The Birth of GeneticsThe study of heredity and

the techniques of genetic analysis

Waseda University, SILS,History of Modern Earth and Life Sciences

The analysis of heredity

Genetics, as a study of the phenomena of heredity, can bethought of as an experimental practice that cuts across manydifferent fields of biology. As an experimental practice, or craft,it relies on the fundamental methodology of hybridization, orsex-crossing. That is, it relies on procedures of interfering withthe normal processes of breeding, or making inferences fromcounterfactual-assumptions.We will see that the analysis of heredity practiced in genetics isa form of methodological reductionism, which perhaps includesepistemological reductionism but does not need to implyontological reductionism. If this is the case, we do not need to beconcerned to articulate a program of reducing the organism tothe gene, and we can instead ask about the changing concept ofthe gene through various forms of genetic analysis.

Title 1 / 36

The hereditarians

In the 19th century, there was a lively debate about whatconstituted a heritable characteristic, and whether or notacquired – or learned – characteristics could be passed onthrough the mechanisms of inheritance.Galton conceived of inheritance as a statistical relation overcharacteristics between populations of successive generations.This definition gives rise to a research program treating thedata of hereditary transmission. In general, in this tradition thetraits were seen to have continuous differentiation.Finally, A. Weismann’s theory of cellular heredity – with thenucleus of the germ cell carrying the hereditary substance –used the results of cytology to explain heredity. This cytologicaltheory, however, originally provided no mechanism forinheritance.

Title 2 / 36

Darwin’s theory of pangenesisIn The Variation of Animals and Plants under Domestication (1886),C. Darwin put forward a theory of heredity – which he calledpangenesis – that was quite influential up to around 1900.Pangenesis was the theory that all the cells in an organism shedminute particles that he called “gemmules,” which are able tocirculate throughout the body and finally congregate in thegerm cells. These gemmules are then transmitted to the nextgeneration and are responsible for the transmission ofcharacteristics from parent to offspring. If any cells of theparent undergo changes as a result of environmental change,they will consequently transmit modified gemmules to theiroffspring. This was a way of accounting for the inheritance ofacquired characteristics.The theory of was set aside after F. Galton failed to transferpigmentation differences between developing rabbits by bloodtransfusion.

Title 3 / 36

Gregor Johann Mendel (1822–1884)

Mendel was an ethnic German, born inthe Austrian Empire (now, CzechRepublic), to a farming family.He was a fiar, but attended universityat both Olomauc and Vienna. (Hisphysics professor was Doppler.)He carried out his experiments inbreeding at St. Thomas’ Abbey.He was a physics teacher and later headof the Abbey.Most of his published work was inmeteorology.We will look at some results fromExperiments on Plant Hybridization, 1865.

Title 4 / 36

Mendel’s laws of heredity in pea plants

Mendel was interested in how hybridization might be relatedto the emergence of new species. He carried out a series ofbreeding experiments on pea plants, involving some 10,000plants over the course of 8 years.He believed that the key to heredity lay in the question ofvariation and advanced the novel proposition that heredity isparticulate, or discrete, in opposition to the blended theories ofinheritance put forward by Darwin and others.The 1865 paper also made a new argument for the idea thatheredity does not operate on acquired characteristics, and thattraits are transmitted unchanged from one generation to thenext.

Title 5 / 36

The Pisum sativum experimentHe chose Pisum sativum as a model system that showed clearpairs of contrasting traits (which he called “elements”), such asstem height, seed shape and color, and bred plants havingopposite traits over a number of generations.He first bred true lines of each character, and then cross-bredthese and counted the results. He was interested the ratios, notthe absolute numbers.He found that in the first generation (F1) one of the traits,which he called dominant, would be found in 100% of theoffspring, but in the next generation (F2), the other trait, whichhe called recessive, would reappear in numbers thatapproximated the rato 3:1, dominant to recessive.In order to explain these results he supposed there was someunderlying object of analysis, which he called the factor orrudiment (Anlage, related to our later genotype), that could bedistinguished from the plant itself (later our phenotype).

Title 6 / 36

Simplification of Mendel’s law of heredity

Title 7 / 36

The reception of Mendel’s ideasMendel’s paper was distributed to 134 libraries and scientificinstitutions. 40 reprints of the paper were made and Mendelsent some of these to prominent biologists. Nevertheless, thepaper was neglected by geneticists for almost 30 years.There are a number of reasons why his contemporaries werenot interested in his work:

Mendel was writing about hybridization, not inheritance.He was interested in the production of species throughhybridization, a degenerating research program at thetime.His contemporaries were interested in the origin of species,but Mendel gave no account of how new species could beproduced by crossing hybrids in this way.He did not believe that the laws that he had discoveredwere universal.Mendel did not belong to a social network of biologists.

