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 acceptedControversies 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