40 Evolutionary Anthropology CROTCHETS & QUIDDITIES Goings on in Mendel’s Garden KENNETH WEISS The honorable monk probably didn’t cheat. But he led us astray in other ways. Gregor Mendel gave us the tools by discussed. But this may have inadver- which to do modern genetics, and we tently led us astray, in ways for which have a century of progress to show for we are paying a price today. The arti- it. We properly credit Mendel and his ficial nature of his experiments lured peas for showing us the particulate us into confusing the inheritance of nature of inheritance, but his work traits with the inheritance of genes. both enabled and disabled evolution- And this in turn has led to an unwar- ary thinking for several decades after ranted phenogenetic (see Note 1) de- its rediscovery. Since the factors he terminism that impairs our under- studied didn’t change over genera- standing of biology. tions, Mendel’s discoveries solved the problem that perplexed Darwin, that BLENDING IN blending inheritance would swamp variation and prevent evolution from Mendel wasn’t trying to explain evo- happening. Yet, for the same reason, lution. He knew of traits that varied Mendel’s work impeded evolutionary continuously in his experimental pea thought because evolution requires species, Pisum sativum, and appeared Figure 1. Mendel. change, and discrete variation was to blend in darwinian fashion from also incompatible with darwinian one generation to the next. But he gradualism. Eventually things were wanted to breed agriculturally valu- worked out, we got our unified theory able strains, so he avoided such traits used pea strains that differed only by (the neodarwinian Synthesis), and it and instead selected strains of pea the single traits he reported for each rested on Mendel’s discoveries. plants with traits that stably bred true experiment; (3) that he never actually Despite his contributions, there has across generations. This enabled him stated his two famous laws, which long been a bit of queasiness about to do his experiments, but it made were only attributed to him afterward; Mendel. Almost as soon as his work his analysis conditional, on the cho- (4) that Mendel detected, but failed to was discovered in 1900, persistent sen characteristics of his particular mention linkage (non-independence suggestion arose that the honorable stocks, an important point we will re- of traits), which would have violated turn to later. Moravian monk had falsified his data. his second principle; and (5) whether A recent analysis of these allegations he was affected by Darwin’s Origin of has helped illuminate the issues, and WHAT’S BEEN SAID OF WHAT Species, a translation of which he is known to have read. they fall entirely to Mendel’s credit. MENDEL DID This vindication is good for Men- The first is the most important is- sue, and I’ll consider it last. As to the del’s reputation, and his principles are Fairbanks and Rytting (2001) have charge that Mendel really didn’t have invoked routinely when inheritance is recently published a fine review of the controversies surrounding Mendel’s plants that differed only by the single work and contribution to genetics. traits as he claimed, Fairbanks and Fairbanks and Rytting describe five Rytting show that individual strains of controversies surrounding Mendel’s peas that are known to have been Kenneth Weiss is Evan Pugh Professor of paper: (1) that Mendel fudged his data available to Mendel at the time, in fact Anthropology and Genetics at Penn State to make his results too close to what bred true, separately, for his traits. University. he expected to be believable as results Mendel did not study multiple traits of the experiments he said he did; (2) in a single experiment and then de- Evolutionary Anthropology 11:40–44 (2002) that he misleadingly claimed that he scribe the results as if each trait were CROTCHETS & QUIDDITIES Evolutionary Anthropology 41 genes affecting his traits are so far apart on the chromosome that recom- bination would have been too infre- quent for Mendel to have detected their slight correlation with his sam- ple sizes. His choice of traits also ap- pears to have been for uncompro- mised reasons: they were of botanical importance and differed between strains. Mendel read Origin of Species and made a number of marginal notes in his copy. These notes show that he thought about Darwin’s message, but he found little in the book that related to his agronomic objectives, and noth- ing suggests that his experiments were influenced by Darwin’s ideas in any material way. RESEARCH INTEGRITY OR FALSE ACCUSATION? As to the cheating scandal, the oft- repeated claim is that Mendel’s results fit his