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Codominance and Incomplete Dominance Quiz Practice
Incomplete Dominance
Up to this point, all of the traits we have studied have been controlled by genes in which there are dominant
and recessive alleles. In these cases, a heterozygous individual has the same phenotype as a homozygous dominant
individual.
There are some genes for which this is not true. For example, in two types of plants (Japanese four-o’clocks
and snapdragons) the gene that controls flower color has an allele coding for red color that is not completely dominant.
This is an example of a trait that is “incompletely dominant.”
Trait
Flower color in fouro’clocks and
snapdragons.
Alleles
Genotypes
Phenotypes
R – Red
RR
Red
r – White
rr
White
Rr
Pink
“both red and white
blend together”—
incomplete dominance
Codominance
In some genes, both alleles in a heterozygote are fully expressed, a phenomenon called codominance.
Codominance can affect the coat color in cows.
* Notice how both alleles are capital letters. This is because there is no one dominant allele! They both are
expressed in physical characteristics. Instead I use the prime (‘) symbol to distinguish between the two alleles.
Trait
Alleles
H – Brown
Genotypes
HH
Phenotypes
Brown
H’ – White
H’H’
White
HH’
Roan (Brown +
white)
“both colors show up
but they don’t blend
together”-codominance
Coat color in cows.
PROBLEMS
1. Describe the difference between incomplete dominance and codominance. You may have to use your book.
Remember to discuss the heterozygotes!
Incomplete Dominance:
Codominance:
2. Practice setting up genotypes for the phenotypes listed in each set. You must make up your own alleles.
Remember the difference between complete dominance, incomplete dominance and codominance
a. Birds can be blue, white, or white with blue-tipped feathers.
b. Flowers can be white, pink, or red.
c. A Hoo can have curly hair, spiked hair, or a mix of both curly and spiked.
d. A Sneech can be long, medium, or short.
e. A Bleexo can be spotted, black, or white.
3. Now, can you figure out in the above list, which of the letters a.)-e.) represent codominant traits and which are
incomplete.
Codominant _____________________________________________________
Incompletely Dominant _____________________________________________
4. In Smileys, eye shape can be starred, circular, or a circle with a star. Write the genotypes for the pictured
phenotypes
_______
________
_______
5. Show the cross between a star-eyed and a circle eyed. 
What are the phenotypes and fractions of the offspring? ___________________
What are the genotypes and fractions? ___________________________
6. Show the cross between a circle-star eyed, and a circle eyed.
How many of the offspring are circle-eyed? ____________
How many of the offspring are circle-star eyed? ____________
7. What are the 4 blood group phenotypes?_____ _____ _____ _____
8. List all of the possible blood group genotypes:
9. Which blood group is the universal donor?__________ Receiver?________
10. If a woman who is type B marries a man and has a child who is type O, what are the possible genotypes of her
husband?