Controversies Controversies about mendel's work Although Mendel is now recognized as the founder of genetics, significant controversy ensued about his work throughout the 20century Fairbanks and Rytting(2001)reviewed five of the most contentious issues Are Mendel's data too good to be true? Is Mendel's description of his experiments fictitious? Did Mendel articulate the laws of inheritance attributed to him? Did mendel detect but not mention linkage? Did Mendel support or oppose Darwi Are Mendel's data too good to be true? 1902, Weldon (x test): Mendels observed ratios were astonishingly close to his expectations. 1936, Fisher: Mendel's results were consistently so close to expected ratios that the validity of those results must be questioned. He concluded that mendel should have misclassified some heterozygotes as homozygotes obtaining 2: I genotypic ratio Mendel chose 100 F2 plants with the dominant phenotype for each of the five plant traits and grew only ten F3 descendents from each of these plants o If all ten F3 descendents had the dominant phenotype: F2 plant as constant (he o F3 descendents had both dominant and recessive phenotypes, the F2 plant as hybrid (heterozygous) o If each offspring has an independent probability, 0.75, of displaying the dominant character, the probability that all ten will do so is 0.75, or 0.0563 Consequently, between 5 and 6 percent of the heterozygous parents were misclassified as homozygotes o The expected ratio of segregating to nonsegregating families should not be 2: 1 but 1.8874: 1. 1126, or approximately 377.5: 222.5 out of 600 A x test of Mendel's observed values of 399 and 201 and Fisher's expected values of 377.5 and 222.5 yields a x value of 3. 3020 with one degree of freedom, which is not statistically significant because its probability is 0.0692(0.05). Piegorsch(1983)calculated the summed x2 value to be 7.6582 with a corresponding probability of 0.2642 with six degrees of freedom. When this probability is halved to 0. 1321, the differences between Mendel's observed ratio d Fishers expected ration becomes nonsignificant 1986, Edwards: Only two of Mendel's 22 experiments yield x values with probabilities <0.5 and six yield x values with probabilities >0.90, -- bias toward expectation in Mendels data Random distribution: P=0.05 to 0.95, nonsignificant and hypothesis is accepted
Controversies 1 Controversies about Mendel’s work Although Mendel is now recognized as the founder of genetics, significant controversy ensued about his work throughout the 20th century. Fairbanks and Rytting (2001) reviewed five of the most contentious issues: • Are Mendel's data too good to be true? • Is Mendel's description of his experimentsfictitious? • Did Mendel articulate the laws of inheritance attributed to him? • Did Mendel detect but not mention linkage? • Did Mendel support or oppose Darwin? Are Mendel's data too good to be true? • 1902, Weldon ( 2 test): Mendel's observed ratios were astonishingly close to his expectations. • 1936, Fisher: Mendel's results were consistently so close to expected ratios that the validity of those results must be questioned. • He concluded that Mendel should have misclassified some heterozygotes as homozygotes in obtaining 2: 1 genotypic ratio. • Mendel chose 100 F2 plants with the dominant phenotype for each of the five plant traits and grew only tenF3 descendents from each of these plants. o If all ten F3 descendents had the dominant phenotype: F2 plant as constant (homozygous). o F3 descendents had both dominant and recessive phenotypes, the F2 plant as hybrid (heterozygous). o If each offspring has an independent probability, 0.75, of displaying the dominant character, the probability that all ten will do so is 0.7510, or 0.0563. o Consequently, between 5 and 6 percent of the heterozygous parents were misclassified as homozygotes. o The expected ratio of segregating to nonsegregating families should not be 2 : 1, but 1.8874 :1.1126, or approximately 377.5 : 222.5 out of 600. o A 2 test of Mendel's observed values of 399 and 201 and Fisher's expected values of 377.5 and 222.5 yields a 2 value of 3.3020 with one degree of freedom, which is not statistically significant because its probability is 0.0692 (> 0.05). o Piegorsch (1983) calculated the summed 2 value to be 7.6582 with a corresponding probability of 0.2642 with six degrees of freedom. When this probability is halved to 0.1321, the differences between Mendel’s observed ratio and Fisher’s expected ration becomes nonsignificant. • 1986, Edwards: Only two of Mendel's 22 experiments yield 2 values with probabilities <0.5 and six yield 2 values with probabilities >0.90, -- bias toward expectation in Mendel's data. o Random distribution: P = 0.05 to 0.95, nonsignificant and hypothesis is accepted
