Genetics Tool· Non-Mendelian Inheritance

Codominance Punnett Square Examples and Practice

Use this codominance Punnett square calculator to combine parent alleles, label the heterozygote, and read the phenotype ratio. It works for roan cattle, MN blood group, sickle cell trait, simplified ABO examples, and teacher-made practice problems.

Codominance Punnett Square Calculator

Select an example or enter your own parent genotypes. Results update as you type.

Quick Examples

A classic animal example where red and white hairs appear together.

Example: RW, MN, AB, or HS

Use two alleles for each parent

RW×RW

4×4 grid  · 16 offspring combinations

RW
RW
RW
RW
RW
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RW
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RW
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RW
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs
RRWWRoan coat, red and white hairs

Roan coat, red and white hairs

100%

16/16

Phenotypic ratio:16(Roan coat, red and white hairs)
Roan cattle showing codominance with separate red and white hairs
Figure 1. Roan coat colour is a codominance example. Heterozygous animals have red hairs and white hairs on the same body. The two traits stay separate, so the heterozygote is not a blended intermediate.

What Is a Codominance Punnett Square?

A codominance Punnett square is a prediction grid for a trait where both alleles are visible in the heterozygote. The grid still follows Mendel's law of segregation. Each parent passes one allele into each gamete. The difference is the phenotype assigned to the heterozygous box.

In complete dominance, a heterozygote often looks like the dominant homozygote. In codominance, the heterozygote gets its own phenotype because both alleles are expressed. Roan cattle show red and white hairs. Type AB blood shows A and B antigens. MN blood group heterozygotes show M and N antigens.

RR

First homozygote

One visible phenotype

RW

Heterozygote

Both phenotypes together

WW

Second homozygote

The other visible phenotype

How to Solve a Codominance Punnett Square

  1. 1
    Write the parent genotypes: Use two allele symbols for the same gene. For roan cattle, red can be R and white can be W, so a roan animal is RW.
  2. 2
    Split each parent into gametes: A parent with genotype RW can make R gametes and W gametes. Put one parent across the top and the other down the side.
  3. 3
    Fill each box in the grid: Combine the top allele with the side allele in every cell. For RW by RW, the boxes are RR, RW, WR, and WW.
  4. 4
    Label the heterozygote correctly: RW and WR are codominant heterozygotes. They show both traits. Do not label them as red, white, or a blended colour.
  5. 5
    Count the ratio: A heterozygote by heterozygote codominance cross usually gives 1 RR, 2 RW, and 1 WW. The phenotype ratio is 1:2:1.

What Does a Codominance Punnett Square Look Like?

Most classroom codominance problems use a 2 by 2 grid. A roan by roan cattle cross is the clearest example because the two alleles stay easy to track.

GametesRW
RRR, redRW, roan
WWR, roanWW, white

This grid shows how to make a Punnett square for codominance. The heterozygote has its own label. That label is the key feature students must show in their answer.

Codominance Punnett Square Examples

Example 1: Roan Cattle Cross

Cross two roan cattle. Use R for red coat and W for white coat. Roan is RW because both red and white hairs are visible.

Cross: RW × RW
25% RR = red coat
50% RW or WR = roan coat
25% WW = white coat
Phenotype ratio: 1 : 2 : 1
This is the standard example of a codominance Punnett square. The middle class is roan, not pink or light red, because both coat colours appear as separate hairs.

Example 2: AB Blood Type as Codominance

In the ABO system, IA and IB are codominant. A person with genotype IAIB has type AB blood because both antigens are present.

The full ABO system has three alleles: IA, IB, and i. For a simple classroom codominance model, use A and B as the visible alleles. A by B can produce an AB heterozygote, and that heterozygote expresses both A and B antigens. For parent blood type probability, use the blood type calculator.

Example 3: MN Blood Group

The MN blood group is controlled by codominant M and N alleles. A heterozygote has both M and N antigens on red blood cells.

Cross: MN × MN
25% MM = M antigen only
50% MN or NM = M and N antigens
25% NN = N antigen only

Codominance Practice Worksheet with Answers

Use these short problems as classroom practice. Students should write the gametes, complete the grid, and then list genotype and phenotype percentages.

