Name: __________________
Hour: __________
Complete Dominance Problems
Complete dominance problems deal with traits that are either dominant or recessive. The dominant
trait is shown in the phenotype if is present (“Qq or QQ), and the recessive trait is only shown if there
are 2 copies of the recessive allele (qq).
Humans have many traits that are characterized by complete dominance. Fill in the punnet squares
for the following scenarios and then answer the questions.
Directions:
1. There are TWO (2) possible phenotypes for each problem.
2. Use the first letter of the dominant trait to determine genotype. (Ex: Hairy = H)
3. Use the same letter to designate the recessive trait, but use LOWER CASE. (Ex: Hairless = h)
Problems:
1. Right-handedness is a dominant trait. Left-handedness is recessive. Cross a homozygous righthanded woman with a homozygous left-handed man.
Key:
H = Right handed
h = left handed
a. What is the genotype of the woman? ________ What is her phenotype? __________________
b. What is the genotype of the man? ________ What is his phenotype? __________________
c. What is the phenotypic ratio of right-handed children to left-handed? _________ : _________
d. What fraction of the children are right-handed? __________________________
2. Having hairy knuckles is a dominant trait. If two parents are heterozygous for the trait, show
their possible offspring.
Key:
______ = ________________
______ = ________________
a. What is the genotype of the parents? _________ What is their phenotype? ______________
b. What is the ratio of hairy-knuckled children to hairless children? ________ : __________
c. Is this a genotypic ratio or phenotypic ratio? Circle one!
d. What is the ratio of homozygous dominant to heterozygous to homozygous recessive
offspring? ________ : __________ : ________.
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3. Being able to roll your tongue into a U-shape is a dominant trait. Both of my parents can roll
their tongues, but I cannot.
Key:
______ = ________________
______ = ________________
a. What is the genotype of my Dad? ________ What is the genotype of my Mom? ___________
b. What is my genotype? __________
c. What fraction of my parent’s offspring are not able to roll their tongues? _______________
4. Dimples are a dominant trait. My mom is heterozygous for the trait, and my dad is
homozygous recessive.
Key:
______ = ________________
______ = ________________
a. What is my Mom’s genotype? ________ What is her phenotype? ______________________
b. What is my Dad’s genotype? ________ What is his phenotype? ______________________
c. What is the genotypic ratio of homozygous dominant to heterozygous to homozygous
recessive offspring? _______ : _______ : _______
d. What are the chances that I have dimples? _______%
5. Freckles are a dominant trait. My dad is homozygous dominant for the trait and my mom is
heterozygous.
Key:
______ = ________________
______ = ________________
a. What is my Mom’s genotype? ________ What is her phenotype? ______________________
b. What is my Dad’s genotype? ________ What is his phenotype? ______________________
c. What are the possible phenotypes of the offspring? __________________________________
d. What is the phenotypic ratio of the offspring? _______ : _______
e. What are the chances that I have freckles? _______%
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6. Neither of my parents have a widow’s peak hairline. Having a widow’s peak is a dominant
trait.
Key:
______ = ________________
______ = ________________
a. What is the genotype of my Mom? ________
b. What is the genotype of my Dad? ________
c. What is the phenotypic ratio of the offspring? _______ : _______
d. What are the chances that I have a widow’s peak? _______%
7. Having your earlobes unattached is a dominant trait. Both of my earlobes are unattached but
both of my parent’s earlobes are attached.
Key:
______ = ________________
______ = ________________
a. What is the genotype of my Mom? ________
b. What is the genotype of my Dad? ________
c. What are the chances that I have unattached earlobes? _______%
d. What is the problem with this? _________________________________________________
e. What is a viable solution for how this occurred (without the parents cheating on each other?
_____________________________________________________
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