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Heredity and Reproduction
Chapter 4
Reference Text:
PCI LIFE SCIENCE
Chapter 4 - Section 1
TRAITS
TRAITS
Have you ever heard
someone say something like,
"You have your mother's eyes,“
or
"You have your father's chin"?
Why do you resemble your parents?
TRAITS
The color of your eyes is a trait.
Traits are the physical
characteristics that an organism
possesses.
Parents pass on their traits to
their children (offspring).
This is called heredity.
TRAITS
What determines which
traits you inherit from
each parent?
Mendel’s Pea Plants
In the 1800s, a scientist
named Gregor Mendel
studied the passing of
traits from parents to
offspring.
He wanted to know why
certain patterns showed
up in living things.
Mendel’s Pea Plants
Mendel had noticed that a
certain trait would appear in
one generation but not in the
next. Then, the trait would
show up again in the next
generation.
For example, a man would
have curly hair, his child
would have straight hair, and
his grandchild would have
curly hair.
Mendel’s Pea Plants
Mendel wanted to
know why this
happened.
He wondered how
traits passed from
parents to
offspring.
Mendel’s Pea Plants
Mendel did some experiments to find
out.
He decided to cross-pollinate pea
plants and see what happened.
Mendel’s Pea Plants
Cross-pollination
happens when
pollen from one is
transferred to the
reproductive
structure of
another plant.
The parent plants
will produce a new
plant.
Mendel’s Pea Plants
Cross-pollination can happen when an insect
sits on a flower and pollen sticks to the
insect.
When the insect flies to another flower, it
deposits the pollen from the first flower
onto the reproductive structure of the
second flower.
Mendel’s Pea Plants
Mendel’s Pea Plants
Mendel conducted
his experiments
with pea plants
because they grow
quickly and there
are a lot of
different kinds of
pea plants.
Mendel’s Pea Plants
For example, pea plants can be short or
tall, can have round or wrinkled seeds,
and can have purple or white flowers.
Mendel’s Pea Plants
In Mendel's experiments, he crosspollinated plants that had different traits
to see what the offspring would look like.
For example, Mendel wanted to know what
would happen if a tall plant and a short plant
had offspring.
Would the offspring be tall, short, or
medium?
What do you think?
Mendel’s Pea Plants
Mendel studied seven characteristics of pea
plants. He wanted answers to the following
questions:
Chapter 4 - Section 2
Dominant and Recessive Traits
Mendel’s Pea Plants
Gregor Mendel cross-pollinated
pea plants to study the
characteristics of their
offspring,
For example, he crosspollinated tall plants with short
plants. These plants are called
the parent generation (P).
Their offspring are called
first-generation (F1) plants.
Mendel’s Pea Plants
In Mendel's experiment, all of the
first-generation plants were tall.
What happened to the short trait?
Would all the offspring always be
tall?
Dominant & Recessive Traits
The same results happened
in every experiment.
One trait, like being tall,
was always there in the
first generation.
The other trait, like being
short, seemed to go away.
Dominant & Recessive Traits
Mendel
described the
trait that was
there in the
first generation
as the dominant
trait.
He described the trait that went
away in the first generation as the
recessive trait.
Dominant & Recessive Traits
A dominant trait is a
trait that is most
likely to appear in
offspring and a
recessive trait is least
likely to appear in
offspring.
Dominant & Recessive Traits
If green pod color is
the dominant trait and
yellow pod color is the
recessive trait, what
will the pod color of
the F1 generation
offspring have?
If you said GREEN, you
are correct!
Dominant & Recessive Traits
Mendel wanted to know
what happened to the
recessive traits.
So, he watched what
happened when the firstgeneration plants
reproduced.
Their offspring were
called second-generation
plants. (F2)
Dominant & Recessive Traits
Mendel noticed that the recessive
traits came back in some of the
offspring.
Most of the second-generation plants
were tall, but some were short.
Dominant & Recessive Traits
This happened with every set of
traits that Mendel studied.
Each recessive trait reappeared in
some of the second-generation
plants.
Dominant & Recessive Traits
Mendel figured out that each plant
had two sets of instructions for
each characteristic.
Dominant & Recessive Traits
These instructions are called genes.
Genes are the parts of DNA that
give instructions for what traits
offspring will have.
Dominant & Recessive Traits
Each parent gives one
set of genes to the
offspring.
