Punnett Square Calculator
Punnett square calculator — enter two parent genotypes (e.g. Aa × Aa or AaBb × AaBb) and get the Punnett square plus genotype and phenotype ratios. Curriculum-aligned.
Punnett Square Calculator
Enter two parent genotypes using gene letters — uppercase for a dominant allele, lowercase for recessive. e.g. Aa (monohybrid) or AaBb (dihybrid).
- Curriculum
- English (global) — Cambridge International + IB
- Built against
- Cambridge IGCSE Biology 0610 + IB Diploma (2023–2025) — Inheritance
- Unit system
- SI primary; US/imperial readout below
- First published
- 2 Jun 2026
- Last updated
- 13 Sep 2026
View authoritative scientific sources
- Mendel G. Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn 1866;4:3-47 — the original crosses
- Punnett RC. Mendelism. 2nd ed. Cambridge: Macmillan and Bowes; 1907 — the square's first appearance in print; it is not in the 1905 first edition (Edwards AWF. Punnett's square. Stud Hist Philos Biol Biomed Sci 2012;43(1):219-224)
- Urry LA et al. Campbell Biology. 12th ed. Pearson; 2020 — Mendelian genetics
- Griffiths AJF et al. An Introduction to Genetic Analysis. 7th ed. New York: W. H. Freeman; 2000 — patterns of inheritance
⚠️ Educational use only — see full disclaimer
EDUCATIONAL USE DISCLAIMER
This calculator is provided for educational and reference purposes only. It is not a substitute for instruction from a qualified teacher, your prescribed textbook, or your school's official curriculum materials.
When preparing for examinations, always cross-check our calculations and notation against your current syllabus and your teacher's guidance. Syllabus conventions and accepted notation vary between curricula and may change between examination years.
If you believe any calculation, notation, or curriculum reference in this tool is inaccurate, please let us know via the feedback button. We review feedback promptly and update tools when verified corrections are needed.
RECATOOLS accepts no liability for academic, examination, professional, or research outcomes arising from use of this tool.
How to Use the Punnett Square Calculator
Pick your curriculum
Use the curriculum pills above to match your syllabus (Cambridge, SEAB, SPM, SBMPTN, 高考, 學測 or HKDSE). Terminology and the whole page follow your selection.
Enter the genotypes
Type each parent's genotype. Use a capital letter for a dominant allele (A) and a lowercase letter for recessive (a). For a dihybrid cross use two genes, e.g. AaBb.
Read the square
The calculator generates the gametes, fills the Punnett square and shows every possible offspring genotype.
Read the ratios
Below the square we show the simplified genotype ratio (e.g. 1:2:1) and phenotype ratio (e.g. 3:1).
The Punnett Square, in Your Curriculum's Words
Punnett Square (monohybrid cross)
Example: Cross two heterozygous tall pea plants (Tt × Tt), where T (tall) is dominant over t (short).
Gametes: T, t from each parent. The square gives TT, Tt, Tt, tt.
Genotype 1 TT : 2 Tt : 1 tt; phenotype 3 tall : 1 short
A Punnett square is a grid that predicts the possible offspring genotypes from a genetic cross. Each parent's gametes are placed along the sides, and every cell combines one allele from each parent. A monohybrid cross (one gene) gives a 2×2 square; a dihybrid cross (two genes) gives a 4×4 square. This calculator generates the gametes, fills the square and works out the ratios for you.
A dominant allele is written with a capital letter and masks the effect of the recessive allele (lowercase). From the genotype we determine the phenotype — the visible trait — and express the genotype and phenotype ratios in their simplest whole-number form. The allele letters are your choice; all calculation happens in your browser — nothing is uploaded.
An Aa × Aa cross always gives a 3:1 phenotype ratio — Mendel's finding from thousands of pea plants, long before genes were known.
It shows probabilities, not outcomes
The Punnett square is named after Reginald Punnett (early 1900s).
A monohybrid cross uses a 2×2 square; a dihybrid uses 4×4.
A dominant allele (uppercase) masks the recessive allele's effect.
An Aa × Aa cross gives a 3:1 phenotype ratio.
A dihybrid AaBb × AaBb cross gives 9:3:3:1.
Gregor Mendel discovered these patterns with pea plants.
A genotype is the gene make-up; the phenotype is the visible trait.
Each parent contributes one allele for each gene.
The square shows probabilities, not guaranteed outcomes.
This calculator runs in your browser — your working stays private.
