thedna that was supposedly directing their assembly。
no one could understand how dna could possibly be getting messages to the proteins。 theanswer； we now know； was rna； or ribonucleic acid； which acts as an interpreter betweenthe two。 it is a notable oddity of biology that dna and proteins don鈥檛 speak the samelanguage。 for almost four billion years they have been the living world鈥檚 great double act； andyet they answer to mutually inpatible codes； as if one spoke spanish and the other hindi。
to municate they need a mediator in the form of rna。 working with a kind of chemicalclerk called a ribosome； rna translates information from a cell鈥檚 dna into terms proteinscan understand and act upon。
however； by the early 1900s； where we resume our story； we were still a very long wayfrom understanding that； or indeed almost anything else to do with the confused business ofheredity。
clearly there was a need for some inspired and clever experimentation； and happily the ageproduced a young person with the diligence and aptitude to undertake it。 his name wasthomas hunt morgan； and in 1904； just four years after the timely rediscovery of mendel鈥檚experiments with pea plants and still almost a decade before gene would even bee a word；he began to do remarkably dedicated things with chromosomes。
chromosomes had been discovered by chance in 1888 and were so called because theyreadily absorbed dye and thus were easy to see under the microscope。 by the turn of thetwentieth century it was strongly suspected that they were involved in the passing on of traits；but no one knew how； or even really whether； they did this。
morgan chose as his subject of study a tiny； delicate fly formally called drosophilamelanogaster； but more monly known as the fruit fly （or vinegar fly； banana fly； orgarbage fly）。 drosophila is familiar to most of us as that frail； colorless insect that seems tohave a pulsive urge to drown in our drinks。 as laboratory specimens fruit flies had certainvery attractive advantages： they cost almost nothing to house and feed； could be bred by themillions in milk bottles； went from egg to productive parenthood in ten days or less； and hadjust four chromosomes； which kept things conveniently simple。
working out of a small lab （which became known inevitably as the fly room） inschermerhorn hall at columbia university in new york； morgan and his team embarked ona program of meticulous breeding and crossbreeding involving millions of flies （onebiographer says billions； though that is probably an exaggeration）； each