About the Punnett Square Calculator
Punnett Square Calculator draws the Punnett square for a genetic cross and counts the results. Type the genotype of each parent as one pair of letters per gene, such as Aa for one gene or AaBb for two, using a capital letter for the dominant allele and a small letter for the recessive one. The page lists the gametes, fills the grid, and gives the number and percentage of each genotype and each phenotype, such as the classic 3 to 1 ratio for Aa × Aa and the 9:3:3:1 ratio for AaBb × AaBb.
Updated October 2026
How to use the Punnett Square Calculator
- 1Type the genotype of parent 1, for example Aa or AaBb.
- 2Type the genotype of parent 2 for the same genes.
- 3Read the Punnett square grid of offspring genotypes.
- 4Read the genotype and phenotype counts and percentages.
- 5Copy the result for your homework or notes.
Reading a Punnett square
Each parent makes gametes (egg or sperm cells) that carry one allele of each gene. The top of the square lists the gametes of one parent and the side lists the gametes of the other. Each cell combines the allele from its column with the allele from its row. Because every gamete is equally likely, every cell is equally likely, so counting cells gives the probabilities.
Common crosses to know
These results come up in almost every genetics course, and you can check each one with the calculator.
- AA × aa: all offspring are Aa and show the dominant trait
- Aa × Aa: 1 AA : 2 Aa : 1 aa, so 3 dominant to 1 recessive
- Aa × aa: half Aa and half aa, a test cross
- AaBb × AaBb: 9 : 3 : 3 : 1 for the four trait combinations
Check your work
When you do a square by hand, count the cells: a monohybrid cross has 4 and a dihybrid cross has 16. The percentages must add up to 100%. If your teacher asks for the ratio, divide each count by the smallest one and write them as integers. The page shows both counts and percentages so you can do that.
Related tools
Estimate blood groups with the Blood Type Calculator and eye colour with the Baby Eye Color Calculator. For populations rather than families use the Hardy-Weinberg Calculator, and to check chance in general try the Probability Calculator.
Features
- One-gene (monohybrid) and two-gene (dihybrid) crosses
- Full grid with gametes on both sides
- Genotype and phenotype ratios in counts and percent
- Complete dominance with capital and lowercase letters
- Clear messages for mistakes in the input
- Private: nothing is uploaded
Frequently asked questions
What is a Punnett square?
A grid that shows every combination of alleles that two parents can pass to their offspring, so you can read off the chances of each genotype and trait.
How do I write the genotypes?
Use two letters for each gene: capital for dominant and small for recessive. Aa is a carrier, AA is homozygous dominant and aa is homozygous recessive. For two genes write both pairs, such as AaBb.
What ratio does Aa × Aa give?
1 AA : 2 Aa : 1 aa for genotypes, which is 3 dominant to 1 recessive for the visible trait (75% and 25%).
What is a dihybrid cross?
A cross that follows two genes at once. For AaBb × AaBb the square has 16 cells and the traits appear in the ratio 9:3:3:1.
What is the difference between genotype and phenotype?
The genotype is the pair of alleles (Aa). The phenotype is the trait you can see, such as purple or white flowers.
Does it handle incomplete dominance or linked genes?
No. It assumes complete dominance and genes on different chromosomes that are inherited independently, which is what school courses use.
Why are some phenotype names generic?
The page does not know your trait, so it says dominant A or recessive a. Replace them with your trait names, such as tall and short.
Is my data stored?
No. The square is drawn in your browser.