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Punnett Square Calculator

Monohybrid cross — predict offspring genotype and phenotype probabilities.

Mother's Genotype
Father's Genotype
💡 Quick Summary

Predict offspring genotype and phenotype probabilities for a monohybrid (single-trait) cross. Enter parental genotypes using preset options or custom input, define your dominant and recessive alleles, and instantly see the Punnett square, genotypic/phenotypic ratios, and a probability chart.

📋 How to Use
  1. Select or type the mother's genotype (e.g. Aa) using the radio buttons or the custom input field.
  2. Select or type the father's genotype (e.g. Aa) using the radio buttons or the custom input field.
  3. Enter the dominant allele letter (e.g. A) and the recessive allele letter (e.g. a). They must be different.
  4. The Punnett square and all results update automatically as you type.
  5. Click Reset to clear all inputs and start a new cross.
🧮 Formulas & Logic
Genotypic ratio
Count of each unique offspring genotype across all 4 cells of the Punnett square
Phenotypic ratio
Dominant-expressing offspring (AA or Aa) : Recessive offspring (aa)
Probability
P(genotype) = number of matching cells ÷ 4 × 100%
📊 Result Interpretation
Homozygous Dominant (AA)

Both alleles are the dominant form. The organism expresses the dominant phenotype and passes only dominant alleles to offspring.

Heterozygous (Aa)

One dominant and one recessive allele. The organism expresses the dominant phenotype but is a carrier of the recessive allele.

Homozygous Recessive (aa)

Both alleles are the recessive form. The organism expresses the recessive phenotype.

3:1 phenotypic ratio

The classic result of an Aa × Aa cross — 75% dominant phenotype, 25% recessive phenotype.

🔬 Applications
  • Predicting the probability of inherited traits in plant and animal breeding
  • Genetic counselling — estimating the probability of offspring inheriting a recessive disorder
  • Teaching Mendelian genetics and the Law of Segregation
  • Determining carrier status probabilities in family pedigree analysis
  • Agricultural genetics — planning crop and livestock breeding programmes
⚠️ Common Mistakes & Warnings
Monohybrid crosses only

This calculator handles one gene with two alleles (monohybrid cross). It does not account for dihybrid crosses, codominance, incomplete dominance, sex-linked traits, or polygenic inheritance.

These are probabilities, not guarantees

The ratios shown are theoretical probabilities based on independent assortment. Actual offspring ratios in small litters can deviate significantly from these predictions.

Allele letters must differ

The dominant and recessive allele labels must be different characters. Using the same letter for both will prevent calculation.

❓ Frequently Asked Questions

What is a Punnett square?
A Punnett square is a diagram used to predict the genotype combinations possible in offspring from two parents. Each row represents a maternal allele and each column a paternal allele; each cell shows the resulting offspring genotype.
What is a monohybrid cross?
A monohybrid cross examines the inheritance of a single trait controlled by one gene with two alleles (e.g. tall vs. short pea plants). Gregor Mendel's foundational experiments were monohybrid crosses.
Why does an Aa × Aa cross give a 3:1 phenotypic ratio?
The four possible offspring are AA, Aa, Aa, and aa. AA and Aa both express the dominant phenotype (3 out of 4), while aa is the only recessive phenotype (1 out of 4), giving a 3:1 ratio.
What is the difference between genotype and phenotype?
Genotype is the actual combination of alleles an organism possesses (AA, Aa, or aa). Phenotype is the observable trait expressed — an organism with AA or Aa genotype shows the dominant phenotype, while only aa shows the recessive phenotype.
Can I use this for sex-linked traits?
No — this calculator assumes autosomal inheritance (traits not linked to sex chromosomes). Sex-linked traits require a different model where one parent carries only one allele at an X-linked locus.

Punnett square predicts offspring genotype probabilities for a monohybrid cross based on Mendel's Law of Segregation.