What is Incomplete Dominance and Codominance?
Incomplete dominance and codominance are two exceptions to simple Mendelian dominance, where neither allele fully masks the other. Together they explain why some traits blend or appear side by side instead of following a strict dominant/recessive pattern.
Incomplete dominance produces a blended intermediate phenotype (e.g., red × white → pink), while codominance shows both alleles fully and separately in the phenotype (e.g., AB blood type).
- •Heterozygote shows a blended, intermediate phenotype
- •Example: red (RR) × white (rr) snapdragon → pink (Rr)
- •Neither allele is fully dominant
- •Phenotype ratio in F2: 1:2:1 (matches genotype ratio)
- •Heterozygote shows both phenotypes fully and simultaneously
- •Example: AB blood type shows both A and B antigens
- •Both alleles are equally expressed, not blended
- •Classic case: human ABO blood group (IA and IB codominant)
Step-by-step worked examples
A red-flowered snapdragon (RR) is crossed with a white-flowered one (rr). The trait shows incomplete dominance. What color are the F1 offspring?
Incomplete dominance blends the two phenotypes RR × rr → all Rr offspring Rr phenotype = pink (intermediate between red and white)
Two pink snapdragons (Rr × Rr) are crossed. What phenotype ratio is expected in the offspring?
Genotypes: 1 RR : 2 Rr : 1 rr With incomplete dominance, phenotype follows genotype directly Phenotype ratio = 1 red : 2 pink : 1 white
A person with genotype IAIB has parents with blood types A (IAIA) and B (IBIB). What is the person's blood type, and why?
IA and IB are codominant — neither masks the other Both A and B antigens are expressed on red blood cells Blood type = AB
Flashcards
Quick quiz
Q1.A red (RR) and white (rr) snapdragon are crossed, producing pink (Rr) offspring. This is an example of:
Q2.AB blood type, where both A and B antigens are expressed, is an example of:
Q3.In incomplete dominance, what is the expected phenotype ratio of an Rr × Rr cross?
Q4.What distinguishes codominance from incomplete dominance?
The full card deck, worked steps and AI-tutor support for “What is Incomplete Dominance and Codominance?” are in Notek — study by hand before your exam.
Common mistakes
Calling pink snapdragons an example of codominance. — Correct: Pink is a blend, so it's incomplete dominance — codominance would show red and white patches or streaks together, not a blend.
Thinking AB blood type is a blend of A and B. — Correct: AB blood type expresses both A and B antigens fully and separately — that's codominance, not a blend.
Assuming incomplete dominance still gives a 3:1 phenotype ratio. — Correct: Because heterozygotes have their own distinct phenotype, incomplete dominance gives a 1:2:1 ratio, matching the genotype ratio.
Believing one allele is still 'dominant' in incomplete dominance. — Correct: In incomplete dominance, neither allele is dominant — both contribute partially to the blended phenotype.
FAQ
What is incomplete dominance?
A form of inheritance where the heterozygote shows a blended, intermediate phenotype, like pink flowers from red and white parents.
What is codominance?
A form of inheritance where the heterozygote shows both alleles' traits fully and separately, like AB blood type.
What are examples of incomplete dominance and codominance?
Snapdragon flower color (incomplete dominance) and human ABO blood type (codominance) are classic examples.
How do you tell incomplete dominance and codominance apart?
If the heterozygote phenotype is a blend (like pink), it's incomplete dominance; if both parental traits appear fully and separately (like AB), it's codominance.