Title 8 / 36

The rediscoveryIn 1900, three botanists – Carl Correns (1864–1933), Hugo deVries (1848–1935) and Erich Tschermak (1871–1962) – claimedto have independently rediscovered Mendel’s ratios, and thento have found his 1856 paper.In fact, however, many people had reported numbers that wereapproximations of Mendel’s ratios, but they did not see thesignificance of this. What seems clear from the evidence is thatCorrens and Tschermak saw that there was somethingsignificant in the numbers they were seeing, and then insearching the literature, they found Mendel’s work. De Vrieshad produced numbers that could have been used to show theratios and the fact of germinal segregation, but it was only afterhe read a copy of Mendel’s paper in 1900, that he understoodthe significance of these numbers.That is, the Mendelian ratios are not an unadulteratedobservational fact, but a theory-laden fact.

Title 9 / 36

De Vries’ “mutations”

Hugo de Vries (1848–1935) was a Dutch botanist who workedin genetics after rediscovering Mendel’s laws (and paper). Hedisagreed with the mechanism of natural selection for explainingevolution and offered, instead, a theory of rapid changes,which he called mutations. These, he thought, would lead torapid changes in species – saltationism.He carried out a series of experiments on Oenothera lamarkiana –evening primrose. He planted seeds in the botanical gardens atAmsterdam, and observed the results. Very quickly he foundvarieties that were different from the original stock. He foundone variety, O. gigas, which was robust and bred true, claimingthat these were new forms that were complete and bred true.

Title 10 / 36

The mutation theory

de Vries,Mutation Theory (1903)“I. New elementary species arisesuddenly, without transitionalforms... II. New elementaryspecies are... absolutely constantfrom the moment they arise... VI.The mutations, to which the originof new species is due appear to beindefinite, that is to say, thechanges may effect all organs andseem to take place in anydirection...”Such mutations are different fromthe modern concept of mutation asoccuring in the genetic material.

Title 11 / 36

New “species”

Oenothera lacta

De Vries believed that he hadobtained new species in this way.de Vries, Species and variation … (1905)“They come into existence at once,fully equipped, without preparationor intermediate steps. No series ofgenerations, no selection, no strugglefor existence was needed. It was asudden leap into another type, a sportin the best acceptation of the word. Itfulfilled my hopes, and at once gaveproof of the possibility of the directobservation of the origin of species,and the experimental control thereof.”

Title 12 / 36

Bateson’s MendelismWilliam Bateson (1861–1926) was a Cambridge natural scientistand zoologist, who founded the department of genetics. Hesupported evolution as a biological fact, but he did not acceptthe mechanism of natural selection. He argued, instead, thatspecies are created by hybridization or drastic mutations. Thiswas partly because it was believed by physicists at the time thatthe earth was fairly young. He wrote a number of booksarguing for the saltationist position.In 1900, Bateson became a Mendelian and had Mendel’s papertranslated into English and republished. He and his colleaguesdiscovered semidominance, showed that there are Mendeliantraits in animals as well, that some genes are linked, etc. In theearly part of the 20th century, the Darwinists and theMendelians were opposed: They had different ideas (gradualvs. sudden change, continuous vs. discrete traits), they haddifferent research methods (observation vs. experimentation),they represented different research traditions.

Title 13 / 36

Drosophila melanogaster, the common fruit flyDrosophila has had a long career inscience; they are ubiquitous and breedeasily and quickly.The fruit fly first came into genetic labsin 1901 as a control for otherexperiments.Gradually, however, researchersnoticed that they show a large numberof easily observable mutations whichdisplay Medelian traits.The fruit fly was made most famous bythe so-called “Fly Room” at ColumbiaUniversity and remains an essentialtool for genetic research andinstruction.

Title 14 / 36

Thomas Morgan (1866–1945), and the “Fly Boys”

Morgan took a PhD in zoology from Johns Hopkins, and taughtat Columbia University before becoming the founding directorof the devision of biology at Caltech. He had a number ofstudents at Columbia, of whom the most famous were AlfredSturtevant (1891–1970), Calvin Bridges (1889–1938), and laterHermann Joseph Muller (1890–1967).Morgan started out to try to prove de Vries correct, but hiswork quickly lead him down a path of studying Mendelianmechanisms in Drosophila. He took on a number of studentswho worked with him in Columbia and who took with themthe experimental methods of Drosophila crossing to their ownlabs. This was an incredibly productive time and they allworked collaboratively to develop the chromosome theory ofinheritance. Researchers came from all over the world to learnthe techniques of genetic analysis developed in the “Fly Room.”

