Moss - What genes cant do - 2003, страница 12

This holds good evenin those frequent cases where the characters in question may have the greatestimportance for the welfare or economic value of the individuals.We are very far from the ideal of enthusiastic Mendelians, viz. the possibility ofdissolving genotypes into relatively small units, be they called genes, allelomorphs, factors, or something else. Personally I believe in a great central “something” as yet not divisible into separate factors. The pomace-flies in Morgan’ssplendid experiments continue to be pomace flies even if they lose all “good”genes necessary for a normal fly-life, or if they be possessed with all the “bad”genes, detrimental to the welfare of this little friend of the geneticist (Johannsen1923).Johannsen is posing a question as to the scope of genetic decomposability. Segregation is the evidence for decomposability in inheritance.

Butdoes the fact of the segragatability of some characteristics, require thatGenesis of the Gene39all inheritance is decomposable? Clearly there is no logical necessity thatone follows the other. It could be that, as Johannsen intimates, onlycertain and perhaps comparatively superficial aspects of the organism aredecomposable in their manner of inheritance. Nor would this be inconsistent with the practice (and success) of Mendelian genetics as a formof instrumental reductionism in areas such as agriculture and medicine.Breeders continue to benefit from the ability to reinforce selectivelycertain desirable features of crops without concern for the decomposibility of those central features of plants that most or all plants havein common.

And, just as in the case of Morgan’s flies—which, despitehowever many genetic aberrations they enjoy, continue to be flies—physicians can (and do) benefit from the ability to identify heritable diseasesand genetic syndromes by use of traditional Mendelian pedigrees (andpenetrance and expressivity “fudge” factors). And this is so even if thecentral features of being human (let alone mammal) never do enter intothe realm of Mendelian segregation.Although a critique of the limits of the scope of decomposability doesnot undermine the intentions of genetics as an instrumental reductionism, the same does not hold for the intentions of a constitutive reductionism. By this latter statement I simply mean a theory which treats theorganism as fundamentally decomposable.

If the word “gene” is meantto denote an entity that is causally responsible for a piece of the phenotype and if genetics taken in the vein of constitutive reductionism requiresthat the whole phenotype is explicable in terms of genes, then therecannot be a limit on the scope of genetic decompossibility for fear thatthe whole enterprise might hit a rocky shore. One might be tempted tosuggest that with the benefits of molecular hindsight we can disregardJohannsen’s concerns because we now understand genes to be segmentsof DNA whose decomposability is inscribed in the start-and-stop codesthat demarcate genetic reading sequences.

But this would be tantamountto a kind of category mistake.Neither the basis for an instrumentally reductionist genetics nor a constitutively reductionist genetics follows from the structure of DNA itself,which does not bring with it either an epigenesist or a preformationistname tag. Nor does the ability to correlate certain instances of classical segregation of traits with certain DNA sequences imply that the40Chapter 1remainder of DNA is going to be amenable to any form of classicalreductionist analysis with respect to its relationship to a phenotype.When DNA segregates during meiosis, it is not the phenotype that is segregating; thus, the structure and dynamics of DNA do not address thequestion of the decomposability of the phenotype.

The very same instrumental critique of classical genes can be replayed with DNA in mind.On the basis of observed patterns of Mendelian inheritance, we can treatcertain molecular genes as if they preformationistically determined phenotypes [something like a BRCA1 or cystic fibrosis gene would be examples (see below)] while understanding that at the mechanistic-causal levelof explanation DNA participates in the construction of the phenotype ina manner not amenable to reductionist decomposition. We have noreason to rule out the possibility that the more “species-typic” and“genus-typic” characteristics which are clearly heritable but never seento segregate are based on structures hierarchically above (or simply otherthan) genes—e.g., chromosomal organization, membrane organization,metabolic dynamics, and so forth.Johannsen’s second question pertains to exactly those genes that dobehave in a classically Mendelian fashion.

With respect to these hewonders what the relationship is between so-called dominant and socalled recessive alternatives, or, for that matter, between alternative genesat all, endorsing our current formulation of “multiple allelos” as “different states in the same locus of a chromosome.”When we regard Mendelian “pairs,” Aa, Bb and so on, it is in most cases anormal reaction (character) that is the “allele” to an abnormal. Yellow in ripepeas[e] is normal, the green is an expression for imperfect ripeness as can easilybe proven experimentally, e.g., by etherization . .

. The rich material from theAmerican Drosophilia-researches of Morgan’s school has supplied many cases ofmultiple allelisms—most of all of them being different “abnormalities” comparedwith the characters of the normal wild fly . . . To my mind the main question inregard to these units is this: Are experimentally demonstrated units anythingmore than expressions for local deviations from the original (“normal”) constitutional state in the chromosomes?Is the whole of Mendelism perhaps nothing but an establishment of very manychromosomal irregularities, disturbances or diseases of enormously practical andtheoretical importance but without deeper value for an understanding of the“normal” constitution of natural biotypes (Johannsen 1923)?It is more than noteworthy that although Johannsen was a critical linkin the chain that led to classical genetics, students of genetics to this dayGenesis of the Gene41understand Mendel’s exemplary work on the pea to mean that agenetic locus typically houses several qualitatively different traitsas opposed to various deviations from a single standard capacity.

Similarly, what percentage of that public for whom “a gene for blue eyes”is famous and canonical understand that said gene possesses no substantive capacity for producing blue color but only an inability toproduce that brown pigment which may mask the blue which is alreadythere?While fantastical notions of alleles as alternative qualitative traitsbespeak a caricaturesque preformationism, the idea of alleles as deviations from a norm is quite compatible with an instrumentalist reductionism. Deviations from a norm may become advantageous when thecontext that determines what is normal becomes shifted. For example,consider agricultural applications where the locus of normality shiftsfrom that of ecological fitness to that of commercial value. Navel orangesand other fruits are bred for seedlessness and become unable to reproduce sexually.

High-yield grains have lost metabolic versatility. Breederscan take advantage of abnormal allelic variants by imposing new conditions of normality. In medicine the context of normality is generallyanchored in the taken-for-granted presuppositions of a culture.

The auraof objectivity becomes problemized with, for example, the emergence ofgroups such as the hearing impaired, who constitute themselves as newfocal points of normativity, that is, as alternative forms of life with distinctive beliefs about what should count as normal. New alleles thatbring about qualitative differences in the phenotype generally do sobecause they entail the loss of some biochemical activity, which resultsin a regrouping at a higher level of organization, be it organismal ororganismal and sociocultural.Finally, Johannsen broached the question of the cytoplasm.

Hisremarks on this topic are brief but not without interest:Chromosomes are doubtless vehicles for “Mendelian inheritance” but Cytoplasmhas its importance too . . . Gametogenesis with chromosome-reductions, accompanied by reformation and, as it were, partial rejuvenescence of cell-structure,must in some way act as if especially organized for obliterating the individual’spersonally “acquired characters,” which as a rule totally disappear in sexualreproduction .