QuestionCrossAnswerPhenotype result
1RW by RW25% RR, 50% RW or WR, 25% WW1 red : 2 roan : 1 white
2RR by WW100% RWAll roan
3MN by NN50% MN, 50% NN1 MN blood group : 1 N blood group
4AB by AA50% AA, 50% AB1 A phenotype : 1 AB phenotype in the simplified model
5HS by HS25% HH, 50% HS, 25% SS1 normal haemoglobin : 2 carrier : 1 sickle haemoglobin class

Dihybrid Codominance Punnett Square

A dihybrid codominance Punnett square tracks two genes at the same time. The grid usually becomes 4 by 4 when both parents are heterozygous at both genes. The setup is the same as any dihybrid cross: make all possible gametes from each parent, place one parent on the top, place the other on the side, and combine alleles in each box.

The special part is phenotype scoring. For any codominant gene, the heterozygote must show both traits. For example, if gene one is coat colour with RW as roan and gene two is blood antigen with MN as M plus N, an offspring with genotype RWMN expresses both codominant pairs. For larger grids, a forked-line method can be easier than drawing every box.

Codominance vs Incomplete Dominance

Students often confuse these two inheritance patterns because both can produce a 1:2:1 phenotype ratio. The correct answer depends on the heterozygote.

FeatureCodominanceIncomplete dominance
HeterozygoteShows both phenotypesShows one blended phenotype
ExampleAB blood type or roan coatPink snapdragon flower
Middle phenotypeTwo traits togetherIntermediate trait
Ratio for RW by RW1:2:11:2:1
Main scoring ruleLabel heterozygote as bothLabel heterozygote as blended

For blended heterozygotes, use the incomplete dominance Punnett square. For a molecular reference on ABO antigens, see the NCBI Blood Groups and Red Cell Antigens resource.

Related Tools

Incomplete Dominance Punnett Square

Genetics

Compare codominance with crosses where heterozygotes show a blended phenotype.

Open Calculator

Blood Type Calculator

Genetics

Model ABO and Rh inheritance with parent blood types and probability results.

Open Calculator

Punnett Square Calculator

Genetics

Build standard monohybrid, dihybrid, and multihybrid Punnett squares.

Open Calculator

Frequently Asked Questions About Codominance Punnett Squares

What is a codominance Punnett square?
A codominance Punnett square is a grid that predicts offspring genotypes and phenotypes for a trait where both alleles are expressed in a heterozygote. It shows each parent gamete on the top and side, then combines those gametes in the boxes.
How to solve a codominance Punnett square?
Write the parent genotypes, split each genotype into gametes, place one parent across the top and the other down the side, fill each box, then count the genotype and phenotype classes. In a heterozygote by heterozygote codominance cross, the usual result is 1:2:1.
How to make a codominance Punnett square?
Make a codominance Punnett square by using two visible allele symbols, such as R and W for roan cattle or A and B for a simplified blood type example. Draw a 2 by 2 grid for a monohybrid cross, write gametes on the axes, and combine one allele from each parent in every cell.
What does a codominance Punnett square look like?
A common codominance Punnett square looks like a 2 by 2 grid. For RW by RW, the four boxes are RR, RW, WR, and WW. RR shows the first phenotype, WW shows the second phenotype, and RW or WR shows both phenotypes together.
How to show codominance in a Punnett square?
Show codominance by giving the heterozygote its own phenotype label. Do not call RW red or white. Label it roan because both red and white hairs are expressed. The heterozygote must show both traits, not a blend.
How to do a codominance Punnett square example?
For a roan cattle example, cross RW by RW. The gametes are R and W from each parent. The boxes are RR, RW, WR, and WW. The result is 25 percent red, 50 percent roan, and 25 percent white.
Can codominance be used in a dihybrid Punnett square?
Yes. A dihybrid codominance Punnett square follows the same gamete rule as any dihybrid cross, but at least one gene has a heterozygote phenotype where both alleles are visible. The grid is usually 4 by 4 when both parents are heterozygous for two genes.