Gregor Mendel and
Genetics


I can define heredity.
I can describe the work of Gregor Mendel.
I can define and give examples of a trait.
I can define and give examples of a gene.
I can define an allele.
I can explain when a dominant trait will
appear vs. a recessive trait.
Question

Think about it!
How would you define a trait?
List three examples of traits.
Share with your table your thoughts, and
have one person write your answer on the
white board.
Heredity

Every living thing has a set of
characteristics inherited from its
parent or parents.
Heredity holds the key to
understanding what makes each
species.
Genetics is the scientific study of
heredity.
Questions

Based on the movie from yesterday:
Who was Gregor Mendel?
Why is his work significant to us
today?

Gregor Mendel is considered
to be the father of genetics.
Mendel was a monk in
Austria.
He worked with garden peas.
Questions

Explain why you think garden peas
were a good organism to study
heredity.
Predict what Mendel would have
learned if he had used humans
instead.

Peas use self-pollination, where the
pollen fertilizes an egg all produced
by the same plant.
The peas that Mendel used were
true-breeding.

True-breeding means that if a plant
is allowed to self fertilize, it will
produce offspring identical to itself.
One stock of Mendel’s peas would
only produce tall plants, and
another only short plants.

Pea plants can also cross pollinate,
this means the pollen and the egg
come from two different pea plants.
To do his research, Mendel did not
allow the plants to self pollinate,
instead he cross pollinated the peas.

Mendel studied 7 different pea
plant traits.
A trait is a specific characteristic
that varies from one individual to
another, like seed color or plant
height.

Mendel crossed plants with each of
the seven different traits and
studied their offspring.
He called each original pair of
plants the P or parental generation.
He called the offspring of the P
generation, the F1 generation.

The offspring of crosses between
parents with different traits are
called hybrids.
The F1 generation in Mendel’s
experiments showed traits of only
one parent.
Questions

Predict why the traits of only one
parent appeared in the offspring of
the first generation.
What happened to the other traits
and why are they not present?

One conclusion Mendel drew from
this is that inheritance is determined
by factors that are passed from one
generation to the next.
Today, we call those factors genes.
Every trait has at least two different
genes controlling it.

The different forms of a gene
are called alleles.
Mendel’s second conclusion is
called the principle of
dominance.
Question

In genetics, predict what
dominance means.
What do you think recessive
means?

The principle of dominance states
some alleles are dominant and
others are recessive.
Dominant alleles always makes
recessive alleles.
Recessive traits only appear when
the dominant allele is not present.

Mendel wanted to know if the
recessive alleles had disappeared or
if they were just masked, so he
performed a second experiment.
In this experiment, he crossed
plants from the F1 generation

The F1 generation plants
produced offspring he referred
to as the F2 generation.
In the F2 generation, the traits
of the recessive alleles
reappeared.
Question

What is a gamete? Give an example
of a gamete.

Mendel concluded that during
the formation of gametes, or
sex cells, the alleles separate
from each other.
In other words, sex cells carry
only 1 copy of each allele
instead of two.
Probability and Punnet
Squares


I can explain probability.
I can explain how a dominant and
recessive allele are represented using
letters.
I can give examples of both homozygous
and heterozygous individuals.
I can fill out a Punnett Square.
I can define and give examples of
phenotypes and genotypes.

In your own words, define
probability.
Predict how you think probability
applies to genetics.

The likelihood that a particular
event will occur is called
probability.
The probability of a coin
ending up heads is ½ or 50%.

To get the probability of getting
heads three times in a row, you
multiply. ½ x ½ x ½ = 1/8
This leads to an important point,
past outcomes do not affect future
outcomes.

Like flipping a coin, the
segregation of alleles is
completely random.
The principles of probability
can be used to predict the
outcomes of genetic crosses.

In your own words, define allele.
If the trait tall is dominant, predict
how it would be written using the
letter t.
If the trait short is recessive, predict
how it would be written using the
letter t.

Gene combinations can be
determined by drawing Punnet
squares.
The letters for dominant alleles are
capital letters.
The letters for recessive alleles are
lower case.

