Incomplete Dominance Calculator
Cross two parents for a gene showing incomplete dominance, where the heterozygote blends into an intermediate phenotype rather than matching either parent, to see the genotype and phenotype ratios. Two 3D diagrams compare red and white homozygous flowers to a blended pink heterozygote, and charts show the resulting 1:2:1 phenotype ratio.
Reviewed by the ToolNestr Editorial Team — July 2026
Red, white, and blended pink
1. Homozygous red and white parents
Pure red (RR) and pure white (rr) — each produces only its own pigment.
2. Heterozygous pink offspring (Rr)
A single blended pink color — the true intermediate, not a mix of separate red and white patches.
Incomplete dominance charts
How it works
The core idea in one line: when neither allele can fully mask the other, a heterozygote ends up expressing an intermediate amount of whatever the gene product does — half as much red pigment as a homozygote, for instance — producing a visibly blended trait rather than one that matches either parent.
RR × rr → all Rr (pink)
a cross between the two homozygous extremes produces uniformly intermediate offspring
Rr × Rr → 1 RR : 2 Rr : 1 rr
genotype ratio matches phenotype ratio exactly: 1 red : 2 pink : 1 white
The Punnett square mechanics behind incomplete dominance are identical to ordinary Mendelian segregation, producing the same 1:2:1 genotype ratio as any Rr × Rr cross. What differs is the phenotype mapping: instead of one allele fully masking the other's contribution, both alleles partially contribute to the same trait (like pigment production), so a heterozygote ends up with an intermediate dose — literally half the pigment of a homozygous red flower and twice that of a homozygous white one — producing the visibly blended pink phenotype.
Worked example 1 — crossing red and white flowers (RR × rr)
Given: A homozygous red-flowered snapdragon (RR) is crossed with a homozygous white-flowered one (rr).
This is the classic experiment (in snapdragons and four o'clock flowers) that first revealed inheritance isn't always simple dominant/recessive — every single offspring shows the blended pink trait.
Worked example 2 — crossing two pink (Rr) offspring
Given: Two of the pink F1 snapdragons (Rr) are crossed with each other.
Unlike simple dominance (which would give 3:1), incomplete dominance produces a 1:2:1 ratio because the heterozygote has its own unique, visible phenotype rather than being hidden behind a dominant trait.
Incomplete dominance vs simple dominance outcomes
Both start from the identical Rr × Rr genotype cross — only the phenotype interpretation differs.
| Pattern | Rr phenotype | Resulting ratio |
|---|---|---|
| Simple dominance | Same as RR (dominant) | 3 : 1 |
| Incomplete dominance ★ | Blended intermediate (pink) | 1 : 2 : 1 |
★ Reference row (worked example 2). The genotype math never changes — only whether the heterozygote's phenotype matches a homozygote or forms its own new, visible category.
Where incomplete dominance actually matters
🌸 Ornamental flower breeding
Flower breeders exploit incomplete dominance deliberately, crossing pure red and white varieties to reliably produce pink hybrids for the ornamental plant market.
🧬 Human cholesterol genetics
Familial hypercholesterolemia shows incomplete dominance — heterozygotes have an intermediate LDL cholesterol receptor function between the two homozygous extremes, with correspondingly intermediate cholesterol levels.
🐔 Poultry feather color
Andalusian chicken feather color is a classic textbook example — black × white parents produce blue-grey (blended) offspring, illustrating incomplete dominance in a way students can visually verify.
🎓 Teaching non-Mendelian inheritance patterns
Incomplete dominance is often the first non-Mendelian pattern students learn, helping build the broader understanding that not every trait follows simple dominant/recessive rules.
Common misconceptions
"Incomplete dominance means the dominant allele is only partly dominant."
Neither allele in incomplete dominance is truly "dominant" in the Mendelian sense at all — both alleles contribute equally and additively to the phenotype, producing a genuinely intermediate blended trait rather than a partially-expressed dominant one.
"Crossing two pink flowers (Rr × Rr) will only ever produce more pink flowers."
Rr × Rr actually produces a 1:2:1 ratio of red, pink, and white offspring — only half of the offspring are expected to be pink; the other half split evenly between the two original homozygous colors.
"Incomplete dominance and codominance are the same phenomenon."
They share the same underlying 1:2:1 genotype ratio, but incomplete dominance blends both alleles into ONE new intermediate trait, while codominance keeps both alleles' traits fully separate and simultaneously visible — the visual result is completely different even though the math matches.
"The blended phenotype means genetic material from the parents literally mixed together."
The alleles themselves stay completely distinct and unchanged (particulate inheritance) — only the resulting protein/pigment output blends into an intermediate phenotype. The genes themselves are never physically blended or diluted.
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. Results are exact probability ratios.
How to use this calculator
Enter both parent genotypes
Use R/r style notation for the two alleles of the incompletely-dominant gene.
View the cross grid
The full grid shows every possible offspring genotype combination.
Read the blended phenotype ratio
Heterozygotes get their own intermediate phenotype category, distinct from either homozygote.
Related tools
Frequently asked questions
What is incomplete dominance?
Incomplete dominance is an inheritance pattern where the heterozygote shows a phenotype intermediate between the two homozygous parents, rather than matching one of them — like pink flowers resulting from a cross between red and white parents.
How is incomplete dominance different from codominance?
Incomplete dominance blends both alleles into a single intermediate trait (pink from red and white). Codominance keeps both traits fully separate and visible at once (like distinct red and white patches, not blended pink) — the underlying math is identical, only the visual outcome differs.
Why does RR × rr produce all pink offspring in incomplete dominance?
Because RR × rr always produces 100% Rr offspring (every child gets one R and one r), and under incomplete dominance, every Rr individual shows the blended intermediate phenotype — pink, in the classic flower-color example.
What happens when you cross two pink (Rr) flowers?
Rr × Rr follows the same 1:2:1 genotype ratio as any monohybrid cross, giving 1 RR (red) : 2 Rr (pink) : 1 rr (white) — since genotype and phenotype ratios match exactly under incomplete dominance.
Are there real human examples of incomplete dominance?
Yes — familial hypercholesterolemia is a human example, where heterozygotes show an intermediate level of blood cholesterol receptor function between the two homozygous extremes.