Controversies The proximity of Mendel's F, progeny data to an incorrect expectation is not as questionable it might seem when viewed in a botanical context Mendel probably sowed more than ten seeds in a space to be occupied by ten plants, then thinned the seedlings to ten to ensure that there were ten F3 progeny from each F2 plant. Indeed, Mendel's description of his method, reported cultivating rather than sowing seeds Had Mendel sown more than ten seeds from each F2 plant, then he could have scored two of the plant traits in seedlings before thinning Differences in stem length, as Mendel noted in his paper, can be easily scored in seedlings a few days after germination Variation for flower and seed-coat color was perfectly correlated with variation for axillary pigmentation in his experiments. Mendel could score F3 plants for the presence or absence of axillary pigmentation as early as two to three weeks after germination Wright (1966)and Beadle(1967)proposed that Mendel might have unconsciously misclassified individuals with questionable phenotypes to favor his expectations o Mendel was careful about misclassification of phenotypes and had mentioned that with a little practice in sorting, however, mistakes are easy to avoid o Mendel identified not only the phenotypes, but also the genotypes of individual seeds through examination of their self-fertilized progeny Mendel might have stopped counting individuals when the numbers were close to the ratios he expected [olby(1985)and Beadle(1967) o The average number of seeds per plant is close to 30(16 590 seeds divided by 550 plants=30 16 seeds per plant from the monohybrid, dihybrid, and trihybrid nts for seed shape and cotyledon color) o Is 30 seeds per plant too low for 19 century pea cultivation? o Dr J. Rasmussen, a pea geneticist wrote to Fisher: " About 30 good seeds per plant is, under Mendel's conditions. o Fairbanks and Rytting(2001)found that average number of seeds per plant was 47.18 among 24 garden-pea varieties grown at the New York Agricultural Experiment Station in Geneva, during 1888(Curtis, 1889) Of the 24 varieties tested, four produced averages of less than 30 seeds In Mendels second letter to Nageli, he referred to his paper as the unchanged reprint of the draft of the lecture mentioned; thus the brevity of the exposition, as is essential for public lecture Fairbanks and rytting(2001)concluded that, although the bias in Mendel's experiments is evident, there are reasonable statistical and botanical explanations for the bias, and insufficient evidence to indicate that Mendel or anyone else falsified the data
Controversies 2 The proximity of Mendel's F3 progeny data to an incorrect expectation is not as questionable as it might seem when viewed in a botanical context. • Mendel probably sowed more than ten seeds in a space to be occupied by ten plants, then thinned the seedlings to ten to ensure that there were ten F3 progeny from each F2 plant. Indeed, Mendel's description of his method, reported cultivating rather than sowing 10 seeds. • Had Mendel sown more than ten seeds from each F2 plant, then he could have scored two of the plant traits in seedlings before thinning. o Differences in stem length, as Mendel noted in his paper, can be easily scored in seedlings a few days after germination. o Variation for flower and seed-coat color was perfectly correlated with variation for axillary pigmentation in his experiments. Mendel could score F3 plants for the presence or absence of axillary pigmentation as early as two to three weeks after germination. • Wright (1966) and Beadle (1967) proposed that Mendel might have unconsciously misclassified individuals with questionable phenotypesto favor his expectations. o Mendel was careful about misclassification of phenotypes and had mentioned that "with a little practice in sorting, however, mistakes are easy to avoid". o Mendel identified not only the phenotypes, but also the genotypes of individual seeds through examination of their self-fertilized progeny. • Mendel might have stopped counting individuals when the numbers were close to the ratios he expected [Olby (1985) and Beadle (1967)]. o The average number of seeds per plant is close to 30 (16 590 seeds divided by 550 plants = 30.16 seeds per plant from the monohybrid, dihybrid, and trihybrid experiments for seed shape and cotyledon color). o Is 30 seeds per plant too low for 19th century pea cultivation? o Dr. J. Rasmussen, a pea geneticist wrote to Fisher: "About 30 good seeds per plant is, under Mendel's conditions." o Fairbanks and Rytting (2001) found that average number of seeds per plant was 47.18 among 24 garden-pea varieties grown at the New York Agricultural Experiment Station in Geneva, during 1888 (Curtis, 1889). Of the 24 varieties tested, four produced averages of less than 30 seeds per plant. • In Mendel'ssecond letter to Nägeli, he referred to his paper as "the unchanged reprint of the draft of the lecture mentioned; thus the brevity of the exposition, as is essential for a public lecture". • Fairbanks and Rytting (2001) concluded that, “although the bias in Mendel's experiments is evident, there are reasonable statistical and botanical explanations for the bias, and insufficient evidence to indicate that Mendel or anyone else falsified the data
