HBio- Genetics: Complete Dominance
Name:____________________Hr:____
Background: Remember back to when we learned about DNA- our chromosomes come in pairs, one from mom
and one from dad. In Biology, when we study genetics, we study the science behind the genes (on our
chromosomes), how we inherit different genes, and we try to figure out what kinds of variations can occur when
our mom and dad’s DNA meet!
Today we are going to learn about complete dominance. Sometimes, the gene we get from our mom is
different than the gene for the same trait we get from our dad. These variable forms of the same genes are
called alleles. When two alleles come together to form a pair of genes for a particular trait, we call this a
genotype. Our bodies must then decide, based on the genotype, how to make us look the way we do. Our
physical appearance, also called our phenotype, is influenced by the interactions between our varying alleles
and genotypes that we get from mom or dad. As you may already be thinking, there are MANY ways these
alleles can interact with each other, but for now let’s stick to one of the most basic- complete dominance.
With complete dominance, one allele for a trait is completely dominant over other alleles for the same trait.
For example: Tongue rolling is a dominant trait. We’ll say that its allele is represented by the letter R. People
who cannot roll their tongues have what is called a recessive allele, which we will say is represented by the
letter r.
So let’s recap: (R)= tongue rolling allele
(r)= non-tongue rolling allele
If we all need two alleles to make our genotype for the tongue rolling gene, what
are the 3 different possible variations of genotypes for this trait? RR, Rr, and rr.
These three genotypes (respectively) are called homozygous dominant,
heterozygous, and homozygous recessive. (One way to remember this is that
“homo-“ means same- with RR and rr, the two alleles, or letters, are the same.
“Hetero-“ means different- Rr are two different letters.)
With Complete Dominance, again, one allele for the tongue rolling trait is completely dominant over the other
allele for the same trait. This means that when is comes to the genotype Rr, even though a person is a carrier
for the non-rolling tongue allele, the R for tongue rolling is DOMINANT and so, that person will be a tongue
roller anyway. So, people with the genotype RR and Rr will be tongue rollers because they posses at least one
dominant allele. People with the genotype rr, will not be able to roll their tongue.
NOW, how do we keep track of who can pass on the tongue rolling phenotype to their offspring and who can’t?
We call this a punnett square:
For example, if we know that 1 parent’s genotype is Rr and the other parent is rr, we can calculate the
probability of what genotypes their children may have by making a punnett square:
We put each parents’ genotype on the top and left edge:
R
r
Rr
r
rr
Parent 2 genotype 
r
Rr
Parent 1 genotype
rr
Genotypes
½ Rr
½ rr
**Inside of the squares are the possible genotypes of the offspring.
Phenotypes
½ rolled tongues
½ not rolled tongues
We can then describe this punnett square in 2 ways: By Genotype and Phenotype. Each child has a 50%
chance of having the heterozygous genotype for tongue rolling and a 50% chance of having the homozygous
recessive genotypes are for tongue rolling. Because of complete dominance if the child is heterozygous it will
have a phenotype of a tongue roller. If the child is homozygous recessive it will have the phenotype of nontongue rollers. Often times you will be asked to write out the phenotypic and genotypic ratios. In this case you
can make a small table such as the one above.
Practice 1: -Show and explain all work- include necessary punnett squares
1. Based on the information on the previous page, is it possible for 2 parents who are tongue rollers to have
a child who is a non-tongue roller? How? Show and explain punnett squares.
2. Curly hair (H) is dominant over the allele for straight hair (h). Two parents have curly hair but are
heterozygous for the hair trait. Draw the punnett square for their offspring:
a. What percentage of their children will have straight hair? Curly hair?
b. What if one of their heterozygous children married and had children with a person
homozygous dominant for curly? Show a punnett square. What percentage of their children
will have curly hair?
3. For cows, the allele for brown fur color (F) is dominant over white fur color (f). A farmer has 2 cattle to
start with: one brown heterozygous male and one brown homozygous dominant female cow. If the
farmer wanted ALL WHITE cows, how many generations of inbreeding cows would it take to get a herd
of all white cows? (Hint, siblings cannot be bred back to parents because of age difference, but siblings
can be bred to siblings). Show all work and punnett squares.
HBio-Genetics: Complete Dominance
Practice 2-Show and explain all work- include necessary punnett squares
1. In peas, yellow seed color is dominant to green seed color. Please cross a homozygous dominant pea
plant with a heterozygous pea plant. Show the Punnett square and give the genotypic and phenotypic
ratios of the offspring.
2. You are raising peas from a stock of peas that is always round. You take a round pea and cross it with a
heterozygous wrinkled pea. The wrinkled pea’s “parents” had baby peas that always came out with
three times as many wrinkles as rounds.
a. Is round or wrinkled dominant?
b. What is the expected genotypic ratio for the cross you made? Show your punnett square.
3. In humans, normal thumbs are dominant over hitchhiker thumbs. Both you and your sister have hitchhiker
thumbs, but your mom and dad have normal thumbs. Your brother has a normal thumb, also. Please give the
genotypes of the following individuals. (Hint: Set up a Punnett square to help you.)
T=
t=
a. your father______
d. your sister_____
b. your mother_____
e. your brother_____
c. you_____
4. Dogs can have spots=S or no spots=s. You have a dog with no spots. Circle all the following
statements that are true.
a. Your dog’s genotype is Ss.
b. Your dog’s parents could have been spotted.
c. Your dog’s parents could have been one with no spots and one with spots.
d. Your dog’s parents could both have no spots.
e. Your dog’s parents could have been one heterozygous spots and one homozygous spots.
HBio-Genetics: Complete Dominance
Practice 3-Show and explain all work- include necessary punnett squares
1. In summer squash, white fruit is dominant over yellow. Suppose that a gardener places the pollen from a
homozygous yellow-fruited plant on the pistil of a heterozygous white-fruited plant, Determine the
genotypic and phenotypic ratios of the possible offspring. What is the probability of getting yellow fruit?
2. In a particular flower there are big and small blooms. For each of the following statements, tell which
trait you think is dominant and why. If you can’t tell which is dominant from the information given,
then put “can’t tell.” Use punnett squares to support your answer.
a. Two big bloomed flowers have a small bloomed offspring. Are big blooms or small blooms
dominant?
b. Two big bloomed flowers have all big bloomed offspring. Are big blooms or small blooms
dominant?
c. A big bloomed flower and a small bloomed flower have ½ big bloomed offspring and ½ small
bloomed offspring. Are big blooms or small blooms dominant?
d. Two small bloomed flowers have hundreds of offspring and approximately ¾ of all of their
offspring are small bloomed and ¼ are big bloomed. Are big blooms or small blooms dominant?