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Inherited traits
Mendel’s Legacy
The “Father” of
genetics.
 Studied inherited traits
in pea plants
 Discovered dominant
and recessive traits
and was able to predict
what pea offspring
would look like!

A Mendelian Genetic Primer
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
.
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Genes come in pairs that
separate in the
formation of gametes, or
sex cells.
The members of the pair
may be identical
(homozygous) or nonidentical
(heterozygous).
Each form of a particular
gene is called an allele
For simplification, only
two alleles of a given
gene are possible in an
individual although
many alleles of a gene
are possible within a
population.
 One allele is dominant
over another (or so
Mendel believed).

A Mendelian Genetic Primer
.
A Mendelian Genetic Primer
 Genotype: What alleles the
organism has.
 Phenotype: How the alleles are
expressed. Often, this means what
. the organism looks like.

Why is this not always the case?
Hold up one or more
of the three response
cards to answer the
following prompts…
Homozygous
Heterozygous
Homozygous
Recessive
One dominant allele
and one recessive
allele
Homozygous
Dominant
Heterozygous
Homozygous
Recessive
Homozygous
Dominant
Heterozygous

A species of rabbit can have either
brown fur, or white fur (there is no inbetween). The brown fur allele is
dominant, and the white fur allele is
recessive.
What genotype(s) can a rabbit with brown fur have?

A species of rabbit can have either
brown fur, or white fur (there is no inbetween). The brown fur allele is
dominant, and the white fur allele is
recessive.
What genotype(s) can a rabbit with white fur have?
Different Genotypes Can Produce the Same Phenotype

Two rabbits are brown and each have one
copy of the gene for brown fur and one of the
gene for white fur.

What are their genotypes?

Two rabbits are brown and each have one
copy of the gene for brown fur and one of the
gene for white fur.

What are the chances that one of their offspring will
have white fur?
We combine the
father’s genotype
with the mother’s
genotype.
 It doesn’t matter
which parent goes on
which part of the
square.

The four squares
represent the 4
possible offspring
these parent can
have with regard to
this one gene.
 Now lets fill in the
squares to see what
offspring are
possible.


What is the genotype
of offspring #1?

What is the genotype
of offspring #2?

What is the genotype
of offspring #3?

What is the genotype
of offspring #4?
Now we see the 4
possible offspring for
this mating.
 These offspring have
3 different
genotypes.
 BUT there are only 2
phenotypes!
 What are they?





3 offspring out of the 4
have brown fur. Hold
up the cards that
represent the
genotype(s) that make
brown rabbits.
BB and Bb
¾ rabbits = 75% of
offspring
If the parents have 40
offspring, how many
would probably be
brown?




1 offspring out of the 4
has whit fur. Hold up
the cards that
represent the
genotype(s) that make
white rabbits.
bb
1/4 rabbits = 25% of
offspring
If the parents have 40
offspring, how many
would probably be
white?
So two brown rabbits
produced some
offspring that are
white!
 What is the genotype
for white rabbits?


Some rabbits may be brown, but carry a copy of
the white gene. The white gene doesn’t show up
because it is recessive. What is the genotype of
these rabbits?

We call organisms like these carriers of an allele because they do not show the phenotype, but
they carry the allele in their genotype.

These rabbits were not white their offspring may
be.

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



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A brown rabbit has two copies of the gene for
brown fur.
What is its genotype?
There is also a white rabbit.
What is its genotype?
The two rabbits mate, what is the genotype
of 100% of their offspring?
Draw a punnett square
Hold up your answer
What color are the offspring rabbits?



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



A brown rabbit has one copy of the gene for
brown fur and one of the gene for white fur.
What is its genotype?
There is also a white rabbit.
What is its genotype?
The two rabbits mate, what are the two
possible genotypes of their offspring?
Draw a punnett square
Hold up your answer
What color are the offspring rabbits?
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What percent of the offspring are heterozygous?
50%
Hold up the card that represents these rabbits.
What color would these rabbits be?
What percent of the offspring are homozygous
for the recessive allele?
 50%
 Hold up the card that represents these rabbits.
 What color would these rabbits be?

What Works for Peas Also Works for Humans
An albino woman

In the cross Bb x Bb, where
B is a dominant allele for
standard pigmentation and
b is a recessive allele for no
pigmentation (albinism).


Are the parents albino?
No, because the standard
pigmentation allele (B) is
dominant. So it is the one
the shows up!
What Works for Peas Also Works for Humans

Are the parents heterozygous
or homozygous?
 Heterozygous (Bb)
 Why do we call the parents
carriers of the albinism gene?

An albino woman
In the cross Bb x Bb, where B
is a dominant allele for
standard pigmentation and b
is a recessive allele for no
pigmentation (albinism).
What Works for Peas Also Works for Humans
An albino woman

In the cross Bb x Bb, where
B is a dominant allele for
standard pigmentation and
b is a recessive allele for no
pigmentation (albinism).

What genotype does a
person need to have to
look albino?
Homozygous recessive
(bb)

What Works for Peas Also Works for Humans
An albino woman

In the cross Bb x Bb, where
B is a dominant allele for
standard pigmentation and
b is a recessive allele for no
pigmentation (albinism)

Draw a punnett square for
this cross, to see what their
children would look like.
Now we see the 4
possible offspring for
this mating.
 These offspring have
3 different
genotypes.
 BUT there are only 2
phenotypes!
 What are they?

What Works for Peas Also Works for Humans

What fraction of the
offspring will be albino?
 ¼ will be albino.
 Hold up the card that
represents a person with
albinism.

An albino woman
In the cross Bb x Bb, where
B is a dominant allele for
standard pigmentation and
b is a recessive allele for no
pigmentation (albinism)
What Works for Peas Also Works for Humans

What fraction of the offspring
will look normal?
 ¾ of offspring will be normal.
 Hold up the card(s) that
represent(s) a person that has
standard skin pigmentation.

An albino woman
In the cross Bb x Bb, where B
is a dominant allele for
standard pigmentation and b
is a recessive allele for no
pigmentation (albinism)
What Works for Peas Also Works for Humans
An albino woman

In the cross Bb x Bb, where
B is a dominant allele for
standard pigmentation and
b is a recessive allele for no
pigmentation (albinism)

What fraction of the
offspring will be carriers?
Hold up the card that
represents a carrier.

What Works for Peas Also Works for Humans

Hold up the card that
represents a homozygous
dominant genotype.
 What fraction of the
offspring will homozygous
dominant?

An albino woman
In the cross Bb x Bb, where
B is a dominant allele for
standard pigmentation and
b is a recessive allele for no
pigmentation (albinism)