Title 15 / 36

Morgan and the “boys”

Marine Biological Laboratory in Woods Hole, Massachusetts

Title 16 / 36

White eyesMorgan was initially skeptical about Mendel’s ideas and beganworking on Drosophila to see if he could find evidence for deVriesian mutation. In the course of these experiments, henoticed a white-eyed male, which he called a “mutation.” Ashe studied and crossed this white-eyed male he noticed that allof the F1 were wild type, but in the F2 generation, white-eyedmales appeared again – as would be expected. When he crossedthese white-eyed F2 males with wild-eyed F1 females, half ofthe males and half of the females were white-eyed, and whenhe crossed these white-eyed females with wild-eyed males, allthe males were white-eyed and all the females were wild-eyed.This lead to the realization that there was some kind of sexlinkage that was effecting the results. That is, Morgan assumedthat the factor for eye color segregated with the factor for sex.This was the impetus for a change in research direction andMorgan and his younger colleagues began to study linkage.

Title 17 / 36

Sex-linkage in white-eyed Drosophila, I

Wild-eyed females crossed with white-eyed males

Title 18 / 36

Sex-linkage in white-eyed Drosophila, II

White-eyed females crossed with wild-eyed males

Title 19 / 36

Linkage analysisThe realization that the factor for white-eyes was linked to sex,lead to the supposition that it was physically located on the sexchromosome. This assumption – along with the observation ofcrossover in meiosis – provided the basis for a far-rangingresearch program to map the genes to the chromosome.The first step in this program was to establish a baseline bymeasuring the “distance” between two genes – as the ratio(percentage) over a large number of crosses of the actual rate ofrecombination to the expected rate for unlinked genes. Thisinvolved separating and counting the progeny of crosses inwhich recombination has and has not occurred.In order to measure the linkage between different genes, purestrains of Drosophila had to be bred, which was itself a long,drawn out process. Bridges and Muller proved to beparticularly good at this.

Title 20 / 36

Morgan in the Fly Room

Drosophilawere kept in emptymilk bottles capped with cottongauze. They were fed bananapulp and could be knocked outwith ether so that they could beinspected, counted and sortedinto new bottles.

Title 21 / 36

Calvin Bridges in the Columbia Fly Room

Title 22 / 36

The chromosome theoryDrosophila have only 4 chromosomes, so the“fly boys” thought it would be relatively easyto map the genes to this structure.Constructing the framework for chromosomes2 and 3 took two years. First the “distance”between two genes – say pink and ebony – wereestablished, and then other genes were relatedto these. It turns out that chromosomes alsohave physical structure that complicates thisprocess, and this also had to be worked outthrough these analytical techniques.In all of this work the fundamental techniquewas genetic analysis – that is, crossing and backcrossing specially prepared stocks of flies andcounting the results.

Bridges’ “Totem pole”

Title 23 / 36

The reception of the theoryA flood of papers from the “Fly Room” and The Mechanism ofMendelian Heredity (1915) by Morgan, Sturtevant, Muller andBridges convinced most biologists that Mendelian geneticscoupled with chromosome theory was the way forward instudying heredity. Morgan lectured widely, many youngresearchers were trained at Columbia and then later Caltech,and the fly group freely gave stocks of specially bred flies toother researchers.This produced a new school of geneticists who took apragmatic and experimental approach to biology. Althoughmany of the older generations of naturalists objected to the“unnatural” constraints of laboratory practice, the youngerresearchers gravitated to these highly productive methods. Themerger of cytology and genetic analysis gave the new geneticsa basis in physical reality that the previous theories of heredityhad lacked.

Title 24 / 36

Biology as a laboratory science

Morgan, Critique of the Theory of Evolution … (1916)“The objection has been raised, in fact, that in the breedingwork with Drosophilawe are dealing with artificial andunnatural conditions. It has been more than implied thatresults obtained from the breeding pen, the seed pan, the flowerpot and the milk bottle do not apply to evolution in the ‘open’,nature ‘at large’ or to ‘wild’ types. To be consistent, this sameobjection should be extended to the use of the spectroscope inthe study of the evolution of the stars, to the use of the test tubeand the balance by the chemist, of the galvanometer by thephysicist. All these are unnatural instruments used to tortureNature’s secrets from her. I venture to think that the realantithesis is not between unnatural and natural treatment ofNature, but rather between controlled or verifiable data on theone hand, and unrestrained generalization on the other.”

Title 25 / 36

The dissemination of Drosophila and geneticsIt was the custom of the drosophilists to share information andfly stocks with each other. Sharing fly stocks was a sign ofmembership in a special community, a guarantee ofparticipation.In 1911–1912, Morgan persuaded professors at small colleges inthe US to take and breed stocks. During this period, Columbiawas the only large center. During the 1920s and 30s, large labsstarted in Cold Spring Harbor, Berlin, and University of Texas,Austin. In 1928, Morgan, Bridges, and Sturtevant moved fromColumbia to Caltech. By the end of the 30s, all largeuniversities kept stocks of Drosophila.The model experimental systems that were produced withthese stocks were freely exchanged and a large percentage ofthe drosophilists’ correspondence was taken up discussing theexchange of stocks. Morgan’s group continued to dominate thisexchange into the 30s.

