Mendel and the Laws of
Inheritance
Biology
Dobson High School
Hope Finzer
Who was Gregor Mendel?
Austrian monk and
scientist
His work marks the start
of modern genetics
Used pea plants to study
the passing of traits from
generation to generation
First person to succeed in
predicting how traits are
transferred from one
generation to the next
Why did Mendel choose
pea plants?
Pea plants have both male and female gametes
in each plant
They reproduce sexually by the transfer of
pollen grains from the male gamete to the
female ovule
The plants reproductive structures are easily
identified
Short 90 day life cycle Distinct traits: seed
shape, seed color, pod shape, pod cover, and
height; are some examples
Pea Traits
How did Mendel control his
experiment?
By manually transferring pollen from one
plant to another plant with traits that he
wanted to test
What is a cross?
Manually transferring pollen from a
designated plant with the desired trait
to another plant with another desired
trait
What is pollination?
The transfer of the male pollen grains
to the female organ of flower, resulting
in a fertilized mature seed
Mendel’s Investigations
His goal was to
explain the patterns
of inheritance at the
time, no explanation
could accurately
explain heredity
What is a hybrid?
An organism that receives different genetic
information for a trait from each parent
Mendel’s Experiment
Pure-bred – organisms
that when allowed to
self-fertilize, only
produce offspring the
parental trait(s)
Mendel started by
crossing pure-bred tall
pea plants with purebred short pea plant
What sort of plants did Mendel
get in his first generation?
He called the first pair the (P) or parental
group and the first generation the first
filial group or the F1 generation
The F1 generation were all tall but each
offspring had both the tall and short
genes
He then allowed the F1 group self-fertilize
and they produced a F2 generation of
3/4th tall plants and 1/4th short plants
Parent, F1 and F2 generations
How did this happen?
What did Mendel determine?
That one trait must be stronger than
the other
What are the types of traits?
Dominant – the
trait that shows in
a hybrid
Recessive – trait
that does not show
in a hybrid
Dominant
&
Recessive
traits
Mendel’s Hypothesis
Each trait is controlled by something he
called a “factor” within the organism
We now call this factor a “gene”
Gene – genetic factor that controls a trait
Since 2 parents contribute to the offspring,
there must be 2 genes that separate when
gametes form
Since traits can disappear for a generation
and return unchanged to the next, they must
be separate and distinct
What is an allele?
Gene form for each variation of a
trait of an organism
Mendel’s pea plants
The pure-bred plants have 2 of the
same alleles; example Mendel’s tall pea
plants had 2 alleles for tallness
The hybrid crosses have alleles of 2
different traits
Example Mendel’s F1 generation had 1
allele for tallness and 1 allele for
shortness
Laws of Segregation
The 2 alleles for each trait must
separate when gametes form
A parent passes on at random only 1
allele for each trait to each offspring
What is a phenotype?
The way an organism looks and behaves –
the outward appearance regardless of
what is in the genetic code
Example Mendel’s F1 generation looked
tall but had the gene for shortness
What is a genotype?
The gene combination an organism
contains; example even though Mendel’s
F1 generation was all tall plants, they all
had for both tallness and shortness
If the organism has the same alleles or
traits; example having both tall alleles,
usually written as TT; then the organism
is considered homozygous
What is a genotype?
If the organism has different alleles or
traits; example having one tall allele and
one short allele, usually written as Tt;
then the organism is considered
heterozygous
What is a Dihybrid cross?
A cross involving two different traits
What happens in the F1
generation?
Mendel took a pea plant
with round, yellow
seeds (these are the
dominant traits) and
crossed this plant with
a pea plant with
wrinkled, green seeds
(these are the
recessive traits)
What happens in the F1
generation?
The genotypes of these are:
RRYY and rryy
The entire F1 generation were
genotype – RrYy
The entire F1 generation had
the phenotype - round, yellow
seeds
What happens in the F2
generation?
Mendel allowed the
plants in the F1
generation to selfpollinate andMendel
found that there
were some plants
with round, yellow
seeds and some
wrinkled, green
seeds
What happens in the F2
generation?
He also found that some
plants had round, green
seeds and some wrinkled,
yellow seed
He found that the traits
appeared in a fixed ratio
of:
9 round, yellow
3 round, green
3 wrinkled, yellow
1 wrinkled, green
What is Mendel’s Law of
Independent Assortment?
Genes for different traits are inherited
independently of each other
What is a Punnett Square?
Short hand way of finding the expected
proportions of possible genotype in
offspring
Monohybrid crosses in the
Punnett Square
The Punnett Square is 2 boxes tall and 2
boxes wide.
One parents’ genotype is separated and
placed on the top of the Punnett Square
and the other parents genotype is
separated and placed along the left side
of the boxes
Monohybrid crosses in the
Punnett Square
The alleles are
then written into
the squares that
are underneath
or beside them
and then all
possible
offspring can be
determined
Examples:
Exceptions to Mendel’s Laws
Incomplete dominance
Codominance
Sex-linked traits
Incomplete Dominance
Shows the appearance of a third
phenotype
Example: crossing a red flowering
snapdragon with a white flowering
snapdragon produces pink flowering
snapdragons in the F1 generation
Incomplete Dominance
Codominance
Both alleles are expressed in the F1
generation
Example: Cross a black feathered
chicken with a white feathered chicken
The F1 generation will show both black
and white feathers in a checkerboard
pattern
Codominance
Sex Linked
Traits
The expression of
traits from genes
found on the sex
chromosomes – X
or Y
Example eye color
in fruit flies
Examples: Incomplete
Dominance
Examples: Codominance
Examples: Sex-linked traits