Transcript 3.2 Dominant, Recessive, Heterozygous, Homozygous

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)

Joshua has one allele for a widows peak, and one
allele for a straight hairline.
 Homozygous
or heterozygous for hairline shape?
Check for Understanding (CFU)

Caleb has two alleles for curly hair.
 Homozygous
or heterozygous for hair type?
Check for Understanding (CFU)

Zach has two alleles for hairy forearms.
 Homozygous
hair?
or heterozygous for amount of forearm
Check for Understanding (CFU)

Lynda has one allele for straight hair, and one
allele for wavy hair.
 Homozygous
or heterozygous for hair type?
Principle of Dominance

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
 Katlyn has two blonde alleles. Her hair color is blonde.
 Alex has two brown alleles. His hair is brown.
 Amber has one brown allele and one blonde allele. Her
hair is BROWN!!!
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Asia
has two dominant alleles
 What type of fingers does she have?
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Tonya
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
 Brice
has two recessive alleles
 What type of fingers does he have?
Check for Understanding (CFU)

Long fingers is dominant, short fingers is recessive
 Jaron
has one dominant allele and one recessive allele
 What type of fingers does he have?
But Mr. LY, 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 that is tt?
What would be the phenotype
of a flower that is Tt?
T and t
Tt
Short
Tall
Whiteboard Practice!
Guided Practice
The bunnies of the forests of Louisiana
can either be brown or black. Brown is
dominant over black.
 Write all the genotypes possible for a
brown bunny.

Guided Practice
The bunnies of the forests of Louisiana
can either be brown or black. Brown is
dominant to black.
 What is the genotype AND phenotype
of a bunny that is heterozygous?

Guided Practice
The bunnies of the forests of Louisiana can
either be brown or black. Brown is
dominant to black.
 What is the genotype AND phenotype of a
bunny that is homozygous recessive?

Guided Practice
People can have
either free or attached
earlobes. Free is
dominant over
attached.
 What letters would
you use to represent
EACH allele?

Guided Practice
People can have either
free or attached
earlobes. Free is
dominant to attached.
 What genotype AND
phenotype of someone
who is homozygous
dominant?

Guided Practice
People can have either
free or attached
earlobes. Free is
dominant to attached.
 What genotype AND
phenotype of someone
who is heterozygous?

Guided Practice
Chickpeas can be either green or yellow.
Yellow is dominant over green.
 List all the possible genotypes for a yellow
chickpea.

Guided Practice
Chickpeas can be either green or
yellow. Yellow is dominant over green.
 What is genotype AND phenotype of a
chickpea that is homozygous recessive?

Guided Practice
Chickpeas can be either green or
yellow. Yellow is dominant over green.
 What is genotype AND phenotype of a
chickpea that is heterozygous?

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!