Title 26 / 36

The fly lab at University of Texas, Austin

T.S. Painter sits tothe left, W.S. Stonestands in the back,C.P. Oliver sitsfront, and Mullerviews fliesthrough ajeweler’s loupe.

Title 27 / 36

The fly lab at Cold Spring Harbor, Long Island, New York

Title 28 / 36

X-rays and mutationIn 1926, now at University of Texas,Muller carried out at series ofexperiments on the effect of x-rayradiation on the rate of mutations inDrosophila. A quantitative correlationbetween the dose of radiation and thenumber of lethal mutations quicklyemerged.There was a media sensation when heannounced these results at aconference in Berlin, and when hisresults were repeated by others hebecame something of a scientificcelebrity.

Title 29 / 36

Drosophila and the genetics of development

In the 1930s, George Beadle (1903–1989) and Boris Ephrussi(1901–1979), working in Morgan’s fly group at Caltech, beganto study the genetics of development in Drosophila bytransplanting tissue – an imaginal disk – from one fly larva intoanother fly larva, using a micropipette, in order to grow thetransplanted tissue in the host larvae.They developed a delicate experimental technique toinvestigate the role of genes in the embryological developmentof eye color. In fact, they showed that although transplantedeyes still became eyes – although not properly located – theygenerally, although not always, took on the coloration of thehost genes not that of the transplanted genes. They argued thatthis means that the mechanism of development is controlled bythe overall – we would say, epigenetic – context, not simply thechromosomal material of the transplanted imaginal disk.

Title 30 / 36

Beadle and Ephrussi at work

Microscope A,used by oneperson, had astand and amechanism for thechange of objects.Microscope B,facing MicroscopeA at 45° and usedby a differentperson, examineda test tubemounted on anadjustable stand.

Title 31 / 36

NeurosporaNeurospora is a mold – used to make oncom – thatis usually cultured on a medium consisting ofsugar, inorganic salts, and the vitamin biotin. Thefungus has a short life cycle, and reproduces bothasexually and sexually – that is, sexualreproduction gives rise to spores. Hence, it couldbe subjected to genetic analysis. In addition,Neurospora possesses only one set of unpairedchromosomes, so that any mutation isimmediately expressed.It can also be cultivated on a minimal medium ofcarbon, salt and biotin. On this medium, thefungus can manufacture for itself the othersubstances that it needs – amino acids, vitaminsand proteins.

Title 32 / 36

Neurospora takes over

The fact that not all transplanted imaginal disk showedautonomous development indicated that the genetics ofmetabolic chainsmight be investigated, but Drosophila proved tobe a clumsy system for these studies.Now at Stanford, Beadle began working on NeurosporawithEdward Tatum (1909–1975). They irradiated the mold toproduce mutants that could not live without the addition ofspecific amino acids, starting with arginine. They producedfour different strains of the mold that could not producearginine and showed that each of these had lost the ability toproduce a certain enzyme that is involved in the production ofarginine. In this way, they set out an experimental system fordemonstrating the effects of genes on metabolism at amolecular level.

Title 33 / 36

The one-gene one-enzyme hypothesis

This work led them to frame the hypothesis that eachindividual gene is responsible for the production of one, andonly one, enzyme. This was the basis of a highly productiveresearch program in biochemical genetics, that helped lay thefoundations for molecular biology.The current position is that one gene produces one protein, orrather polypeptide (protein component).EnzymeA protein capable of producing certain chemical changes bycatalytic action. In the 19th century, what we call enzymes wereknown as “ferments.”

Title 34 / 36

What is a gene?We tend to think we have a clear idea of what a gene is, but thisis actually tricky to pin down. In fact, we generally have twodifferent things in mind, and it is not clear that they are thesame, or map one-to-one to each other.Gene-p refers to something, or set of things, that has thefunction of producing a certain trait in the phenotype –including the genetic material of the phenotype, the genome.But the idea that there is actually a certain thing that functionsin this way is unclear, and we speak of such gene-ps only froman instrumentalist perspective.On the other hand, when we try to take a realist perspective wemean gene-d, and refer to some molecular sequence in thechromosome, in the genome. But, in fact, gene-d isindeterminate with respect to the phenotype, because thecontext in which it operates – the epigenome, and the organismin general – is also crucial.

Title 35 / 36

Overview

We have discussed the rise of classical genetics withMendel and the Columbia Fly Room.We have discussed the rise of developmental, orbiochemical genetics.We have underlined some philosophical questionspertaining to the nature of the gene, or the gene concept.

Title 36 / 36