The offspring has two
forms of the same
gene for each
characteristic.
Each member of a gene
pair is called an allele.
Genotype and Phenotype
The combination of alleles in an
organism controls what traits
the organism will have.
For example, there is one allele
for shortness in pea plants and
one allele for tallness.
If the offspring's gene for
height is made up of two alleles
for tallness, the plant will be
tall.
If the offspring's gene for
height is made up of two alleles
for shortness, the plant will be
short
Genotype and Phenotype
This combination of genes
passed on to offspring is called
genotype.
Dominant alleles are written as
uppercase letters.
So, the genotype for a plant
with two dominant alleles for
height is TT.
Recessive alleles are written as
lowercase letters.
So, the genotype for a plant
with two recessive alleles for
height is tt.
TT
tt
Genotype and Phenotype
If an organism has
one dominant allele
for height and one
recessive allele for
height, its genotype
is Tt.
Genotype and Phenotype
Phenotype is the way
an organism looks.
An organism's
phenotype is decided
by its genotype.
Genotype and Phenotype
If a dominant allele is
present in the organism's
genotype, that trait will
appear in the organism.
If a plant has one dominant
allele for height, the plant
will be tall.
A recessive allele is hidden
if a dominant allele is
present.
Genotype and Phenotype
An organism will only
display a recessive
trait when two
recessive alleles are
present and there are
NO dominant alleles.
Genotype and Phenotype
Imagine that two pea plants are crosspollinated.
One pea plant has the genotype TT,
so it is tall.
The other pea plant has the genotype tt,
so it is short.
Genotype and Phenotype
Because the allele for
tallness (T) is dominant, a
plant with the genotype TT
or Tt will be tall.
Remember, a recessive
allele is hidden if a dominant
allele is present.
Offspring with the
genotype tt have two
recessive alleles.
These offspring will be
short.
Punnett Squares
Biologists use a diagram
called a Punnett square
to figure out the
probability of an
offspring expressing a
certain genotype.
Probability is the
mathematical chance
that something will
happen.
Punnett Squares
Look at the Punnett square below.
One parent's genotype for height is recorded on
the top row (TT).
The other parent's genotype for height is recorded
in the left column (tt).
T
t
t
T
Punnett Squares
Remember that a child gets one
allele from each parent.
MOM
T
t
DAD
t
T
Punnett Squares
The four examples in the Punnett square show what
will happen if the two parents have offspring.
In this case, every combination of alleles from the
two parents results in the genotype Tt.
So, 100 percent of the offspring will be tall.
T
T
t
Tt
Tt
t
Tt
Tt
Punnett Squares
What happens when one of the
parents has a dominant allele and a
recessive allele?
T
t
t
t
Punnett Squares
Look at the Punnett square below.
In this case, two offspring have the genotype
Tt and two have the genotype tt.
Two offspring will be tall, and two will be
short.
T
t
t
Tt
tt
t
Tt
tt
Punnett Squares
What happens when both parents
have a dominant allele and a
recessive allele?
T
T
t
t
Punnett Squares
Look at the Punnett square below.
In this case, one offspring has the genotype TT,
two have the genotype Tt, and one has the
genotype tt.
How many of the offspring will be tall, and how
many will be short?
T
t
T
TT
Tt
t
Tt
tt
Traits in Humans
A combination of
alleles makes you look
the way you do.
You get traits from
your parents, just like
pea plants do.
Traits in Humans
For example, the trait of earlobe shape in
humans is inherited.
There is an allele for free earlobes and an
allele for attached earlobes.
The allele for free earlobes is dominant, and
the allele for attached earlobes is recessive.
Free Earlobes
Attached Earlobes
Traits in Humans
If your gene for earlobes is
made up of two alleles for
free earlobes, you will have
free earlobes.
If you have two alleles for
attached earlobes, you will
have attached earlobes.
If you have one allele for
attached earlobes and one
allele for free earlobes,
what will your earlobes look
like?
Traits in Humans
The characteristics of all organisms are
controlled by the alleles that make up their genes.
Have you ever looked at a newborn baby and
noticed that it resembles its parents?
You are observing the effects of alleles.
Traits in Humans
Alleles determine who has a cleft
chin, curly hair, blue eyes, and many
other physical characteristics.
Chapter 4 - Section 3
Heredity & Reproduction