Gregor Mendel The Man and The Myth

The Myth Detailed Studies of Garden Pea Austrian Monk, Father of Genetics Detailed Studies of Garden Pea Particulate Nature of Inheritance Law of Segregation Law of Independent Assortment Ahead of his Time

Who was Gregor Mendel? 1822 Johann Mendel born at Hyncie 1831-40 Elementary & Secondary Sch 1840-43 Institute of Philosophy 1844-48 Novitiate & Theology study at St Thomas Monastery, Brno (Gregor) 1851-53 Studied Physics & Natural History at U. of Vienna 1868 Elected Abbot of Monastery 1884 Died January 6

Experiments on Plant Hybrids (1865) By Gregor Mendel Mendel published this 47 page description of his eight year study of hybridization in Pisum in the Proceedings of the Bruun Natural History Society.

1. Introductory Remarks Experiences with artificial fertilization Literature review- no general laws, thus the need for detailed studies Purpose: Determine the different forms of hybrid offspring in each generation and ascertain their statistical relations. This study may help answer questions on the “history of the evolution of organic forms”.

2. Selection of the Experimental Plants Constant, differentiating characters Fertile hybrids Pisum flower structure Ease of artificial fertilization 22 pure strains (varieties/species)

3. Division and Arrangement of Expeiments “The object of the experiment was to observe these variations (in the progeny of hybrids) in the case of each pair of differentiating characters, and to deduce the law according to which they appear in successive generations.”

3. Division and Arrangement of Expeiments 1 round vs wrinkled seeds 2 yellow vs green seeds 3 grey vs white seed coat 4 inflated vs constricted seed pods 5 green vs yellow seed pods 6 axial vs terminal flowers 7 long vs short stems

4. The Forms of the Hybrids (F1) Reciprocal crosses gave equivalent results. Yellow(pollen) X Green(seed bearer) Green(pollen) X Yellow(seed bearer) “Dominating” character expressed in the hybrids, recessive form absent 8th character, early and late flowering time, exhibited an intermediate hybrid.

5. The First Generation from the Hybrids (F2) 1 round vs wrinkled seeds 7324 (2.96:1) 2 yellow vs green seeds 8023 (3.01:1) 3 grey vs white seed coat 929 (3.15:1) 4 inflated vs constrict pods 1181 (2.95:1) 5 green vs yellow seed pods 580 (2.82:1) 6 axial vs terminal flowers 858 (3.14:1) 7 long vs short stems 1064 (2.84:1) Average ratio (2.982:1) (14,949/5010 = 2.984)

6. The Second Generaion from Hybrids (F3) Summary F2 recessives breed true F2 dominants are of two forms: 2/3 yield offspring in 3:1 ratio 1/3 yield only the dominant trait Seed color 372 vs 193 (1.93:1) Seed shape 353 vs 163 (2.13:1) “The average ratio of 2:1 appears, therefore, as fixed with certainty.”

7. The Subsequent Generaions from Hybrids (F4 – F7) “…the terms in the series for the progeny of the hybrids of two differentiating characters.” : A + 2Aa + a (1:2:1) A X a  Aa (X)  1A + 2Aa +1a

7. continued “… that hybrids are inclined to revert to parental forms, is also confirmed by the experiments described.” Gen A Aa a 1 1 2 1 2 6 4 6 3 28 8 28 10 1023 2 1023

8. The Offspring of Hybrids in which several Differentiating Characters are Associated Experiment 1. P round, yellow X wrinkled, green F1 round, yellow (X) F2 315 round, yellow 101 winkled, yellow 108 round, green 32 wrinkled, green 556

Experiment 1. continued Plants from 556 (F2) seeds: round, yellow (9) wrinkled, yellow (3) 38 AB 28 aB 65 ABb 68 aBb 60 AaB 138 AaBb round green (3) wrinkled, green (1) 35 Ab 30 ab 67 Aab

Experiment 1. continued Mendel’s “combination series” (A + 2Aa + a)(B + 2Bb + b) AB+Ab+aB+ab+2ABb+2aBb +2AaB+2Aab+4AaBb “…the relation of each pair of different characters in hybrid union is independent of other differences in the two original parental stocks.”

9. The Reproductive Cells of the Hybrids Back Crosses: Seed parent X Pollen parent AaBb X AB  1:1:1:1 AaBb X ab  1:1:1:1 AB X AaBb  1:1:1:1 ab X AaBb  1:1:1:1

9. continued “Experimentally, therefore, it is confirmed that the pea hybrids form egg and pollen cells which, in their constitution, represent in equal numbers all constant forms which result from the combination of the characters united in fertilization.” AaBb  pollen & eggs ¼ AB, ¼ Ab, ¼ aB, ¼ ab

9. continued Pollen cells A + A + a + a Egg cells A + A + a + a Progeny A + A + a + a A a A a “This represents the average result of self fertilization of the hybrids …” A + 2Aa + a

Section 9. coninued “The law of combinations of different characters”: “In the process of fertilization each pollen form unites on an average equally often with each egg cell form”…to produce the following combinations: AB + AB +…+ ab + ab  16 terms AB Ab aB ab (Remember the 4X4 Punnett Square?)

10. Experiments with Hybrids of other Species of Plants Expt 1 Phaseolus vulgaris X P. nanus For three pairs of contrasting characters, “the ratios … were the same as with Pisum.” No data were reported. Expt 2 P. nanus, L. X P. multiflorus 3:1 ratios for two characters, but hybrid of white X purple produced 1 white and 30 variously colored plants. Mendel proposed a model involving two or three independent color traits to explain the flower color data.1/16, 1/64 F2 white

11. Concluding Remarks Pisum model vs Kolreuter, Gartner and Wichura hybridization results a. Hybrids resemble one or the other parent or are intermediate. b. Offspring of hybrids are variable or are constant. “In the meantime we may presume that in material points an essential difference can scarcely occur, since the unity in the developmental plan of organic life is beyond question.”

Section 11. continued c. Transformation of one species into another by backcrossing Species transformation model based on Pisum results: Expt I ABCDE X abcde AaBbCcDdEe X abcde … Expt 2 ABCDE X abcde AaBbCcDdEe X ABCDE …

Section 11. continued By selecting the desired backcross offspring, Mendel was able to transform “species” A to B in 2-4 generations. Gartner concluded species have fixed limits, and he was in opposition to those who assume continuous evolution of plant forms. Although Mendel indicated that “this opinion cannot be unconditionally accepted”, he agreed that Gartner’s statement held for cultivated plants.

Summary of Mendel’s Findings Segregation A X a  Aa  1A+2Aa+1a Independent assortment (1A+2Aa+1a)(1B+2Bb+b)… Biparental patterns of inheritance Elements or characters in pollen/eggs Mendel was a hybridist

The key point of Mendel’s discovery as per Hartl and Orel 1992 “…pea hybrids form germinal and pollen cells that in their composition correspond in equal numbers to all of the constant forms resulting from the combination of traits united through fertilization.” Mendel p. 29

“The Rediscovery 1900” Hugo de Vries U of Amsterdam Carl Correns U of Tubingen Erich von Tschermak William Bateson- the myth maker

Additional Hybridizations 10 Letters to Carl von Nageli 1866-1873 Frustrations with hybridizations with Hieracium species