Codominance Calculator
Cross two parents for a codominant gene, where heterozygotes show both alleles fully rather than one masking the other, to see the genotype and phenotype ratios. Two 3D diagrams compare a homozygous roan-cattle-style cross to a heterozygous one, and charts break down the resulting phenotype distribution.
Reviewed by the ToolNestr Editorial Team — July 2026
Roan cattle coat color example
1. Homozygous red (R¹R¹)
All hair-color spheres are red — only one allele type present, only one pigment produced.
2. Heterozygous roan (R¹R²)
Red AND white spheres both present and distinct — both alleles' pigments show up fully, side by side, not blended.
Codominance charts
How it works
The core idea in one line: codominance means a heterozygote doesn't hide either allele behind the other — both gene products get made and both show up fully and separately in the phenotype, so the underlying 1:2:1 genotype ratio translates directly into a matching 1:2:1 phenotype ratio.
R¹R² × R¹R² → 1 R¹R¹ : 2 R¹R² : 1 R²R²
genotype ratio — identical math to any monohybrid cross
1:2:1 genotype → 1:2:1 phenotype
for codominance, each genotype is its own distinct, fully visible phenotype
The Punnett square mechanics for a codominant gene are identical to any other monohybrid cross — gametes combine according to the law of segregation, producing the same 1:2:1 genotype ratio as Aa × Aa. The only difference is interpretation: instead of one allele's product masking the other's, both alleles' gene products (like both red and white hair pigments, or both blood-type antigens) are produced and visible simultaneously in the heterozygote, giving it its own unique, fully distinct phenotype rather than resembling either homozygous parent.
Worked example 1 — roan cattle coat color
Given: Cross two roan cattle (R¹R², heterozygous for coat color, where R¹=red hair allele and R²=white hair allele, both codominant).
Unlike simple dominance, the roan phenotype is not "hidden" — it's a fully distinct, visibly mixed-hair-color phenotype that only heterozygotes show.
Worked example 2 — human ABO blood type (IA vs IB)
Given: A parent with genotype IAIB (blood type AB) crosses with another IAIB parent.
IAIB individuals show type AB blood — both A and B antigens present simultaneously on red blood cells — a textbook demonstration of codominance in humans.
Codominance vs simple dominance vs incomplete dominance
The same 1:2:1 genotype ratio from Aa × Aa produces very different phenotype outcomes depending on the inheritance pattern.
| Pattern | Heterozygote phenotype | Phenotype ratio |
|---|---|---|
| Simple dominance | Same as dominant homozygote | 3 : 1 |
| Codominance ★ | Both traits shown fully, separately | 1 : 2 : 1 |
| Incomplete dominance | Blended intermediate trait | 1 : 2 : 1 |
★ Reference row. Codominance and incomplete dominance share the same 1:2:1 ratio numerically, but the heterozygote phenotype looks completely different — distinct-and-separate versus blended.
Where codominance actually matters
🩸 Blood typing and transfusion medicine
Understanding the codominant relationship between IA and IB alleles (producing type AB blood) is essential for blood banks and transfusion compatibility testing.
🐄 Cattle breeding for coat color
Cattle breeders use codominance predictions to anticipate roan, red, or white coat outcomes when planning breeding pairs.
🧬 Immunogenetics research
Many immune system genes (like HLA genes) show codominant expression, where an individual expresses proteins from both inherited alleles simultaneously — important in organ transplant matching.
🎓 Teaching non-Mendelian inheritance
Codominance is one of the clearest ways to teach students that not all inheritance follows simple dominant/recessive rules, broadening their understanding beyond basic Punnett squares.
Common misconceptions
"Codominance and incomplete dominance are the same thing."
They produce the same 1:2:1 ratio numerically but look completely different — codominant heterozygotes show BOTH traits fully and distinctly (like red and white hairs both visible), while incomplete-dominance heterozygotes show a single BLENDED intermediate trait (like pink).
"In codominance, one allele is still slightly stronger than the other."
By definition, codominant alleles are expressed completely equally — neither one is stronger or weaker; both are fully and independently visible in the heterozygote's phenotype.
"AB blood type means a mix or dilution of A and B blood types."
AB blood type means a person has BOTH the A antigen AND the B antigen fully present on their red blood cells simultaneously — it is not a diluted, weakened, or blended version of either.
"Codominance changes how many genotypes are possible from a cross."
The genotype possibilities and their ratios are determined purely by Mendelian segregation, exactly the same as any other cross — codominance only changes how those genotypes map onto observable phenotypes.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Biology 2e — Chapter 12, "Mendel's Experiments and Heredity" (free, peer-reviewed). openstax.org
- • Klug, Cummings & Spencer, Concepts of Genetics — Chapter 4, Extensions of Mendelian Genetics.
- • Campbell & Reece, Biology — Chapter 14, Mendel and the Gene Idea (Extending Mendelian Genetics).
Underlying genotype math is identical to standard Mendelian segregation — only phenotype interpretation differs for codominant alleles. Results are exact probability ratios.
How to use this calculator
Enter both parent genotypes
Use two distinct allele labels for the codominant gene (e.g. R1/R2 or A/B).
View the cross grid
The full grid shows every possible offspring genotype combination.
Read the phenotype ratio
Each genotype maps to its own distinct, fully-visible phenotype.
Related tools
Frequently asked questions
What is codominance?
Codominance is an inheritance pattern where both alleles in a heterozygote are fully and separately expressed, rather than one dominant allele masking the other. A classic example is roan cattle coat color, where a heterozygote shows both red and white hairs distinctly (not blended pink).
How is codominance different from incomplete dominance?
In codominance, both alleles show up fully and separately (like red AND white hairs in a roan). In incomplete dominance, the two alleles blend into an intermediate phenotype (like pink flowers from red and white parents). Codominance keeps both traits distinct; incomplete dominance mixes them.
What is the classic human example of codominance?
ABO blood type is the textbook example — the IA and IB alleles are codominant with each other, so a person with genotype IAIB has type AB blood, showing both A and B antigens on their red blood cells simultaneously.
Does codominance change the genotype ratio from a normal cross?
No — the underlying genotype ratio from a cross (like 1:2:1 for Aa × Aa) is identical to any other inheritance pattern. Only the phenotype interpretation changes, since codominant heterozygotes get their own distinct, unmasked phenotype category instead of matching one of the homozygotes.
Why does codominance produce a 1:2:1 phenotype ratio instead of 3:1?
Because the heterozygote has its own unique, fully visible phenotype (unlike simple dominance, where it's hidden behind the dominant trait) — so all three genotypes (homozygous type 1, heterozygous, homozygous type 2) show up as three distinct phenotypes, matching the 1:2:1 genotype ratio exactly.