Controversies Is Mendel s description ofhis experiments fictitious? Bateson(1913): "It is very unlikely that Mendel could have had seven pairs of varieties such that the members of each pair differed from each other in only one considerable character According to Fairbanks and Rytting there is little foundation for claims that Mendel's experiments were fictitious when viewed from a botanical perspective o Much of the confusion on this issue arises from the fact that pisum sativum is a domesticated species and among the many cultivated varieties there are several nt phenotype Three general categories of pea varieties Garden varieties(also called shelling varieties), field varieties, and sugar varieties Garden varieties display o white seed coats, white flowers and no axillary pigmentation, inflated pods, green pods, and axillary flowers they vary for seed shape, cotyledon color, and stem length Field varieties display o dominant phenotypes for all seven traits Sugar varieties display o dominant phenotypes for all traits except pod shape. Pod shape Mendel's monohybrid experiment for pod shape may have included a field variety and a sugar variety as parents Seed-coat color: There are several possibilities for Mendel's monohybrid experiment for seed-coat color. He could have hybridized a field variety with a garden variety that differed only for seed-coat color Flower position: The most readily available terminal-flowered variety in the 19 century was called the Mummy Pea and it is probably the variety that Mendel used Pod color: Garden and sugar varieties, called gold varieties, have yellow pods Mendel could have matched any one of the gold varieties with another ar variety in a monohybrid exp nt for vellow pod A mathematical minimum of eight varieties is required for seven monohybrid experiments Fairbanks and Rytting (2001)concluded that Mendel could have easily designed and conducted seven monohybrid experiments with 22 varieties at his disposal Evidence, however, is insufficient to advocate for either w
Controversies 3 Is Mendel's description of his experimentsfictitious? • Bateson (1913): “It is very unlikely that Mendel could have had seven pairs of varieties such that the members of each pair differed from each other in only one considerable character.” • According to Fairbanks and Rytting there is little foundation for claims that Mendel's experiments were fictitious when viewed from a botanical perspective. o Much of the confusion on this issue arises from the fact that Pisum sativum is a domesticated species and among the many cultivated varieties there are several different phenotypes. Three general categories of pea varieties: • Garden varieties (also called shelling varieties), field varieties, and sugar varieties. • Garden varieties display o white seed coats, white flowers and no axillary pigmentation, inflated pods, green pods, and axillary flowers; they vary for: seed shape, cotyledon color, and stem length • Field varieties display o dominant phenotypes for all seven traits. • Sugar varieties display o dominant phenotypesfor all traits except pod shape. • Pod Shape: Mendel's monohybrid experiment for pod shape may have included a field variety and a sugar variety as parents. • Seed-coat color: There are several possibilities for Mendel's monohybrid experiment for seed-coat color. He could have hybridized a field variety with a garden variety that differed only forseed-coat color. • Flower position: The most readily available terminal-flowered variety in the 19th century was called the Mummy Pea and it is probably the variety that Mendel used. • Pod color: o Garden and sugar varieties, called gold varieties, have yellow pods. Mendel could have matched any one of the gold varieties with another garden or sugar variety in a monohybrid experiment for yellow pod. A mathematical minimum of eight varieties is required for seven monohybrid experiments. Fairbanks and Rytting (2001) concluded that Mendel could have easily designed and conducted seven monohybrid experiments with 22 varieties at his disposal. Evidence, however, is insufficient to advocate for either way
Controversies Did Mendel articulate the laws of inheritance attributed to him? The two laws of inheritance most often attributed to Mendel are segregation and independen assortment 1. The law of segregation, stated in modern terms, is that during meiosis two alleles of a single locus, one inherited from each parent pair with each other, and then segregate from one another into the germ cells so that each germ cell carries only one allele of that locus. Segregation in heterozygous individuals produces in equal proportions two different types of gametes, each with one of the two alleles 2. The law of independent assortment, stated in modern terms, is that the segregation of alleles of a single locus has no influence on the segregation of alleles at another locus. The result is completely random and uniform combinations of alleles of different loci in the self-fertilized progeny of dihybrid (or multihybrid )individuals The law ofsegregation: Genetic terms, such as allele, locus, and chromosome, had not been coined in Mendel's day nor was the cellular process of meiosis understood Mendel referred to segregation of hereditary elements several times in his paper. He concluded: .the differing elements succeed in escaping from the enforced association only at the stage at which the reproductive cells develop. In the forma f these cells, all elements