Gregor Mendel

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Gregor Mendel

• The Man and The Myth

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The Myth

• 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

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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

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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.

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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.

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2. Selection of the Experimental Plants

• Constant, differentiating characters
• Fertile hybrids
• Pisum flower structure
• Ease of artificial fertilization
• 22 pure strains (varieties/species)

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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.

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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

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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.

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5. The First Generation from the Hybrids (F2)

• 1 round vs wrinkled seeds 7324 (2.961)
• 2 yellow vs green seeds 8023 (3.011)
• 3 grey vs white seed coat 929 (3.151)
• 4 inflated vs constrict pods 1181 (2.951)
• 5 green vs yellow seed pods 580 (2.821)
• 6 axial vs terminal flowers 858 (3.141)
• 7 long vs short stems 1064 (2.841)
• Average ratio (2.9821)
• (14,949/5010 2.984)

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6. The Second Generaion from Hybrids (F3)

• Summary
• F2 recessives breed true
• F2 dominants are of two forms
• 2/3 yield offspring in 31 ratio
• 1/3 yield only the dominant trait
• Seed color 372 vs 193 (1.931)
• Seed shape 353 vs 163 (2.131)
• The average ratio of 21 appears, therefore, as
  fixed with certainty.

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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 (121)
• A X a ? Aa (X) ? 1A 2Aa 1a

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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

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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

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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

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Experiment 1. continued

• Mendels combination series
• (A 2Aa a)(B 2Bb b)?
• ABAbaBab2ABb2aBb
• 2AaB2Aab4AaBb
• the relation of each pair of different
  characters in hybrid union is independent of
  other differences in the two original parental
  stocks.

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9. The Reproductive Cells of the Hybrids

• Back Crosses
• Seed parent X Pollen parent
• AaBb X AB ? 1111
• AaBb X ab ? 1111
• AB X AaBb ? 1111
• ab X AaBb ? 1111

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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

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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

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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 formto produce the following combinations
• AB AB ab ab ? 16 terms
• AB Ab aB ab
• (Remember the 4X4 Punnett Square?)

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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
• 31 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

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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.

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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?

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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
  Gartners statement held for cultivated plants.

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Summary of Mendels Findings

• Segregation
• A X a ? Aa ? 1A2Aa1a
• Independent assortment
• (1A2Aa1a)(1B2Bbb)?
• Biparental patterns of inheritance
• Elements or characters in pollen/eggs
• Mendel was a hybridist

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The key point of Mendels 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

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The Rediscovery 1900

• Hugo de Vries U of Amsterdam
• Carl Correns U of Tubingen
• Erich von Tschermak
• William Bateson- the myth maker

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Additional Hybridizations

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

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