Punnett Square — Monohybrid Cross
2×2 grid. One trait, two alleles. Predict genotype and phenotype ratios. Mendelian inheritance.
Why This Biology Metric Matters
Why: Punnett squares predict offspring genotypes and phenotypes from parent alleles. Foundation of genetics education.
How: Place parent gametes on axes. Fill cells with allele combinations. Count genotypes (1:2:1 for Aa×Aa) and phenotypes (3:1 if dominant).
- ●Aa × Aa → AA:Aa:aa = 1:2:1 genotype, 3:1 phenotype (if A dominant).
- ●Test cross: A_ × aa reveals heterozygote vs homozygote.
- ●Named after Reginald Punnett (1905).
Punnett Square Calculator
2×2 Mendelian inheritance — predict genotypes and phenotypes.
Sample Scenarios — Click to Load
Inputs
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Punnett Square Results
Trait Inheritance
Homozygous Dominant × Homozygous Dominant
All offspring will be homozygous dominant (AA). All will express the dominant trait.
Genotypic Ratio
Phenotypic Ratio
Carrier Probability
Dominant Phenotype
2×2 Punnett Square
Mother A A | A | A |
|---|---|---|
Father A | AA Dominant 25% | AA Dominant 25% |
A | AA Dominant 25% | AA Dominant 25% |
Genotype Probabilities
Homozygous Dominant (AA)
All 4 offspring
Heterozygous (Aa)
0 out of 4 offspring
Homozygous Recessive (aa)
0 out of 4 offspring
Phenotype Probabilities
Dominant (Dominant)
All 4 offspring
Recessive (Recessive)
0 out of 4 offspring
Visual Analysis
Genotype Distribution
Phenotype Probability
Calculation Breakdown
Mother's genotype: AA
Father's genotype: AA
Mother's alleles: A and A
Father's alleles: A and A
Place mother alleles on top, father alleles on left
Fill each cell by combining alleles from row and column
AA: 4 out of 4 (100%)
Aa: 0 out of 4 (0%)
aa: 0 out of 4 (0%)
Dominant (Dominant): 4 out of 4 (100%)
Recessive (Recessive): 0 out of 4 (0%)
Genotypic ratio: 1
Phenotypic ratio: 1:0
Carrier probability: 0%
Educational Disclaimer: This calculator demonstrates basic Mendelian inheritance patterns using a 2×2 Punnett square. Real-world genetics is more complex, involving multiple genes, incomplete dominance, codominance, and environmental factors. This tool is for educational purposes only and should not be used for medical or genetic counseling decisions. Always consult with a genetic counselor or healthcare professional for personalized genetic information.
For educational use only. Always confirm dosages and care with a licensed veterinarian.
🧬 Biology Facts
2×2 grid for one gene. Rows/cols = gametes from each parent.
— Structure
Aa×Aa → 1 AA : 2 Aa : 1 aa. Genotype ratio.
— Genotype
3:1 phenotype if complete dominance. 1:2:1 if codominance.
— Phenotype
Mendel's pea experiments. Law of segregation.
— History
What is a Punnett Square?
A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. Named after Reginald Punnett, this tool visualizes how alleles (gene variants) are inherited from parents to offspring. It's fundamental to understanding Mendelian inheritance patterns.
Predictive Tool
Shows all possible genetic combinations from parent crosses, helping predict offspring traits.
Probability Calculation
Calculates the probability of each genotype and phenotype appearing in offspring.
Educational Foundation
Essential for understanding basic genetics, inheritance patterns, and genetic counseling.
How Mendelian Inheritance Works
1. Alleles and Genes
Each gene has two alleles (one from each parent). Dominant alleles (A) mask recessive alleles (a) in phenotype expression.
2. Genotype vs Phenotype
Genotype: The genetic makeup (AA, Aa, aa). Phenotype: The observable trait (dominant or recessive expression).
3. The 2×2 Square
A 2×2 Punnett square shows all four possible combinations when each parent contributes one allele. Each cell represents a 25% probability.
4. Ratios and Probabilities
Count genotypes and phenotypes to determine ratios (e.g., 1:2:1 for AA:Aa:aa, or 3:1 for dominant:recessive phenotypes).
Understanding the Formula
The 2×2 Punnett square combines each parent's gametes (one allele per parent). Each cell = 25% probability. Count identical genotypes for ratios (e.g., 1 AA : 2 Aa : 1 aa → 1:2:1).
Formulas
- Genotypic ratio = count(AA) : count(Aa) : count(aa)
- Phenotypic ratio = count(dominant) : count(recessive)
- Carrier probability = P(Aa) among offspring
Key Considerations
- Assumes simple Mendelian (one gene, two alleles, complete dominance)
- Real genetics: incomplete dominance, codominance, multiple genes
- Carrier = heterozygous (Aa) expressing dominant phenotype
- For genetic counseling, consult a professional
When to Use Punnett Squares
Single Trait Inheritance
Predicting inheritance of one gene with two alleles
Genetic Counseling
Understanding risk of genetic disorders in offspring
Breeding Programs
Planning crosses to achieve desired traits in plants/animals
Educational Purposes
Teaching basic genetics and inheritance patterns
Carrier Testing
Determining probability of being a carrier for recessive disorders
Pedigree Analysis
Understanding family inheritance patterns
Genotype vs Phenotype Explanation
| Genotype | Description | Phenotype Expression |
|---|---|---|
| AA | Homozygous Dominant | Expresses dominant trait |
| Aa | Heterozygous (Carrier) | Expresses dominant trait, carries recessive allele |
| aa | Homozygous Recessive | Expresses recessive trait |
Dominant vs Recessive Traits
Dominant Traits (A)
- • Expressed when at least one allele is present (AA or Aa)
- • Masks the recessive allele in phenotype
- • Examples: Tongue rolling, widow's peak, detached earlobes
- • Represented by uppercase letter (A)
Recessive Traits (a)
- • Only expressed when both alleles are recessive (aa)
- • Hidden when dominant allele is present
- • Examples: Cystic fibrosis, attached earlobes, inability to roll tongue
- • Represented by lowercase letter (a)
Common Genetic Traits Examples
| Trait | Dominant | Recessive |
|---|---|---|
| Tongue Rolling | Can roll tongue | Cannot roll tongue |
| Earlobe Attachment | Detached earlobes | Attached earlobes |
| Hairline | Widow's peak | Straight hairline |
| Cystic Fibrosis | Unaffected | Affected |
| Flower Color | Red flowers | White flowers |
Best Practices
- Use uppercase (A) for dominant, lowercase (a) for recessive
- Order genotypes consistently: AA, Aa, aa
- Label phenotypes clearly (e.g., Red vs White for flower color)
- Educational use only—not for medical/genetic counseling
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