Download 5.2 Dominant, Recessive, Heterozygous

Dominant, Recessive, Heterozygous,
Homozygous

Catalyst:
 Toby has a deep voice.
 Is
this genotype or phenotype?
 HOW DO YOU KNOW?
 Paula
has one allele for being tall, and one allele for
being short.
 Is
this genotype or phenotype?
 HOW DO YOU KNOW?
Write in complete sentences!
Don’t talk during the Catalyst!
Objectives

By the end of today, all SWBAT…
 Differentiate
between homozygous and
heterozygous genotypes
 Determine the phenotype of an organism,
based on its genotype
Quick Review

All living organisms have two alleles for every gene
 Gene
controls one trait
 Allele = version of a gene


Genotype = which alleles you got from parents
Phenotype = physical expression of the genotype
New Vocabulary

Key Point #1: Genotypes are either heterozygous
or homozygous.
 What
do you think these words mean?
 Homo
= “same”
 Hetero = “different”
 Homozygous
= two of the SAME alleles
 Heterozygous = two DIFFERENT alleles

Examples
 Tierra
has two alleles for brown eyes. She is homozygous
for eye color.
 Reginald has one allele for green eyes, and one allele
for brown eyes. He is heterozygous for eye color.
Check for Understanding (CFU)

Michael has one allele for a widows peak, and one
allele for a straight hairline.
 Homozygous
or heterozygous for hairline shape?
Check for Understanding (CFU)

Adam has two alleles for curly hair.
 Homozygous
or heterozygous for hair type?
Check for Understanding (CFU)

Byron has two alleles for hairy forearms.
 Homozygous
hair?
or heterozygous for amount of forearm
Check for Understanding (CFU)

Jordan has one allele for straight hair, and one
allele for wavy hair.
 Homozygous
or heterozygous for hair type?
Principle of Dominance


The two types of alleles are dominant and
recessive.
Key Point #2: If an organism is heterozygous, a
dominant allele will mask the recessive allele.
 Example:
 The
hair color (gene)
brown allele is dominant, the blonde allele is recessive
 Yashira has two blonde alleles. Her hair color is blonde.
 Melvin has two brown alleles. His hair is brown.
 Jelisa has one brown allele and one blonde allele. Her hair
is BROWN!!!
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Aisha
has two dominant alleles
 What type of fingers does she have?
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Ros’Joan
has one dominant allele and one recessive
allele
 What type of fingers does she have?
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Dweldon
has two recessive alleles
 What type of fingers does he have?
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Kody
has one dominant allele and one recessive allele
 What type of fingers does he have?
But Mr. J, I’m Lazy!


Let’s abbreviate things.
Key Point #3: There are three rules for
abbreviating genotype
1.
2.
3.

Dominant allele = capital letter
Recessive allele = lowercase letter
BOTH alleles use the first letter of the dominant trait
Long tails in dogs are dominant, short tails are
recessive


Long = L
Short = l
Dominant and Recessive Genotypes


For every gene you have two alleles, and each of
those could be dominant or recessive.
So a dog could have THREE different GENOTYPES
for tail length:
LL
(homozygous dominant)
Ll (heterozygous)
ll (homozygous recessive)
Dominant and Recessive Phenotypes


But dogs could only have two different tail lengths.
Remember that a dominant allele will mask a
recessive one. So for this genotype you get….
LL
(long tail)
Ll (long tail)
ll (short tail)
Let’s Practice More

Pea plants can have two alleles for the gene for
height: tall and short. Tall is dominant to short.
What
letters would represent
the two alleles?
What would be the genotype
of a heterozygous flower?
What would be the phenotype
of a flower with TT?
What would be the phenotype
of a flower with Tt?
T, t
Tt
Tall
Tall
Whiteboard Practice!
Guided Practice




Hitchhiker’s thumb is a trait in humans. Normal thumbs
are dominant to Hitchhiker’s thumb. Using the letter “H,”
show an individual who is homozygous recessive for the
gene for hitchhiker’s thumb.
What is this person’s phenotype?
Write the genotype for someone heterozygous.
What is this person’s phenotype?
Guided Practice




The bunnies of the forests of Louisiana can either be
brown or black. Brown is dominant to black.
Write all the genotypes possible for a brown bunny.
What would the phenotype be for a bunny with the
genotype of Bb?
What would be the phenotype of a bunny with the
genotype of BB?
Guided Practice





People can have either free or
attached earlobes. Free is
dominant to attached.
What letters would you use to
represent the alleles?
What genotype is someone who
is Ff?
What phenotype is someone
who is ff?
What phenotype is someone
who is homozygous dominant?
Guided Practice





Chickpeas can be either green or yellow. Yellow is
dominant to green.
What is the genotype of a green pea? Explain.
What is the genotype of a yellow pea? Explain.
What is phenotype of a pea plant that is YY?
What is the phenotype of a heterozygous pea plant?
Independent Practice (IP)
Key Point Wrap-Up

Key Point #1: Genotypes are either heterozygous
or homozygous.
Homozygous = two of the SAME alleles
 Heterozygous = two DIFFERENT alleles



Key Point #2: If an organism is heterozygous, a
dominant allele will mask the recessive allele.
Key Point #3: There are three rules for abbreviating
genotype
1.
2.
3.
Dominant allele = capital letter
Recessive allele = lowercase letter
BOTH alleles use the first letter of the dominant trait
Exit Question

The gene for fur color in mice has alleles
for brown and white. Brown is dominant
to white.
 What
letters are used for the alleles?
 Write the genotype AND phenotype for a
mouse that is homozygous recessive.
Write in complete sentences!
Don’t talk during the Exit Question!