Transcript Genetics: The Work of Gregor Mendel

Mendel’s Principles
• Who was Gregor Mendel?
• He was an Austrian monk born in
1822.
• He is known as the Father of
Genetics for his experiments and
observations of the inherited traits
of pea plants.
• He cross-pollinated pea plants
which were true-breeding
(producing offspring identical to
the parents) for several traits.
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What are Traits?
• A trait is a specific characteristic that varies from
one individual to the next.
• Mendel chose 7 pea plant traits to study:
– seed shape, seed color, flower color, pod shape, pod
color, flower position, and plant height.
• HOWEVER, he studied only ONE TRAIT at a
time! This is what helped make his results so
conclusive!
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Mendel’s Experiment!
• He wanted to fertilize seeds by joining male
& female gametes from 2 different plants
with different traits (ex: Tall & short plants).
• To prevent self-pollination, he cut away the
pollen-bearing male parts from one plant.
• Then, dusted pollen from another plant onto
the egg-bearing female part of the first plant.
• This process is referred to as crosspollination.
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• Mendel cross-pollinated true-breeding tall plants with truebreeding short plants.
• He called these original pair the “parent “ plants or P
generation.
• Their offspring, or the F1 generation were all tall.
• The offspring of crosses between parents with different traits
are called hybrids.
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• Mendel then let the F1 generation self-pollinate to create the second
generation (F2 generation).
• Some of the F2 offspring were tall and some were short.
• The ratio consistently added up to 3 tall: 1 short.
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Mendel’s Results
Based on Mendel’s work, we now know that:
• A gene is a segment of DNA that
determines a trait.
– Ex: eye color
• Alleles are different forms of a gene.
– Ex: brown, blue, green, or hazel eyes
Mendel’s results led him to three basic
principles…
Mendel’s
Discoveries
& Results
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The Principle of Dominance:
• In general, one allele is dominant to another.
• The recessive allele will be expressed only if the
dominant allele is not present.
– Expressed as the lower case letter of the dominant trait.
– Ex: Short is a recessive trait: tt
• The dominant allele is always expressed even if
combined with the recessive allele. It masks the
recessive one.
– Expressed with a CAPITAL letter.
– Ex: Tall is a dominant trait: TT or Tt
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The Principle of Dominance
continued…
• The combination of alleles is called the
genotype.
– If both alleles are the same, the individual is
homozygous.
– If the alleles are different, the individual is
heterozygous.
• The physical appearance, or observable
trait is the phenotype.
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The Principle of Segregation
• When gametes form, the alleles MUST
separate from each other.
• Each gamete carries only a single allele
for each gene.
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Parents (P)
1
2
TT
tt
3
gametes
F1
4
T
T
Tt
gametes
t
t
Tt
7
8
T
t
Tt
t
T
10
TT
Tt
Tt
tt
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12
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Circle #
Genotype
Hetero/Homozygous
1
TT
homozygous
3
T
7
Tt
tt
14
6
Tt
9
F2
5
Phenotype
tall
Generation
Gamete?
(y / n)
no
n/a
P
n/a
yes
heterozygous
tall
F1
no
homozygous
short
F2
no
n/a
Principle of Independent
Assortment
• When examining 2 different traits at the same time
(height & seed color), Mendel noticed that all possible
combinations were expressed in the F2 generation.
– Ex: tall & green, tall & yellow, short & green, short & yellow
• Different traits are inherited independently of each
other, so that there is no relation, for example,
between a plant’s height and its seed color.
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Animation