Dihybrid Cross Punnett Square- Complete Examples and Guide
What is a Dihybrid Cross?
A dihybrid cross tracks two different traits at the same time. Instead of watching just one gene (like flower color), you're watching two genes together—like plant height and seed shape.
Each parent carries two alleles for each gene. When you combine them, you get a 4x4 Punnett square with 16 possible offspring combinations.
This is where most students start drowning. Don't. It's just two monohybrid crosses happening at once.
Dihybrid vs Monohybrid: The Difference
A monohybrid cross looks at one trait. One gene. Four boxes. Simple.
A dihybrid cross looks at two traits. Two genes. Sixteen boxes. More complex, but the logic is identical—you're just doing it twice.
| Feature | Monohybrid Cross | Dihybrid Cross |
|---|---|---|
| Traits tracked | 1 | 2 |
| Punnett square size | 2x2 (4 boxes) | 4x4 (16 boxes) |
| Possible gametes | 2 | 4 |
| Genotypic ratios | 1:2:1 | 9:3:3:1 |
How to Determine Gametes for a Dihybrid Cross
Before you draw anything, you need to know what gametes each parent can produce.
Use FOIL to find allele combinations:
- First allele from gene 1 + First allele from gene 2
- Outer allele from gene 1 + Outer allele from gene 2
- Inner allele from gene 1 + Inner allele from gene 2
- Last allele from gene 1 + Last allele from gene 2
Example: Parent genotype is TtYy
Alleles for T gene: T, t
Alleles for Y gene: Y, y
Your four possible gametes: TY, Ty, tY, ty
Every gamete must have one allele from each gene. No exceptions.
Step-by-Step Dihybrid Cross Example
Let's cross two pea plants:
Parent 1: Round seeds, Yellow color (heterozygous for both) → RrYy
Parent 2: Round seeds, Yellow color (heterozygous for both) → RrYy
R = Round (dominant), r = Wrinkled (recessive)
Y = Yellow (dominant), y = Green (recessive)
Step 1: Determine Gametes
Both parents are RrYy, so both produce: RY, Ry, rY, ry
Step 2: Set Up the Punnett Square
Write one parent's gametes across the top (4 columns). Write the other parent's gametes down the side (4 rows).
Step 3: Fill in the Boxes
Combine alleles from the column header with the row header in each box.
| RY | Ry | rY | ry | |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
Understanding Genotype vs Phenotype Ratios
From that cross, you get the classic 9:3:3:1 ratio.
Phenotypic Ratio (what they look like):
- 9 — Round, Yellow (dominant traits show)
- 3 — Round, Green (one dominant, one recessive)
- 3 — Wrinkled, Yellow (one recessive, one dominant)
- 1 — Wrinkled, Green (both recessive)
Genotypic Ratio (their genetic makeup):
This one is messier. You get 9 different genotypes:
- 1 RRYY
- 2 RRYy
- 2 RrYY
- 4 RrYy
- 1 RRyy
- 2 Rryy
- 1 rrYY
- 2 rrYy
- 1 rryy
The phenotypic ratio is what teachers usually ask for. Memorize 9:3:3:1—it applies to every dihybrid cross between two heterozygotes.
Common Mistakes to Avoid
❌ Only putting one allele per gamete. Every gamete needs one allele from EACH gene. A gamete can't be "R" alone—it must be "Ry" or "RY."
❌ Confusing genes with alleles. You have 2 genes (R and Y), but 4 alleles total in a heterozygote (R, r, Y, y).
❌ Forgetting that dominant alleles hide recessive ones. A plant with genotype "Rryy" has round seeds (R is dominant) and green seeds (yy means green).
❌ Writing gametes incorrectly. If a parent is RRYY, it can only produce one gamete type: RY. Homozygous genes don't create variety in gametes.
Quick Reference: Gamete Combinations by Parent Genotype
| Parent Genotype | Possible Gametes |
|---|---|
| AABB | AB (only one—homozygous for both) |
| AABb | AB, Ab |
| AaBB | AB, aB |
| AaBb | AB, Ab, aB, ab (all four) |
| aaBb | aB, ab |
| Aabb | Ab, ab |
When both genes are heterozygous (AaBb), you get 4 different gametes. When either gene is homozygous, you get fewer combinations.
Practice: Try One Yourself
Cross: TtBB × Ttbb
T = Tall (dominant), t = short (recessive)
B = Brown (dominant), b = white (recessive)
Parent 1 (TtBB) gametes: TB, tB
Parent 2 (Ttbb) gametes: Tb, tb
Your Punnett square is 2×2 with 4 boxes. Offspring genotypes: TTBB, TTbb, ttBB, ttbb, TtBB, Ttbb (twice).
Phenotypic ratio: 3 Tall/Brown : 1 Short/White
Notice the ratio changed. That's because Parent 2 is homozygous for the b allele. The classic 9:3:3:1 only appears when both parents are heterozygous for BOTH genes.
Getting Started: Your Dihybrid Cross Checklist
- Identify the two genes and their dominant/recessive relationships
- Write out each parent's genotype using the gene letters
- Determine each parent's possible gametes using FOIL (one allele per gene in each gamete)
- Draw your 4×4 grid with gametes on axes
- Fill every box by combining row and column gametes
- Count phenotypes to find your ratio
- Count genotypes if the problem asks for it
That's it. No magic. Just systematic work.