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Chapter 5 Lesson 1
Mendel and His Peas
Gregor Mendel
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Young priest that worked in the
garden at a monastery in
Vienna.
Considered the “Father of
Genetics”
Observed traits in hundreds of
pea plants.
Mendel’s Work - Vocabulary
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Traits
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Heredity
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Physical characteristics of organisims
Passing of traits from parent to offspring
Genetics
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Scientific study of heredity
Mendel’s Peas
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Why was Gregor Mendel lucky he chose
peas to study?
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Many traits exist only in two forms
Peas produce many offspring in one
generation.
How did Mendel make his
crosses?
White Plant
Purple Plant
Mendel’s Experiments
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True Breeding (Purebred) Plant
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One that always produces offspring with the
same form of a trait as the parent
i.e. purebred short plants will only produce
short offspring.
Purebred purple plants will only produce purple plants
X
White flower would mean the parents are not purebred
X
Mendel’s Experiments
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Mendel crossed plants with opposite forms of
a trait
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i.e. purple flowers crossed with white flowers
Look at Figure 3 on page 152
Explain why all offspring are purple in the
first generation (F1)
What happens in the second generation (F2)
Mendel’s Experiments
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Mendel studied flower color followed by six
other “opposite” traits.
Look at table 1 on page 154.
What do you notice about the ratio of traits
in the second generation (F2)
Let’s Look at another example
Genes and Alleles
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Genes
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Factors that control traits
Alleles
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Different forms of a gene
One allele is inherited from each parent.
Alleles can be dominant or recessive
Dominant and Recessive Alleles
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Dominant allele
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Always seen if present
Indicated by a capitol letter (T)
Recessive allele
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Masked or hidden if dominant allele is present
Indicated by a lower case letter (t)
Hybrid vs Purebred
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Hybrid
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Organism has two different alleles for the trait – one
dominant and one recessive.
Purebred
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Organism has two identical alleles for a trait – two
recessive OR two dominant.
Alleles are represented with capitol and
lower case letters.
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T = Tall allele (dominant)
t = Short allele (recessive)
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Purebreds
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TT – two dominant alleles – plants will be tall
tt – two recessive alleles – plants will be short
Hybrid
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Tt – one dominat allele and one recessive allele –
plants will be tall.
Phenotypes and Genotypes
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Phenotype
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Genotype
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Physical appearance or visible trait
Example is flower color, stem height etc.
The genetic makeup or allele combination
Examples are TT or Tt
Genotypes determine phenotypes
Homozygous and Heterozygous
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Homozygous
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An organism that has two identical alleles for a trait
TT – homozygous dominant
tt – homozygous recessive
Heterozygous
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An organism that has two different alleles for a trait
Tt – heterozygous – will show the dominant trait
Probability
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Probability
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The likelihood that a particular event
will occur
Example: coin toss
Punnett Squares
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Punnett Squares
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chart that shows all possible
combinations of alleles that can
result from a genetic cross. Used
to determine the probability of a
particular outcome.
Using a punnett square
Tt
(Tall)
t
T
T
Tt
(Tall)
Tall
Tall
TT
Tt
Tall
Short
t
Tt
tt
Codominance
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Codominance – alleles are neither
dominant or recessive therefore both
alleles are expressed in the offspring
Codominance
The heterozygous offspring will express (show) both phenotypes
Consider a rabbit with black fur and the allele combination (Bb).
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What is the rabbit’s phenotype for fur color?
Black
_________
What is the rabbit’s genotype for fur color?
(Bb)
__________
Is the rabbit heterozygous or homozygous for the
heterozygous
fur color trait? _______________
If black is dominant over white, what genotype
(bb)
would produce a white rabbit? _______
If fur color in rabbits was a codominant trait
instead of dominant/recessive trait, what color fur
White and black
would the (Bb) rabbit have? ________________