Dihybrid Cross Punnett Square Calculator

Generate a 4x4 Punnett square for a dihybrid cross. Analyze genotype and phenotype ratios for two traits inherited simultaneously.

Trait Configuration

Trait 1 Name

Dominant Allele Letter (Trait 1)

Trait 2 Name

Dominant Allele Letter (Trait 2)

P1 Parent 1

Genotype for Trait 1

Genotype for Trait 2

P2 Parent 2

Genotype for Trait 1

Genotype for Trait 2

Cross Summary

4x4 Punnett Square

Genotype Ratios

Phenotype Ratios

Summary Statistics

What is a Dihybrid Cross?

A dihybrid cross is a genetic experiment that examines the inheritance of two different traits simultaneously. It tracks how alleles for two genes are passed from parents to offspring, revealing whether the traits are inherited independently (as predicted by Mendel's Law of Independent Assortment) or are linked.

In a dihybrid cross between two organisms that are heterozygous for both traits (AaBb x AaBb), the classic expected phenotype ratio in offspring is 9:3:3:1.

Understanding Genetics Terminology

Gene - A segment of DNA that codes for a specific trait.

Allele - A variant of a gene (e.g., A or a).

Genotype - The combination of alleles an organism has (e.g., AaBb).

Phenotype - The observable expression of the genotype.

Homozygous - Having two identical alleles (AA or aa).

Heterozygous - Having two different alleles (Aa).

Dominant - An allele that is expressed when present (shown as uppercase).

Recessive - An allele that is only expressed when homozygous (shown as lowercase).

Gamete - A reproductive cell (sperm or egg) containing one allele for each gene.

Mendel's Laws

Law of Segregation

Each parent has two alleles for each gene, and they separate during gamete formation so each gamete contains only one allele for each gene.

Law of Independent Assortment

Alleles for different genes are distributed independently during gamete formation (assuming genes are on different chromosomes). This is why a dihybrid cross is possible.

How to Construct a Dihybrid Punnett Square

Step 1: Identify the genotypes of both parents. Example: Parent 1 = AaBb, Parent 2 = AaBb.
Step 2: Determine possible gametes for each parent. Parent 1: AB, Ab, aB, ab. Parent 2: AB, Ab, aB, ab.
Step 3: Create a 4x4 grid with one parent's gametes across the top and the other down the side.
Step 4: Fill in each cell by combining the row gamete with the column gamete.
Step 5: Count and categorize the offspring genotypes and phenotypes.

The 9:3:3:1 Ratio

When crossing two double heterozygotes (AaBb x AaBb):

9/16 show both dominant traits (A_B_)
3/16 show dominant trait 1 and recessive trait 2 (A_bb)
3/16 show recessive trait 1 and dominant trait 2 (aaB_)
1/16 show both recessive traits (aabb)

This is the hallmark ratio of independent assortment.

Real-World Examples

Pea Plant Example (Mendel's Classic Experiment)

Trait 1: Seed shape (R = round, r = wrinkled)
Trait 2: Seed color (Y = yellow, y = green)
Cross: RrYy x RrYy
Expected: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Dog Coat Example

Trait 1: Coat color (B = black, b = brown)
Trait 2: Coat length (S = short, s = long)
Cross: BbSs x BbSs
Expected: 9 black short : 3 black long : 3 brown short : 1 brown long

When the 9:3:3:1 Ratio Doesn't Apply

Linked genes: Genes on the same chromosome may not assort independently.
Incomplete dominance: Heterozygotes show an intermediate phenotype.
Codominance: Both alleles are fully expressed.
Epistasis: One gene affects the expression of another.
Different parental genotypes: The 9:3:3:1 ratio only applies to AaBb x AaBb crosses.

Frequently Asked Questions

Q: What is a dihybrid cross?

A: A cross examining the inheritance of two different traits simultaneously, typically shown using a 4x4 Punnett square.

Q: What is the expected ratio for a dihybrid cross?

A: For AaBb x AaBb, the phenotype ratio is 9:3:3:1.

Q: How many gamete types can AaBb produce?

A: Four types: AB, Ab, aB, and ab (each with equal probability of 25%).

Q: What is the difference between a monohybrid and dihybrid cross?

A: A monohybrid cross examines one trait (2x2 Punnett square), while a dihybrid cross examines two traits (4x4 Punnett square).

Q: How do I know if I have a homozygous or heterozygous parent?

A: Homozygous has identical alleles (AA or aa). Heterozygous has different alleles (Aa). If unknown, test crosses with recessive individuals can reveal the genotype.