Punnett Square Calculator (Simple Dominance)
This calculator models a monohybrid cross for a single autosomal gene with complete dominance. Enter the genotype of each parent (homozygous dominant AA, heterozygous Aa, or homozygous recessive aa) and the calculator computes the probability that offspring will express the dominant phenotype versus the recessive phenotype.
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Formula
P(dominant) = 1 − P(aa) = 1 − p(a from P1) × p(a from P2)
The probability of receiving a recessive allele from a parent equals (number of recessive alleles that parent carries) divided by 2. P(aa offspring) is the product of both parents' probabilities of donating a recessive allele. The probability of expressing the dominant phenotype is 1 minus the probability of being homozygous recessive.
How to use the Punnett Square Calculator (Simple Dominance)
- 1
Enter your parent 1 dominant alleles (0=aa, 1=aa, 2=aa)
- 2
Enter your parent 2 dominant alleles (0=aa, 1=aa, 2=aa)
- 3
Read your results instantly
Results update in real time as you type.
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How a Punnett square works
A Punnett square is a grid-based method for predicting the genotypic and phenotypic ratios of offspring from a cross between two parents. Each parent contributes one allele per gene to each offspring, and each allele is donated independently (Mendel's Law of Segregation). For a single gene, the square has two rows (one allele from each parent on the top) and two columns (one allele from each parent on the side), producing four cells representing all possible combinations. With simple dominance, any cell containing at least one dominant allele produces the dominant phenotype. The calculator encodes this logic: if a parent has 2 dominant alleles (AA), the probability of donating a recessive allele is 0; for Aa it is 0.5; for aa it is 1.
Limits of simple dominance
This calculator assumes complete dominance — one allele fully masks the other — which is the simplest model but not always biologically accurate. Many traits show incomplete dominance (heterozygote has an intermediate phenotype), codominance (both alleles are expressed, as in ABO blood type), or are influenced by multiple genes (polygenic traits). Environmental factors can also modify gene expression. For these more complex situations, this calculator's probabilities will not apply. Additionally, this model covers only autosomal (non-sex-linked) genes. For X-linked traits, males only have one copy of the X chromosome and the inheritance ratios differ between sexes.
Tips & Insights
Enter genotype as number of dominant alleles
Think of the input as a count: AA = 2 dominant alleles, Aa = 1 dominant allele, aa = 0 dominant alleles. This numerical encoding lets the formula compute probabilities directly without needing text input.
The classic Aa × Aa cross gives 75% dominant
When both parents are heterozygous (Aa × Aa), the expected offspring ratio is 1 AA : 2 Aa : 1 aa, meaning 75% express the dominant phenotype and 25% express the recessive phenotype. This is Mendel's 3:1 phenotypic ratio.
These are probabilities, not guarantees
For small litter sizes, observed ratios often deviate significantly from predicted probabilities. The predictions are most reliable for large populations or many crosses. A single litter of four pups from an Aa × Aa cross might all show the dominant phenotype by chance.
Worked Examples
Aa × Aa (both heterozygous parents)
75% dominant phenotype, 25% recessive — the classic 3:1 Mendelian ratio.
AA × aa (homozygous dominant × homozygous recessive)
100% dominant phenotype — all offspring are Aa (carriers) but express the dominant trait.
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Frequently Asked Questions
What does 'simple dominance' mean?
Simple or complete dominance means one allele (dominant, represented by a capital letter) completely masks the effect of the other allele (recessive, lowercase) in heterozygotes. A person with one copy of the dominant allele (Aa) looks the same as a person with two copies (AA).
How do I model a carrier × carrier cross?
Set both parents to 1 (Aa × Aa). The result will be 75% dominant phenotype and 25% recessive. Of the 75% with the dominant phenotype, one-third are AA and two-thirds are Aa carriers.
Can I use this for X-linked traits?
No. This calculator assumes autosomal inheritance. X-linked traits follow different rules because males are hemizygous — they have only one X chromosome. For X-linked recessive traits, affected males inherit the allele from carrier mothers, and the recessive phenotype is much more common in males.
What is the difference between genotype and phenotype ratios?
Genotype refers to the actual allele combination (AA, Aa, or aa); phenotype refers to the observable trait. In a monohybrid cross between two Aa parents, the genotype ratio is 1 AA : 2 Aa : 1 aa, but the phenotype ratio is 3 dominant : 1 recessive, because AA and Aa look identical under complete dominance.
What if both parents are homozygous for the same allele?
If both are AA (inputs: 2 and 2), all offspring are AA — 100% dominant. If both are aa (inputs: 0 and 0), all offspring are aa — 100% recessive. There is no variation when both parents are the same homozygote.
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