Codominance Quiz
Questions: 16 · 10 minutes
1. A red-blood-cell sample reacts with both anti-A serum and anti-B serum. Under the standard ABO inheritance model, which genotype best explains the reaction pattern?
IᴬIᴮ, which produces both A and B antigens
ii, which produces neither A nor B antigen
Iᴬi, which produces the A antigen but not the B antigen
Iᴮi, which produces the B antigen but not the A antigen
2. Which statement best defines codominance?
One allele completely masks the expression of the other allele.
Both alleles in a heterozygote are distinctly expressed in the phenotype.
The heterozygote displays an intermediate blend of the two homozygous phenotypes.
Environmental conditions replace the effects of both inherited alleles.
3. A flower species has allele Cᴿ for red patches and Cᵂ for white patches. A CᴿCᵂ flower displays separate red and white patches. What phenotype would most reasonably be expected for CᴿCᴿ under this model?
A uniform intermediate pink color
White patches only
Separate red and white patches
Red patches only
4. In the MN blood-group system, Lᴹ and Lᴺ are codominant. Two LᴹLᴺ parents have a child. What is the probability that the child will have the MN phenotype?
50%
25%
75%
0%
5. In a simplified cattle example, RR animals have red hair and WW animals have white hair. If RW animals are roan, which observation most directly supports codominance?
Every hair is a uniform pale-pink color.
Hair color changes from red to white as the animal ages.
The white-hair allele is present but produces no visible effect.
Distinct red and white hairs are both present in the coat.
6. A molecular assay measures RNA made from two distinguishable alleles in a heterozygous cell. Which result is most consistent with codominant expression at the measured level?
No RNA from either allele is detected.
Only one allele's RNA is detected in every sample.
RNA products from both inherited alleles are detected.
A third, blended allele appears in place of the inherited alleles.
7. Why is codominance important when connecting genotype to phenotype?
It proves that every gene has exactly two alleles in a population.
It shows that a heterozygote can visibly or measurably express products associated with both alleles.
It means inherited traits are unaffected by environmental conditions.
It guarantees that heterozygotes are healthier than homozygotes.
8. A person with genotype IᴬIᴮ has blood type AB. What makes this a standard example of codominance?
Both A and B antigens are expressed on the red blood cells.
The A allele changes into the B allele during development.
The two alleles blend to produce a new antigen called AB.
Neither allele affects the red blood cells because they cancel each other.
9. The ABO blood-group gene has Iᴬ, Iᴮ, and i alleles in the population. How many of these alleles can one person normally carry at that gene location?
One, because only one allele can be active at a time
Four, because each parent contributes two alleles
All three, because all are present in the population
At most two, one inherited from each parent
10. In a simplified chicken model, BB birds are black, WW birds are white, and BW birds have distinct black and white feathers. What offspring are expected from a BW × BB cross?
All BW with black and white feathers
Half BW with black and white feathers and half BB with black feathers
Half WW with white feathers and half BB with black feathers
One-quarter BB, one-half BW, and one-quarter WW
11. Two traits are controlled by different two-allele genes. At Gene 1, an A₁A₂ heterozygote shows only trait A₁. At Gene 2, a B₁B₂ heterozygote distinctly shows both B₁ and B₂ traits. Which comparison is correct?
Both genes show incomplete dominance.
Gene 1 shows codominance, while Gene 2 shows complete dominance.
Gene 1 shows complete dominance, while Gene 2 shows codominance.
Both genes show complete dominance.
12. Using the standard ABO inheritance model, what blood types can children inherit from one parent with genotype IᴬIᴮ and another with genotype ii?
AB or O only
A or B only
A, B, AB, or O
AB only
13. How does incomplete dominance usually differ from codominance in a heterozygote?
Incomplete dominance requires three alleles, whereas codominance requires two.
Incomplete dominance occurs only in plants, whereas codominance occurs only in animals.
Incomplete dominance prevents either allele from being inherited by offspring.
Incomplete dominance produces an intermediate phenotype rather than distinct expression of both traits.
14. A cross produces an F1 generation that distinctly displays both parental traits. When F1 individuals are crossed, the F2 phenotypes occur in a 1:2:1 ratio: one parental type, both traits, and the other parental type. Which model best fits these observations?
A trait determined only by the offspring's environment
Complete dominance of the first parental allele
Codominance between the two alleles
Inheritance in which neither parent contributes an allele
15. Researchers want to distinguish codominance from incomplete dominance in patterned insects. Which finding would offer the clearest evidence for codominance?
Heterozygotes display both homozygous color patterns in distinct areas.
Heterozygotes have a color midway between the two homozygous colors.
Heterozygotes survive longer than either homozygote.
One homozygous color appears in every heterozygote while the other is hidden.
16. Black-feathered birds have genotype BB, white-feathered birds have WW, and BW birds show distinct black and white feathers. If a BB bird is crossed with a WW bird, what is expected among their offspring?
Half BB and half WW
One-quarter BB, one-half BW, and one-quarter WW
All BW with both black and white feathers
All birds with uniformly gray feathers