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Mendel’s Pea Plant
Experiments
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Gregor Johann Mendel
Austrian monk,
mathematician by trade
Studied the inheritance of
traits in pea plants
Developed the laws of
inheritance
Mendel's work was not
recognized until the turn of
the 20th century
Between 1856 and 1863, Mendel
cultivated and tested some
28,000 pea plants
He found that the plants'
offspring retained traits of the
parents
Called the “Father of Genetics"
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Genetic Terminology
Trait - any characteristic
that can be passed from
parent to offspring by
genes(DNA) and their
alleles(specific code
sequences)
Heredity - passing of
traits from parent to
offspring
Genetics - study of
heredity
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Mendel’s Experimental Methods
•
•Mendel hand-pollinated
flowers using a paintbrush
•He snipped the stamens to
prevent self-pollination
•Covered each flower with
a cloth bag
•He traced 8 traits through the
several generations
•Ultimately he produced
pure strains for each
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Eight Pea Plant Traits
GENE
ALLELES
GENE
ALLELES
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Mendel’s Flowers
Mendel’s questions:
• If traits are known, can the inheritance of the traits in
offspring be predicted?
• What is the ‘mechanism’ for inheritance?
With his pure strains Mendel began systematically
crossing plants, observing one trait at a time:
Monohybrid Crosses = 1 gene and its 2 alleles
He let the plants self-pollinate during these experiments
He observed the offspring produced for each trait, in
each generation, for any patterns that appeared.
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PURE STRAINS
First filial generation
of offspring
WHY no white?
What happened to the
white trait?
Second filial generation
White trait reappeared
in this generation,
so there must be some
‘factor’ in the purple
flower that masked it
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Mendel’s Conclusions
 Dominant – trait that was expressed if present; represented by a
capital letter (R)
 Recessive – trait that is expresses only if the dominant is not present;
represented by a lowercase letter (r)
 Factors: sequence of DNA that codes for a trait, today we know those
are the GENES
 Alleles –variant(protein code) of a gene (dominant and/or recessive)
Ex: Trait(Gene)=Flower Color
Alleles=Purple/White
Gene Flower Color
Maternal
Chromosome
Paternal
Chromosome
Regardless of the trait, results were the same and could be predicted in
future generations…Mendel developed his first law of genetics;
The Law of Dominance
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Law of Dominance
In a cross of parents(P) that are
pure for contrasting traits, only one
form of the trait will appear in the
next generation(F1).
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr
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Let’s see how it works,
by adding the allele notations…
RR
rr
Rr
Rr x Rr
RR
Rr
Rr
rr
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GENOTYPEs
Possible allele combinations for a trait
Homozygous – allele combination involving
two of the same alleles; can be either
 Homozygous dominant: RR
 Homozygous recessive: rr
Heterozygous – allele combination of one
dominant & one recessive allele
 Heterozygous: Rr
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PHENOTYPE
 the physical features resulting from the
genotype; am I…
PURPLE
or, am I
WHITE
Phenotype also refers to your chemical features as well;
enzymes and functional proteins, like hemoglobin, are also
determined by genes and allele combinations
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Genotype & Phenotype in Seed Color
Genotype of alleles:
R = yellow seed
r = green seed
In the F2
The possible combinations are:
Genotypes
RR
Phenotypes YELLOW
Rr
rr
YELLOW
GREEN
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Punnett Squares
• Diagram that is used to predict an outcome of a
particular cross or breeding experiment
• Illustrate Mendel’s Second Law of Genetics:
Law of Segregation
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Homozygous
Dominant Parents
R
R
2
Homozygous
Recessive Parents
r
r
3
HDP X HRP
P1
R
R
R
RR
RR
r
rr
rr
r
Rr
Rr
R
RR
RR
r
rr
rr
r
Rr
Rr
G: 4:4
4
R
r
G: 4:4
Ph: 4:4
Heterozygous
Parents F2
R
r
RR
Rr
Rr
rr
G: 1:2:1
Ph: 3:1
5
R
r
Ph: 4:4
G: 4:4
HDP X Hetero
R
R
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Ph: 4:4
HRP X Hetero
r
r
RR
RR
R
Rr
Rr
Rr
Rr
r
rr
rr
G: 2:4
Ph: 4:4
LOUNGE CHAIR
G: 2:4
Ph: 2:4
1
Homozygous
Dominant Parents
R
R
2
Homozygous
Recessive Parents
r
r
3
HDP X HRP
P1
R
R
R
RR
RR
r
rr
rr
r
Rr
Rr
R
RR
RR
r
rr
rr
r
Rr
Rr
G: 4:4
4
R
r
G: 4:4
Ph: 4:4
Heterozygous
Parents F2
R
r
RR
Rr
Rr
rr
G: 1:2:1
Ph: 3:1
5
R
r
Ph: 4:4
G: 4:4
HDP X Hetero
R
R
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Ph: 4:4
HRP X Hetero
r
r
RR
RR
R
Rr
Rr
Rr
Rr
r
rr
rr
G: 2:4
Ph: 4:4
G: 2:4
Ph: 2:4
Predictable Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT,Tt,Tt,tt
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