present participate in completely free and uniform fashion, and only those that differ separate from each other. In this manner the production of as many kinds of germinal and pollen cells would be possible as there are combinations of potentially formative elements This paragraph, however, is a remarkably lucid summary of the law of segregation o Mendel's reference to"potentially formative elements" implies the existence of invisible particulate determinants of inherited traits. We might well view the term"element, he equivalent of the modern term"allele o Mendel's reference to the "enforced association"of differing elements indicates that he perceived the differing elements as being paired in hybrids(heterozygotes) o His statement that differing elements "separate from each other"shows a clear understanding of segregation that is similar to the modern view o He also correctly recognized that segregation takes place "only at the stage at which the reproductive cells develop"(i.e, during meiosis) This paragraph, however, reveals one aspect of Mendel's perception that differs from the modern concept of segregation o Mendel perceived segregation as an anomaly restricted to hybrids(heterozygotes) o He called the differing elements"antagonistic elements"whose association in the hybrid is a"compromise, and wrote that only those elements that differ separate from one Mendel represented heterozygotes with a two-letter designation(Aa), as modern geneticists usually represent them, he consistently represented homozygotes with a single letter (A or a rather than the two letters(Aa or aa) used today
Controversies 4 Did Mendel articulate the laws of inheritance attributed to him? The two laws of inheritance most often attributed to Mendel are segregation and independent assortment. 1. The law of segregation,stated in modern terms, is that during meiosis two alleles of a single locus, one inherited from each parent, pair with each other, and then segregate from one another into the germ cells so that each germ cell carries only one allele of that locus. Segregation in heterozygous individuals produces in equal proportions two different types of gametes, each with one of the two alleles. 2. The law of independent assortment, stated in modern terms, is that the segregation of alleles of a single locus has no influence on the segregation of alleles at another locus. The result is completely random and uniform combinations of alleles of different loci in the self-fertilized progeny of dihybrid (or multihybrid) individuals. The law of segregation: Genetic terms, such as allele, locus, and chromosome, had not been coined in Mendel's day, nor was the cellular process of meiosis understood. Mendel referred to segregation of hereditary elements several times in his paper. He concluded: • “the differing elements succeed in escaping from the enforced association only at the stage at which the reproductive cells develop. In the formation of these cells, all elements present participate in completely free and uniform fashion, and only those that differ separate from each other. In this manner the production of as many kinds of germinal and pollen cells would be possible as there are combinations of potentially formative elements.” This paragraph, however, is a remarkably lucid summary of the law of segregation. o Mendel's reference to "potentially formative elements” implies the existence of invisible particulate determinants of inherited traits. We might well view the term "element,” as the equivalent of the modern term "allele." o Mendel's reference to the "enforced association" of differing elements indicates that he perceived the differing elements as being paired in hybrids (heterozygotes). o His statement that differing elements "separate from each other" shows a clear understanding of segregation that is similar to the modern view. o He also correctly recognized that segregation takes place "only at the stage at which the reproductive cells develop" (i.e., during meiosis). This paragraph, however, reveals one aspect of Mendel's perception that differs from the modern concept of segregation. o Mendel perceived segregation as an anomaly restricted to hybrids (heterozygotes). o He called the differing elements "antagonistic elements" whose association in the hybrid is a "compromise," and wrote that "only those elements that differ separate from one another." Mendel represented heterozygotes with a two-letter designation (Aa), as modern geneticists usually represent them, he consistently represented homozygotes with a single letter (A or a), rather than the two letters (AA or aa) used today