Example: Tt x Tt

What does the prefix hetero- mean?
If I told you a plant was heterozygous for
the tall trait, what would you predict the
alleles to look like?
What does the prefix homo-mean?
If I told you a plant was homozygous for
the tall trait, what would you predict the
alleles to look like?

Organisms that have two
identical alleles for the same
trait like TT are homozygous.
Organisms with two different
alleles for the same trait like Tt
are heterozygous.

Homozygous organisms are
true breeding for a particular
trait.
Heterozygous organisms are
hybrid for a particular trait.

Using the letter t, give two
examples of a homozygous trait.
Using the letter t, give one example
of a heterozygous trait.

The phenotype is the physical
characteristic you see.
The genotype is the actual
combination of alleles.
You can’t always predict genotypes.
The genotype for a tall plant could
be TT or Tt.

Explain why you think it is difficult
to predict a genotype for a
dominant trait.
Give an example of 3 human
phenotypes seen at your table.

The genotype for a short plant can
only be tt.
Punnett squares also help to predict
ratios.
The ratio of tall to short plants in
the above example is 3:1.

3 of the plants would be tall
and 1 of the plants would be
short.
These are only predictions for
probability.

In order to get the predicted ratios,
large numbers of organisms need to
be used.
Summary: Punnett squares show
expected genotypes and help
predict ratios for phenotypes.
Multiple Alleles


I can explain the principle of independent
assortment.
 I can define and give examples of
incomplete dominance.
I can define and give examples of
codominance.

Gametes are sex cells like the egg
and sperm.
Mendel wanted to find out if
segregation of one pair of alleles
affected the segregation of another
pair of alleles.
For example, must a round seed
also be yellow?
Questions

Does a blonde haired person
always have blue eyes? Why or why
not?
Does a round seed always have to
be yellow or can it be green? Why?

Mendel performed an
experiment to follow 2 different
genes.
Mendel’s experiment is known
as the two-factor cross or
dihybrid.

He crossed homozygous pea
plants with round, yellow seeds
with homozygous pea plants
with wrinkled, green peas.
This was his F1 generation and
produced heterozygous
offspring.

Prediction: Will the offspring of the
F1 generation show the dominant or
recessive trait? Be prepared to
explain why.

The heterozygous offspring
had genotypes of RrYy.
Mendel was trying to see if the
two dominant genes would
always be together.

Mendel’s experiment resulted
in seeds that were not like
either of the parents.
From this experiment, Mendel
concluded that alleles segregate
independently.
Vocabulary

In science, what does the word
segregate mean? Please define in
your own words.

This became known as the principle
of independent assortment.
The principle of independent
assortment states: genes for
different traits segregate
independently during gamete
formation.

Although Mendel’s work was
important, not all traits are just
dominant or recessive.
Many traits are controlled by
multiple alleles or genes.
Think about it

Give an example of a human trait
that you think might be controlled
by more than one gene.

Cases in which one allele is not
completely dominant over another
are called incomplete dominance.
In incomplete dominance, the
heterozygous phenotype is a
mixture of the two traits.

If you cross a white flower with a
red flower, how would the traits
mix?
Using the letter R, predict what the
genotype of the mixed flower would
be.

Example: Red and White 4 o’clock
flowers will make pink flowers.
Codominance is similar to
incomplete dominance.
In codominance, both alleles
contribute to the phenotype.

Example: In some chickens
black and white feathers are
codominant. Heterozygous
chickens are speckled black and
white.

Compare and contrast an
incomplete dominant trait and a
trait that is codominant.

Many genes have more than
two different alleles.
They are said to have multiple
alleles.

Example: A rabbit’s coat color
is determined by a single gene
with four known alleles. There
are four different coat colors
possible. The same is true for:
blood type.

List the different blood types that
are possible for humans.

Many traits are produced by
the interaction of several genes.
Traits controlled by two or
more genes are said to be
polygenic traits.

Polygenic traits means more
than 1 gene.
Polygenic traits usually show a
wide variety of phenotypes.

Example: There are at least
four different genes that play a
role in determining human skin
color.