AP Biology : Understand co-dominance
Study concepts, example questions & explanations for AP Biology
All AP Biology Resources
Example Questions
Example Question #1 : Understand Co Dominance
A mother with type A blood and a father with type B blood have a child. What blood type is impossible for that child to have?
AB
O
A
All are possible
B
All are possible
The mother's possible genotypes for blood are AO and AA, while the father's are BO and BB; therefore, the child could have any blood type because we could receive an O allele from either parent.
The full possibilities are:
A from mother, O from father - blood type A
A from mother, B from father - blood type AB
O from mother, B from father - blood type B
O from mother, O from father - blood type O
Example Question #1 : Understand Co Dominance
In a specific type of flower the genes coding for color display codominance. What would you expect the phenotype to be for a cross between a flower homozygous for white coloration and a flower homozygous for red coloration?
Pink
Red
Distinct red and white spots
White
Distinct red and white spots
Codominance is a phenomenon in which the phenotypes associated with both alleles will be expressed in their entirety. This expression pattern results in mottled expression, creating distinct red and white spots for the flower. This is different than incomplete dominance, in which the two phenotypes appear to blend together.
Example Question #1 : Understand Co Dominance
In humans, blood type is determined by three alleles: A, B, and O. Both the A and B alleles are dominant to the O allele. When both A and B are present, however, a combined phenotype of AB results. In AB individuals, both A and B antigens are fully expressed on the cell surface. This is an example of __________.
incomplete dominance
Mendelian dominance
nondominance
complete dominance
codominance
codominance
Since the A and B alleles both seem to exert a form of dominance, this is clearly not our common example of a complete dominance scenario. We can conclude that blood type is determined by either incomplete dominance or codominance.
In incomplete dominance, both alleles exert influence to a lesser degree resulting in a "blended" phenotype. In blood type, both alleles exert their full influence together. Instead of yielding a blended phenotype, this situation results in a phenotype that is functionally equivalent to having both A and B blood types at once. The A and B alleles are codominant.
Example Question #301 : Evolution And Genetics
Scientists are trying to figure out the dominance hierarchy for a newly discovered plant. They have found that when a true-breeding yellow plant is crossed to a true-breeding green plant, the resulting offspring have distinct spots of yellow and green. What is the most likely explanation for this result?
Yellow is the dominant allele
Incomplete dominance
Green is the dominant allele
Codominance
Codominance
In the resulting offspring, both phenotypes are displayed equally. This is a classic example of codominance. If an intermediate phenotype was observed, incomplete dominance would be the correct answer.
In codominance, both alleles are considered dominant. This means that both alleles will be fully expressed in different regions, resulting in spots. In incomplete dominance neither allele is fully dominant, so both can be expressed simultaneously in a given area. The result is a blending of both alleles.
Example Question #1 : Understand Co Dominance
The shorthorn cattle coat color exhibits codominance. If a homozygous red individual and homozygous white individual produce an offspring, what will the resulting coat color be?
White
Roan
Red
Pink
Roan
In cases of codominance, the offspring have both alleles expressed at the same time. Thus, the coat color will be roa, which contains both white and red hair.
Example Question #5 : Understand Co Dominance
Blood type exhibits codominance. 
A
A
AB
O
AB
In cases of codominance, the offspring have both alleles expressed at the same time. As both 
Certified Tutor
All AP Biology Resources
