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 or heterozygous for amount of forearm hair?

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: hair color (gene)

The 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 genotype1. Dominant allele = capital letter2. Recessive allele = lowercase letter3. 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 genotype1. Dominant allele = capital letter2. Recessive allele = lowercase letter3. 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!