Gregor Mendel

The Man and The Myth

The Myth

Austrian Monk, Father of GeneticsDetailed Studies of Garden PeaParticulate Nature of InheritanceLaw of SegregationLaw of Independent AssortmentAhead of his Time

Who was Gregor Mendel?

1822 Johann Mendel born at Hyncie1831-40 Elementary & Secondary Sch1840-43 Institute of Philosophy1844-48 Novitiate & Theology study at St

Thomas Monastery, Brno (Gregor)1851-53 Studied Physics & Natural History

at U. of Vienna1868 Elected Abbot of Monastery1884 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 fertilizationLiterature review- no general laws, thus

the need for detailed studiesPurpose: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 charactersFertile hybridsPisum flower structureEase of artificial fertilization22 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

Expeiments1 round vs wrinkled seeds2 yellow vs green seeds3 grey vs white seed coat4 inflated vs constricted seed pods5 green vs yellow seed pods6 axial vs terminal flowers7 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)

SummaryF2 recessives breed true

F2 dominants are of two forms: 2/3 yield offspring in 3:1 ratio 1/3 yield only the dominant traitSeed 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 a1 1 2 1 2 6 4 6 3 28 8 28

10 10232 1023

8. The Offspring of Hybrids in which several Differentiating

Characters are Associated

Experiment 1.P round, yellow X wrinkled, greenF1 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 aB65 ABb 68 aBb60 AaB138 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 + aEgg cells A + A + a + aProgeny 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 termsAB Ab aB ab(Remember the 4X4 Punnett Square?)

10. Experiments with Hybrids of other Species of Plants

Expt 1 Phaseolus vulgaris X P. nanusFor three pairs of contrasting characters, “the

ratios … were the same as with Pisum.” No data were reported.

Expt 2 P. nanus, L. X P. multiflorus3:1 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+1aIndependent assortment (1A+2Aa+1a)(1B+2Bb+b)…Biparental patterns of inheritanceElements or characters in pollen/eggsMendel 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 AmsterdamCarl Correns U of TubingenErich von Tschermak

William Bateson- the myth maker

Additional Hybridizations

10 Letters to Carl von Nageli 1866-1873

Frustrations with hybridizations with Hieracium species