Gregor Mendel And The Genetic Revolution

©1999 Timothy G. Standish

Gregor MendelGregor MendelAnd The Genetic RevolutionAnd The Genetic Revolution

Timothy G. Standish, Ph. D.


Introduction- Gregor MendelIntroduction- Gregor Mendel

Father of classical genetics. Born Johan Mendel in 1822 to peasant

family in the Czech village of Heinzendorf part of the Austro-Hungarian empire at the time.

Austrian Augustinian monk (Actually from Brunn which is now in the Czech Republic).


Gregor Mendel - WorkGregor Mendel - Work Starting in 1856 Mendel studied peas which he grew in a

garden out side the Abbey he lived in. Showed that the traits he studied behaved in a precise

mathematical way and disproved the theory of "blended inheritance.”

Mendel’s work was rediscovered in 1900 by three botanists:– Carl Correns (Germany)

– Erich von Tschermak (Austria)

– Hugo de Vries (Holland)


Chromosomes:Chromosomes:The Physical Basis of InheritanceThe Physical Basis of Inheritance 1866 Mendel published his work 1875 Mitosis was first described 1890s Meiosis was described 1900 Mendel's work was rediscovered 1902 Walter Sutton, Theodore Boveri and

others noted parallels between behavior of chromosomes and alleles.


Why Peas?Why Peas? Mendel used peas to study inheritance because: True breeding commercial strains were availible Peas are easy to grow Peas have many easy to observe traits including:

– Seed color - Green or yellow– Seed shape - Round or wrinkled– Pod color - Green or yellow– Pod shape - Smooth or constricted– Flower color - White or purple– Flower position - Axial or terminal– Plant size - Tall or dwarf


Why Peas?Why Peas? Pea flowers are constructed in such a way

that they typically self fertilize Because of this, it is relatively easy to

control crosses in peas

Pea flower


Why Peas?Why Peas? Pea flowers are constructed in such a way

that they typically self fertilize Because of this, it is relatively easy to

control crosses in peas

StigmaPea flower

Anthers


Why Peas?Why Peas? By removing the anthers of one flower and

artificially pollinating using a brush, crosses can be easily controlled in peas.







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Mendel’s ResultsMendel’s Results

When crossing purple flowered peas with white flowered peas, Mendel got the following results:

In the first filial (F1) generation all offspring produced purple flowers

In the second generation (second filial or F2):

– 705 purple– 224 white

Approximately a 3:1 ratio of purple to white


Interpreting Mendel’s ResultsInterpreting Mendel’s Results Because the F1 generation did not produce light purple

flowers and because white flowers showed up in the F2 generation, Mendel disproved blended inheritance.

Mendel said that the parents had two sets of genes thus two copies of the flower color gene

Each gene has two varieties called alleles In the case of the flower color gene the two alleles are

white and purple


Interpreting Mendel’s ResultsInterpreting Mendel’s Results

CC

Cc

Cc

cc

In the F1 generation, the white allele was hidden by the purple “dominant” allele

In the F2 generation, 1/4 of the offspring wound up with two copies of the white allele thus they were white

C c

C

c

F2 GenerationF2 Generation

Cc

Cc

Cc

Cc

C C

c

c

F1 GenerationF1 GenerationGametes from the P generation

Heterozygous parents make gametes either one or the other allele

The F1 Generation is all heterozygous

Homozygous parents can only make gametes with one type of allele


Mendel’s ResultsMendel’s ResultsTraitSeeds

round/wrinkledyellow/greenfull/constricted

Podsgreen/yellowaxial/terminal

Flowersviolet/white

StemTall/dwarf

TraitSeeds

round/wrinkledyellow/greenfull/constricted

Podsgreen/yellowaxial/terminal

Flowersviolet/white

StemTall/dwarf

F1 Results All RoundAll YellowAll Full

All GreenAll Axial

All Violet

All Tall

F1 Results All RoundAll YellowAll Full

All GreenAll Axial

All Violet

All Tall

F2 Results 5,474 Round 1,850 wrinkled6,022 Yellow 2,001 green 882 Full 299 constricted

428 Green 152 yellow651 Axial 207 terminal

705 Violet 224 white

787 Tall 277 dwarf

F2 Results 5,474 Round 1,850 wrinkled6,022 Yellow 2,001 green 882 Full 299 constricted

428 Green 152 yellow651 Axial 207 terminal

705 Violet 224 white

787 Tall 277 dwarf

Dominent traits mask recessive traits

Masked recessive traits reappear


Mendel’s ResultsMendel’s ResultsF2 Results Seeds5,474 Round 1,850 wrinkled6,022 Yellow 2,001 green 882 Full 299 constricted

Pods428 Green 152 yellow651 Axial 207 terminal

Flowers705 Violet 224 white

Stem787 Tall 277 dwarf

F2 Results Seeds5,474 Round 1,850 wrinkled6,022 Yellow 2,001 green 882 Full 299 constricted

Pods428 Green 152 yellow651 Axial 207 terminal

Flowers705 Violet 224 white

Stem787 Tall 277 dwarf

F2 Ratios Seeds2.96:1 Round:wrinkled3.01:1 Yellow:green2.95:1 Full:constricted

Pods2.82:1 Green:yellow3.14:1 Axial:terminal

Flowers3.15:1 Violet:white

Stem2.84:1 Tall:dwarf

F2 Ratios Seeds2.96:1 Round:wrinkled3.01:1 Yellow:green2.95:1 Full:constricted

Pods2.82:1 Green:yellow3.14:1 Axial:terminal

Flowers3.15:1 Violet:white

Stem2.84:1 Tall:dwarf

Ratios are not exactly 3:1

How do we decide if the ratios are close enough to 3:1 to support and not reject our theory?


Independent AssortmentIndependent Assortment When Mendel crossed peas and looked at two different traits, he discovered that the traits assorted independently In other words, if he was looking at the height of the plants and the color of the flowers, all four possible

combinations of height and flower color were produced: Tall Purple Tall white dwarf Purple dwarf white


Tc

tC

tc

TC

t ct CT cT C

Independent AssortmentIndependent Assortment

TtCcTtCCTTCcTTCC

TtccTtCcTTccTTCc

ttCcttCCTtCcTtCC

ttccttCcTtccTtCc

As long as genes are on different chromosomes, they will assort independently


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Gregor Mendel And The Genetic Revolution