Perspectives in Biology and Medicine

PSYCHOLOGICAL, HISTORICAL, AND ETHICAL REFLECTIONS ON THE MENDELIAN PARADOX MOTI NISSANI* Evidence That Mendel's Reported Data Are Too Good to Be True By 1911 [1], R. A. Fisher sensed some statistical irregularities in Mendel 's classic paper [2]. Twenty-five years later, Fisher published an analysis of Mendel's experiments, concluding that "the data of most, if not all, of the experiments have been falsified so as to agree closely with Mendel's expectations" [3, p. 164]. In private, Fisher referred to his discovery that Mendel data had been "faked" as "abominable" and a "shocking experience" [4, pp. 296—297]. Fisher's dispassionate analysis created a storm [1] which, fifty-seven years after the event, shows no signs of subsiding. Fisher's indictment has received the closest possible attention from a great number of scholars , second only, perhaps, in the genetics literature, to Mendel's own paper. Despite this considerable attention, the subject remains every bit as controversial today as it was in 1936. By now, the charge that Mendel's paper does not faithfully report his data stems from four lines of evidence: 1 . A cursory look at Mendel's various observations soon makes a statistically literate person notice that they come, over and over again, uncomfortably close to Mendel's expectations. As Edwards put it, "one can applaud the lucky gambler; but when he is lucky again tomorrow, and the next day, and the following day, one is entitled to become a little suspicious" [5]. The precise calculations are still under dispute, but the best current estimate suggests that results as close as or closer to expectations as the ones reported by Mendel would occur in only 1 out of 33,000 The author thanks J. F. Crow, A. W. F. Edwards, Donna Hoefler-Nissani, and J. B. Michels for their critical reviews. *Interdisciplinary Studies Program, Wayne State University, 6001 Cass Avenue, Detroit, Michigan 48202.© 1994 by The University of Chicago. All rights reserved. 003 1-5982/94/3702-086010 1 .00 182 Moti Nissani ¦ Reflections on the Mendelian Paradox replications [6, p. 921]. In other words, it is virtually inconceivable that Mendel obtained his "good" results by pure chance. 2.In one subset of the experiment discussed above, Mendel tested the genetic makeup of F2 plants showing the dominant characteristic. His theory led him to the correct expectation that the actual ratio of heterozygotes (Aa) to homozygotes (AA) in such cases is 2: 1. However, because he only tested 10 progeny per plant, a small fraction of the heterozygotes would have been missed, leading to misclassification of Aa plants as AA and to an observed ratio of 1.7 Aa to 1 AA. Mendel apparently overlooked this, wrongly expecting a 2 : 1 observed ratio. Amazingly, his reported results agree closely with the naive expectation of a 2: 1 ratio. The overall discrepancy from the correct 1.7 to 1 ratio in this subset of the experiment "is strongly significant, and so low a value could scarcely occur by chance once in 2,000 trials" [3, p. 162]. Fisher sums up the evidence here: A serious and almost inexplicable discrepancy has, however, appeared, in that in one series of results the numbers observed agree excellently with the two to one ratio, which Mendel himself expected, but differ significantly from what should have been expected had his theory been corrected to allow for the small size of his test progenies. [3, p. 164] 3.Mendel obtained all 128 (27) possible pair combinations of his seven pea characters and reported that each character segregated independently of all the others. This assertion presents two complications. First, it is possible that two of Mendel's seven characters were in fact linked [7, 8]. In that case, Mendel's claim—that all possible permutations "gave approximately equal results" [2, p. 22] as the ones he reported for independent assortment—is questionable. Second, even if all his seven characters were unlinked, there is still the question of his failure to report linkage. It seems unlikely that by 1865 Mendel would not have encountered linkage in any of his extensive experiments on species other than peas (see below). It seems even less probable that...