Controversies o For example, Mendel represented the genotypic ratio of the F2 generation of a monohybrid experiment as A+ 2Aa+ a, instead of AA 2Aa+aa Mendel may have concluded that like elements(alleles do not pair with one another and do not segregate in plants that are not hybrids (i. e, are not heterozygotes), and that therefore a single letter was an accurate way to represent such plants Mendel' s view of segregation occurring only in the heterozygotes (i.e, with different alleles) could easily be defended as being completely consistent even with the modern use of the term."-[Fairbanks and rytting(2001)] The law ofindependent assortment: The following statement immediately after Mendel's presentation of his di- and trihybrid experiments is the often-quoted statement of independent assortment The progeny of hybrids in which several essentially different traits are united represent the terms of a combination series in which the series for each pair of differing traits are combined. This also shows at the same time that the behavior of each pair of differing traits in a hybrid association is independent of all other differences in the two parental plants. He articulated the laws of inheritance attributed to him insofar as was possible given the information he had. "--[Fairbanks and Rytting(2001) Did mendel detect but not mention linkage? o Mendel concluded"the behavior of each pair of differing traits in a hybrid association o Dunn(1965), under the assumption that Mendel studied one gene on each of the seven chromosomes, calculated the probability of doing so as 6/7 X 5/7X 4/7X 3/7 X 2/7X 1/7 0.0061(<1%), again calling Mendel's experimental results into question Mendelstudiedhttp://www.ic.bbsrcac.uk/germplas/pisum/zgs4fhtm o two genes on chromosome l(a and i o no genes on chromosomes 2 and 3, either two genes (fa and le or three genes (fa, le, and v)on chromosome 4 o one gene(gp)on chromosome 5, o possibly one gene(p)on chromosome 6 o and one gene(r)on chromosome 7 o The gene in doubt is the one that governs pod shape. Recessive alleles of the v gene on chromosome 4 and the p gene on chromosome 6 both confer constricted(unparchmented) pods when homozygous, and it is not known which of the two Mendel studied Genes on the same chromosome are said to be linked only if they are so close to one another that the frequency of crossovers between them is significantly less than the frequency of recombination for independent assortment In most testcross experiments(the most reliable type of linkage experiment), linkage often cannot be distinguished from independent assortment for genes located more than m 60 cM The two genes that Mendel studied on chromosome 1, i, which governs seed color, and a, which governs flower color, are 204 cM apart, so distant that they assort independently
Controversies 5 o For example, Mendel represented the genotypic ratio of the F2 generation of a monohybrid experiment as A + 2Aa + a, instead of AA + 2Aa + aa. Mendel may have concluded that like elements (alleles) do not pair with one another and do not segregate in plants that are not hybrids (i.e., are not heterozygotes), and that therefore a single letter was an accurate way to represent such plants. "Mendel's view of segregation occurring only in the heterozygotes (i.e., with different alleles) could easily be defended as being completely consistent even with the modern use of the term." – [Fairbanks and Rytting (2001)]. The law of independent assortment: The following statement immediately after Mendel's presentation of his di- and trihybrid experiments is the often-quoted statement of independent assortment: “The progeny of hybrids in which several essentially different traits are united represent the terms of a combination series in which the series for each pair of differing traits are combined. This also shows at the same time that the behavior of each pair of differing traits in a hybrid association is independent of all other differencesin the two parental plants.” “He articulated the laws of inheritance attributed to him insofar as was possible given the information he had.” -- [Fairbanks and Rytting (2001)]. Did Mendel detect but not mention linkage? o Mendel concluded "the behavior of each pair of differing traits in a hybrid association is independent of all other differences in the two parental plants". o Dunn (1965), under the assumption that Mendel studied one gene on each of the seven chromosomes, calculated the probability of doing so as 6/7 x 5/7 x 4/7 x 3/7 x 2/7 x 1/7 = 0.0061 (<1%), again calling Mendel's experimentalresults into question. Mendel studied: http://www.jic.bbsrc.ac.uk/germplas/pisum/zgs4f.htm o two genes on chromosome 1 (a and i), o no genes on chromosomes 2 and 3, either two genes (fa and le) or three genes(fa, le, and v) on chromosome 4, o one gene (gp) on chromosome 5, o possibly one gene (p) on chromosome 6, o and one gene (r) on chromosome 7. o The gene in doubt is the one that governs pod shape. Recessive alleles of the v gene on chromosome 4 and the p gene on chromosome 6 both confer constricted (unparchmented) pods when homozygous, and it is not known which of the two Mendelstudied. Genes on the same chromosome are said to be linked only if they are so close to one another that the frequency of crossovers between them is significantly less than the frequency of recombination for independent assortment. In most testcross experiments (the most reliable type of linkage experiment), linkage often cannot be distinguished from independent assortment for genes located more than 60 cM. • The two genes that Mendel studied on chromosome 1, i, which governs seed color, and a, which governs flower color, are 204 cM apart, so distant that they assort independently