How to make a dihybrid Punnett square, step by step
Table of contents
A dihybrid Punnett square predicts the offspring of a cross that follows two genes. It’s a 4 × 4 grid with 16 boxes. Every box is one equally likely combination of a gamete from each parent. Here’s a method that works every time.
Step 1: Write each parent’s genotype
Use one letter per gene. Upper case is the dominant allele and lower case is the recessive one. For this example we’ll cross AaBb × AaBb.
Step 2: Find each parent’s gametes with FOIL
A gamete carries one allele of each gene. To list them, treat the genotype like two pairs, (Aa)(Bb), and use FOIL:
| Step | Pair | Gamete |
|---|---|---|
| First | A + B | AB |
| Outer | A + b | Ab |
| Inner | a + B | aB |
| Last | a + b | ab |
A homozygous gene gives the same allele in each pair. An AABb parent produces AB, Ab, AB, Ab, which is only two different gametes, each with ½ probability. Keep all four when you draw the grid so every box stays equally likely.
Step 3: Draw the grid
Write one parent’s four gametes across the top and the other parent’s down the side.
| AB | Ab | aB | ab | |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
Step 4: Fill each box
Combine the row gamete and the column gamete. Group the alleles by gene and write the dominant one first. So Ab + aB becomes AaBb, not AbaB. Most mistakes happen at this step.
Step 5: Count genotypes
| Genotype | Count |
|---|---|
| AABB | 1 |
| AABb | 2 |
| AAbb | 1 |
| AaBB | 2 |
| AaBb | 4 |
| Aabb | 2 |
| aaBB | 1 |
| aaBb | 2 |
| aabb | 1 |
There are nine genotypes in a 1:2:1:2:4:2:1:2:1 ratio.
Step 6: Count phenotypes
Any box with at least one A shows the dominant first trait. Any box with at least one B shows the dominant second trait.
- A_B_ (both dominant): 9
- A_bb: 3
- aaB_: 3
- aabb (both recessive): 1
That’s 9:3:3:1.
Common mistakes
- Putting two alleles of the same gene in one gamete.
Aais never a gamete. Each gamete has one A-gene allele and one B-gene allele. - Dropping duplicate gametes. If a parent is homozygous for one gene, still use four headers so the 16 boxes stay equally likely.
- Mixing up genotype and phenotype ratios.
AABBandAaBbhave different genotypes but the same phenotype. - Applying it to linked genes. The square assumes independent assortment. If the genes are close together on one chromosome, you need recombination frequencies.
Check your answer
Pick the same genotypes in the dihybrid cross calculator and compare its square with yours, box by box. Want the theory first? Read dihybrid cross explained simply.