Chapter 3 Section 1
Mendel’s Work
Gregor Mendel



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

Traits

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Heredity

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Physical characteristics of organisims
Passing of traits from parent to offspring
Genetics

Scientific study of heredity
Mendel’s Peas

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?
Tall Plant
Short Plant
Mendel’s Experiments

Purebred Plant


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 pink plants will only produce pink plants
X
White flower would mean the parents are not purebred
X
Mendel’s Experiments

Mendel crossed plants with opposite forms of
a trait
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
i.e. short plants crossed with tall plants
Look at Figure 2 on page 82
Fig 2 page 82
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What did Mendel get
when he crossed
purebred tall plants with
pure bred short plants?
 All Tall plants
What did Mendel get
when he let the F1
plants self pollinate?
 75% tall and 25%
short.
Mendel’s Experiments

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Mendel studied stem height followed by six
other “opposite” traits.
Look at Figure 3 on page 83.
What other “opposite” traits did Mendel
study?
Genes and Alleles

Genes


Factors that control traits
Alleles



Different forms of a gene
One allele is inherited from each parent.
Alleles can be dominant or recessive
Dominant and Recessive Alleles

Dominant allele

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Always seen if present
Indicated by a capitol letter (T)
Recessive allele


Masked or hidden if dominant allele is present
Indicated by a lower case letter (t)
Hybrid vs Purebred

Hybrid

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Organism has two different alleles for the trait – one
dominant and one recessive.
Purebred

Organism has two identical alleles for a trait – two
recessive OR two dominant.
Alleles are represented with capitol and
lower case letters.

T = Tall allele (dominant)
t = Short allele (recessive)

Purebreds



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TT – two dominant alleles – plants will be tall
tt – two recessive alleles – plants will be short
Hybrid

Tt – one dominat allele and one recessive allele –
plants will be tall.
Phenotypes and Genotypes

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

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
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? ________________
Probability and Genetics
Chapter 3 - Section 2
Probability

Probability

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The likelihood that a particular event
will occur
Example: coin toss
Punnett Squares

Punnett Squares

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
Mendel’s Work Review Worksheet

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What trait in pea plants is being studied in
the cross above?
 Plant height (short/tall)
What are the two alleles for this trait?
 Tall (T)
 Short (t)
Which allele is the dominant allele? Explain
how you know.
 Dominate allele is Tall (T).
 Know because all offspring in F1 are tall.
Mendel’s Work Review Worksheet


Which allele is the recessive allele? Explain
 Recessive is short (t)
 Know because this trait is hidden in the F1
What alleles do the F1 offspring have? Explain
which allele was inherited from which parent.
 All F1 offspring are hybrids with the alleles
(Tt)
 (T) was inherited from the tall parent and
(t) was inherited from the short parent.
Codominance

Codominance – alleles are neither
dominant or recessive therefore both
alleles are expressed in the offspring
Codominance
The heterozygous offspring will express (show) both phenotypes