expectations too well and must have been cooked. The argument is somewhat intricate, but I can illus- trate it with one example (Figure 3). Let A denote the dominant effect, rel- ative to recessive a, so only aa individ- uals show the recessive trait. Mendel first crossed AA and aa plants, produc- ing all Aa offspring, with the domi- nant trait. Crossing these offspring with each other generates a next gen- eration with a 2:1 mix of Aa to AA types among dominant plants (plus some recessive aa’s). However, Mendel had no way to discriminate type among these domi- nant plants, so he self-fertilized them and sampled 10 offspring from each. If a plant produced 10 dominant off- Figure 2. Cover page of Mendel’s famous paper. Facsimile courtesy Electronic Schol- spring, he assigned them to the AA arly Publishing (www.esp.org). class. This is fine for AA’s. But for an Aa parent, there is a 1 chance that a 4 studied separately. He did what he de- Mendel understood the principles and given offspring will be aa and have the scribed. their importance. recessive trait, but this means that Mendel’s two famous basic princi- In some experiments, Mendel inten- there is a chance of 1 Ϫ (3)10 ϭ 5.6% 4 ples are: Segregation, that the two tionally studied multiple traits simul- that an Aa parent produces no reces- copies of a gene in a parent separate taneously. In most of these, the genes sive offspring among the 10 that Men- and only one is transmitted to any for each trait happened to have been del scored. He would thus assign these given offspring; and Independent As- on separate chromosomes. But there cryptic Aa’s mistakenly to the AA sortment, that different genes are were instances in which two traits class. Even if the true expected Aa to transmitted independently. Fairbanks were in fact linked (Table 1). Had he AA is 2:1, Mendel’s scoring system and Rytting show that Mendel articu- detected that it could have violated his should have produced a 1.89:1.11 ob- lated both principles, and that the al- second second law. One allegation is served ratio. Thus, his results were not legation that he didn’t is based on that he observed co-inherited traits just improbably close to 2:1 but that is searches through his paper to find but failed to report them. However, the wrong answer to be close to. Must them in modern genetic terms. But Fairbanks and Rytting show that the he have cheated to get them? 42 Evolutionary Anthropology CROTCHETS & QUIDDITIES TABLE 1. MENDEL’S GENES Trait Gene Chromosome Notes Seed shape r 7 Smooth dominant to wrinkled Cotyledon color i 1 Yellow dominant to green Seed coat color a 1 Opaque dominant to colorless Pod shape v or p 4 or 6 Inflated dominant to constricted; Which gene Mendel used is not clear Pod color gp 5 Green dominant to yellow Flower position fa 4 Axillary position dominant to apical Stem length le 4 Long dominant to short Source: Fairbanks and Rytting. Fairbanks and Rytting analyze peas. And even if he or his assistant Around 1850, Mendel attended lec- these experiments and conclude that selectively pitched (or nibbled) some tures in chemistry from a professor Mendel’s results were at most only peas, well, before I put blueberries on Redtenbacker, who had done research slightly unlikely, and can be explained my cereal I discard the green ones, but on the integral atomic weight of car- without cheating, in terms of what is I still say that blueberries are blue. bon. I think (see Note 2) it quite plau- known about pea biology. This largely We should also consider Mendel’s sible that Mendel was predisposed to exonerates Mendel, even from the historical context. In 1815 William think of integral units of causation for most serious charge. But I think the Prout suggested that the weights of what he observed in plants. Some- whole issue has been greatly over- pure substances were integral num- thing discrete had to be passed to the blown in the first place. The reasons to bers of the weight of hydrogen, of gametes of his plants, and indeed lighten up on the poor monk have to which he hypothesized other sub- Mendel called them “elements”.He do with his context, with statistical stances were composed. Supporters was after a principle of nature, not a considerations, and with what science of this view stuck to their guns even in parameter estimate. Everyone knew is all about. the presence of non-integral relative there was error in experiments (Men- Mendel was apparently good at an- weights such as that for chlorine, esti- del identified sources of uncertainty in alytical thinking and quantification mated as 35.83. On various rationales, scoring pea traits). New scientific the- but he worked in a pre-statistics age.