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Dihybrid Cross — 9:3:3:1 Ratio

4×4 Punnett square. Two traits, independent assortment. AaBb×AaBb → 9:3:3:1 phenotype.

Concept Fundamentals
Dihybrid
4×4
Phenotype
9:3:3:1
Combinations
16
Assortment
Independent
Calculate Dihybrid Cross9:3:3:1 ratio

Why This Biology Metric Matters

Why: Dihybrid crosses demonstrate independent assortment. Two unlinked genes segregate independently.

How: 4×4 grid. Gametes: AB, Ab, aB, ab from each parent. Fill 16 cells. Phenotype 9 dom-dom : 3 dom-rec : 3 rec-dom : 1 rec-rec.

  • ●9:3:3:1 for AaBb×AaBb with dominance. Mendel's pea color and shape.
  • ●Independent assortment: genes on different chromosomes.
  • ●Linked genes deviate from 9:3:3:1.
Sources:NCBIKhan Academy

📋 Sample Examples

🌱 Pea Plant (Mendel's Classic)

Round/wrinkled seeds + Yellow/green color

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👤 Human Hair Traits

Curly/straight + Dark/light color

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🌸 Flower Color and Height

Purple/white flowers + Tall/short plants

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🐕 Animal Coat Traits

Black/brown coat + Solid/spotted pattern

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👁️ Eye Color and Hair Texture

Brown/blue eyes + Curly/straight hair

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Enter Parent Genotypes

🧬 Trait Information

👩 Mother's Genotypes

👨 Father's Genotypes

Results Summary

Dihybrid Cross Results

Seed Shape × Seed Color

Total Offspring

16

Possible combinations

Genotypic Ratio

AAbb:1

Phenotypic Ratio

1

4×4 Punnett Square

Mother's GametesAb
Ab
AAbb
Round / Green
6.3%

Genotype Distribution

GenotypeCountPercentagePhenotype
AAbb1/166.3%Round / Green

Phenotype Distribution

PhenotypeCountPercentageGenotypes
Round / Green1/166.3%AAbb

Visualizations

Genotype Distribution

Phenotype Distribution

Step-by-Step Calculation Breakdown

Step 1: Determine Parent Genotypes
Mother: AaBb
Father: AaBb
Step 2: Determine Possible Gametes
Mother's gametes for Trait 1 (Seed Shape): A
Mother's gametes for Trait 2 (Seed Color): b
Father's gametes for Trait 1 (Seed Shape): A
Father's gametes for Trait 2 (Seed Color): b
Step 3: Create Gamete Combinations
Mother can produce 1 types of gametes: Ab
Father can produce 1 types of gametes: Ab
Step 4: Construct 4×4 Punnett Square
Each cell represents one possible offspring genotype (1/16 probability)
Total possible offspring combinations: 1 × 1 = 16
Step 5: Count Genotypes
AAbb: 1/16 (6.3%)
Step 6: Determine Phenotypes
Round / Green: 1/16 (6.3%) - Genotypes: AAbb
Step 7: Calculate Ratios
Genotypic Ratio: AAbb:1
Phenotypic Ratio: 1

For educational use only. Always confirm dosages and care with a licensed veterinarian.

🧬 Biology Facts

📐

4×4 grid. 4 gamete types per parent. 16 offspring combinations.

— Structure

🧬

9:3:3:1 = both dom : A dom : B dom : both rec.

— Phenotype

📊

Independent assortment. Unlinked genes on different chromosomes.

— Law

⚖️

Mendel's second law. Dihybrid pea experiments.

— History

What is a Dihybrid Cross?

A dihybrid cross is a breeding experiment between two organisms that are heterozygous (or have different genotypes) for two different traits. This type of cross allows geneticists to study how two traits are inherited together and whether they follow Mendel's law of independent assortment.

🧬

Two Traits

Simultaneously tracks inheritance of two different genetic traits, each controlled by separate genes.

📊

4×4 Grid

Creates a 16-box Punnett square showing all possible offspring genotype combinations.

🎯

9:3:3:1 Ratio

Classic Mendelian ratio when both parents are heterozygous for both traits.

How to Create a 4×4 Punnett Square

Step 1: Determine Parent Genotypes

Identify the genotype of each parent for both traits. For example:

  • Mother: AaBb (heterozygous for both traits)
  • Father: AaBb (heterozygous for both traits)

Step 2: Determine Possible Gametes

Each parent can produce gametes (sperm or egg) containing one allele for each trait. For a heterozygous parent (AaBb), the possible gametes are:

  • AB, Ab, aB, ab (4 different gamete types)

This follows the law of independent assortment - alleles for different traits segregate independently during gamete formation.

Step 3: Set Up the Grid

Create a 4×4 grid where:

  • Top row: Father's 4 possible gametes
  • Left column: Mother's 4 possible gametes
  • Each cell: Combination of one maternal and one paternal gamete

Step 4: Fill in the Cells

For each cell, combine the alleles from the corresponding gametes:

  • Maternal gamete AB + Paternal gamete AB = Offspring genotype AABB
  • Maternal gamete Ab + Paternal gamete aB = Offspring genotype AaBb

Each cell represents 1/16 (6.25%) of the total possible offspring.

Step 5: Count Genotypes and Phenotypes

Count how many times each genotype appears, then determine the phenotype for each genotype:

  • Genotypes: AABB (1), AABb (2), AAbb (1), AaBB (2), AaBb (4), Aabb (2), aaBB (1), aaBb (2), aabb (1)
  • Phenotypes: Dominant/Dominant (9), Dominant/Recessive (3), Recessive/Dominant (3), Recessive/Recessive (1)

When Are Dihybrid Crosses Used?

🧬 Plant Breeding

Breeders use dihybrid crosses to predict offspring traits when selecting for multiple desirable characteristics, such as disease resistance and high yield.

🔬 Genetic Research

Scientists use dihybrid crosses to study gene linkage, test for independent assortment, and understand inheritance patterns.

👨‍⚕️ Medical Genetics

Genetic counselors use dihybrid crosses to predict the probability of offspring inheriting multiple genetic conditions.

📚 Education

Dihybrid crosses are fundamental to understanding Mendelian genetics and are commonly taught in biology courses.

Mendel's Law of Independent Assortment

The law of independent assortment states that alleles for different traits segregate independently of one another during gamete formation. This means that the inheritance of one trait does not influence the inheritance of another trait, as long as the genes are located on different chromosomes (or far apart on the same chromosome).

Key Points:

  • Applies when genes are on different chromosomes or far apart on the same chromosome
  • Each gamete receives one allele for each trait randomly
  • Results in the classic 9:3:3:1 phenotypic ratio for heterozygous × heterozygous crosses
  • Does NOT apply when genes are linked (close together on the same chromosome)

Understanding the 9:3:3:1 Ratio

When both parents are heterozygous for both traits (AaBb × AaBb), the offspring show a predictable phenotypic ratio of 9:3:3:1:

PhenotypeCountPercentageGenotypes
Dominant / Dominant956.25%AABB, AABb, AaBB, AaBb
Dominant / Recessive318.75%AAbb, Aabb
Recessive / Dominant318.75%aaBB, aaBb
Recessive / Recessive16.25%aabb

This ratio occurs because:

  • Each trait independently follows a 3:1 ratio (dominant:recessive)
  • When combined: (3:1) × (3:1) = 9:3:3:1
  • 9 = both dominant traits (3/4 × 3/4)
  • 3 = first dominant, second recessive (3/4 × 1/4)
  • 3 = first recessive, second dominant (1/4 × 3/4)
  • 1 = both recessive traits (1/4 × 1/4)
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