Incomplete dominance & codominance ratios (single-gene)
This calculator models a single-gene cross with two alleles (for example A and a).
The genetics mechanics are the same as a monohybrid cross, but the phenotype mapping is different:
in incomplete dominance and codominance, the heterozygote has its own phenotype label.
What changes compared with complete dominance?
In complete dominance, AA and Aa often share the same phenotype. Here, you usually treat all three genotypes as potentially distinct:
AA, Aa, and aa can each map to a different phenotype label.
That is why the calculator lets you edit phenotype labels for each genotype.
Definitions
Incomplete dominance: the heterozygote has an intermediate phenotype (example: red × white → pink).
Codominance: the heterozygote expresses both allele effects at the same time (example: roan coat, or “both colors present”).
The probability calculations do not change between the two models—only the phenotype interpretation does.
Assumptions used in this calculator
This is an intro-level model: one gene, two alleles, random fertilization, and no linkage, epistasis, lethality, or selection affecting offspring classes.
Each parent produces gametes according to its genotype, and offspring genotypes form by combining one gamete from each parent.
Step 1: Determine gametes from each parent
Each parent contributes one allele in a gamete. Gamete probabilities depend on the parent genotype:
| Parent genotype |
Gametes produced |
Gamete probabilities |
| AA |
A only |
\(P(A)=1\) |
| Aa |
A and a |
\(P(A)=\frac{1}{2}\), \(P(a)=\frac{1}{2}\) |
| aa |
a only |
\(P(a)=1\) |
Step 2: Build the 2×2 Punnett square
The Punnett square combines gametes from Parent 1 (rows) and Parent 2 (columns).
Each cell represents one fertilization outcome. In a standard 2×2 display, each cell is treated as
\(\frac{1}{4}\) of the outcomes (duplicated rows/columns appear when a parent is homozygous, but the visual stays 2×2 for consistency).
For each cell, the offspring genotype is the allele combination. If you show phenotypes inside cells,
the calculator also displays the phenotype label assigned to that genotype.
Step 3: Convert cell outcomes into genotype probabilities
Genotype probability is the fraction of Punnett cells that yield that genotype (out of 4 cells).
For a heterozygote cross \(Aa \times Aa\), the classic result is:
\[
\begin{aligned}
P(AA) &= \frac{1}{4} \\
P(Aa) &= \frac{2}{4}=\frac{1}{2} \\
P(aa) &= \frac{1}{4}
\end{aligned}
\]
You can also see the same result using multiplication inside cells (AND rule) and summing paths (OR rule):
\[
\begin{aligned}
P(AA) &= P(A)\cdot P(A)=\frac{1}{2}\cdot\frac{1}{2}=\frac{1}{4} \\
P(Aa) &= P(A)\cdot P(a) + P(a)\cdot P(A)
= \frac{1}{2}\cdot\frac{1}{2} + \frac{1}{2}\cdot\frac{1}{2}
= \frac{1}{2} \\
P(aa) &= P(a)\cdot P(a)=\frac{1}{2}\cdot\frac{1}{2}=\frac{1}{4}
\end{aligned}
\]
Step 4: Convert probabilities to ratios
The calculator reports ratios using the Punnett cell counts (out of 4) and then simplifies them.
For \(Aa \times Aa\), the genotype counts are \(1,2,1\), so the simplified genotype ratio is:
\[
\text{Genotype ratio }(AA:Aa:aa)=1:2:1
\]
In incomplete dominance and codominance, if your phenotype labels are distinct for AA, Aa, and aa,
the phenotype ratio matches the genotype ratio:
\[
\text{Phenotype ratio }(\phi_{AA}:\phi_{Aa}:\phi_{aa})=1:2:1
\]
If you intentionally give the same phenotype label to two genotypes (for example, if AA and Aa share a label),
then the calculator will still show genotype probabilities, but phenotype categories effectively merge in interpretation.
Quick reference examples
Example 1 (incomplete dominance): \(Aa \times Aa\) with AA = “Red”, Aa = “Pink”, aa = “White”
gives phenotype probabilities \(25\%\), \(50\%\), \(25\%\) and phenotype ratio \(1:2:1\).
Example 2 (codominance): \(AA \times aa\) always produces \(Aa\) offspring.
That means \(P(Aa)=1\) and phenotype ratio is \(0:1:0\) (all heterozygote phenotype).
How to read the calculator output
The calculator displays:
(1) genotype probabilities for AA, Aa, aa,
(2) phenotype probabilities using your labels,
(3) simplified ratios,
(4) a Punnett square showing genotype and/or phenotype in each cell,
and (5) a three-bar chart summarizing the phenotype distribution.