Frequently Asked Questions
- Use a pair of letters for each gene — a capital for the dominant allele and the same lowercase letter for recessive. For example Aa is heterozygous, AA is homozygous dominant and aa is homozygous recessive. For a dihybrid cross use two genes, e.g. AaBb. The calculator accepts whichever letters you choose.
- The genotype is the combination of alleles an organism carries (e.g. Aa), while the phenotype is the observable trait that results from it (e.g. purple flowers). A dominant allele determines the phenotype when present, so AA and Aa show the same dominant phenotype.
- In a monohybrid cross of two heterozygotes (Aa × Aa), on average three of every four offspring show the dominant trait and one shows the recessive trait. It is a probability, not a guarantee — small samples can deviate from the ideal ratio.
- Yes. Enter two genes for each parent, e.g. AaBb × AaBb, and the calculator builds the 4×4 square of 16 cells and gives the classic 9:3:3:1 phenotype ratio.
- The genetics is the same worldwide. What changes is the terminology — "dominant/recessive" is written 显性/隐性 in simplified Chinese and 顯性/隱性 in traditional, while SPM students see "dominan/resesif". The square and the calculated ratios are identical.
- Yes. A capital letter means the dominant allele and lowercase means the recessive allele of the same gene. "A" and "a" are two alleles of one gene, so make sure you use the same letter for the dominant and recessive pair.
- This tool covers simple Mendelian dominant-recessive inheritance (monohybrid and dihybrid). Codominance, incomplete dominance and sex-linked inheritance follow extra rules that are not modelled here.
- The Tool Information block lists the exact syllabus for your selected curriculum (e.g. SEAB O-Level Biology 6093 or SPM Biologi 4551). It is a study aid, not a substitute for your official syllabus or teacher.
- It holds only when the two genes assort independently — they sit on different chromosomes, or far apart on the same one — and each shows complete dominance. Closely linked genes travel together, so real counts drift away from 9:3:3:1, and the tighter the linkage the larger the drift; that departure is exactly what Morgan and his students used to map the fruit-fly chromosomes from 1911 onwards. This calculator assumes independent assortment and says so; it does not model linkage.
Method & sources
How it computes
Mendelian crosses under complete dominance: each parent genotype is split into loci, the gametes are the Cartesian product of one allele per locus (two for one heterozygous gene, four for two), and every gamete pair is combined to fill the square. Genotypes are tallied from the cells and a locus shows the dominant phenotype whenever one capital allele is present, both reduced to lowest whole-number ratios — 1:2:1 and 3:1 for Aa × Aa, 9:3:3:1 for AaBb × AaBb.
What this tool implements
- Mendel's laws as taught: segregation — one allele per gamete — and independent assortment of separate loci, which is what makes the dihybrid square the product of two monohybrid squares.
- Complete dominance only, written as a capital for the dominant allele and the same letter in lower case for the recessive. Codominance, incomplete dominance, epistasis, sex linkage, genetic linkage and crossing over are NOT modelled, and every language arm of the page says so.
- Both parents must use the same gene letters; a mismatch (Aa × Bb) is refused rather than silently reinterpreted, and a genotype with an odd number of letters or a gene given once is rejected.
- The square gives expected probabilities over many offspring, not the outcome of one cross — the facts heading and the 3:1 FAQ both lead with that, in all seven curriculum profiles.
Sources
- Mendel G. Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn 1866;4:3-47. The original crosses, the 3:1 and 9:3:3:1 ratios and the F2 counts behind them.
- Punnett RC. Mendelism. 2nd ed. Cambridge: Macmillan and Bowes; 1907 — the square's first appearance in print; with Edwards AWF. Punnett's square. Studies in History and Philosophy of Biological and Biomedical Sciences 2012;43(1):219-224, which dates it and shows it is absent from the 1905 first edition.
- Griffiths AJF, Miller JH, Suzuki DT, Lewontin RC, Gelbart WM. An Introduction to Genetic Analysis. 7th ed. New York: W. H. Freeman; 2000. Patterns of inheritance and the use of the Punnett square.
- Urry LA, Cain ML, Wasserman SA, Minorsky PV, Orr RB. Campbell Biology. 12th ed. New York: Pearson; 2020. Mendel and the gene idea — monohybrid and dihybrid crosses, genotype and phenotype.
What can make this go out of date
- None at runtime — combinatorics on the letters the reader types. The only thing that moves is the curriculum each profile names (Cambridge IGCSE 0610, SEAB 6093, SPM 4551, SNBT, 高考生物, 108課綱, HKDSE), which are checked when a syllabus is revised, not on a schedule.
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