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.
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!
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.
Mendel cross-pollinated true-breeding tall plants with
true-breeding 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.
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.
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
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
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.
The Principle of Segregation
When gametes form, the alleles MUST
separate from each other.
Each gamete carries only a single allele
for each gene.
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
11
12
13
14
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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