Punnett Square Calculator
Type the genotypes of two parents, such as Tt and Tt, and the calculator draws the Punnett square for one gene. It then gives the genotype and phenotype ratios and the probability of each outcome as a fraction, a percentage and a number out of 4, with the working written in plain English so you can check your own method.
It covers single-gene (monohybrid) crosses, which is what the GCSE Biology specifications ask for. The traits in the examples are invented, so there are no real families or medical conditions to worry about.
- Single-gene crosses
- Fractions, percentages and ratios
- Working in plain English
- Free, no sign-up
- Years 9 to 11
Work out a cross
One gene at a time. This calculator does single-gene (monohybrid) crosses: a capital letter is the dominant allele and the same small letter is the recessive allele, for example R and r. The traits in the examples are invented.
1. Type the two parents’ genotypes
Two letters, such as Rr.
Use the same letter as parent 1.
Name the two characteristics (optional)
If you say what each allele gives, the results use your words instead of “dominant” and “recessive”. Use any invented or textbook trait.
Or try an example (invented traits)
2. Your Punnett square
| Parent 2 gametes are the row headings, parent 1 gametes the column headings. | Parent 1 gamete T | Parent 1 gamete t |
|---|---|---|
| Parent 2 gamete T | TTtall | Tttall |
| Parent 2 gamete t | Tttall | ttshort |
Genotypes
| Genotype | Boxes | Fraction | Percent | In 4 |
|---|---|---|---|---|
| TT homozygous dominant | 1 | 1/4 | 25% | 1 in 4 |
| Tt heterozygous | 2 | 1/2 | 50% | 2 in 4= 1 in 2 |
| tt homozygous recessive | 1 | 1/4 | 25% | 1 in 4 |
Genotype ratio: TT : Tt : tt = 1 : 2 : 1
Phenotypes
| Phenotype | Boxes | Fraction | Percent | In 4 |
|---|---|---|---|---|
| tall (dominant) | 3 | 3/4 | 75% | 3 in 4 |
| short (recessive) | 1 | 1/4 | 25% | 1 in 4 |
Phenotype ratio: tall : short = 3 : 1
These are the chances for each offspring on its own. A real family of four from parents like these will not always come out exactly as the ratio says.
Step-by-step working
- Write the genotypes. Parent 1 is Tt, which is heterozygous: its two alleles are different. Parent 2 is Tt, which is heterozygous: its two alleles are different. T is the dominant allele (capital letter) and t is the recessive allele (small letter).
- Split each parent into gametes. Parent 1 (Tt) makes gametes that carry T or t, half of each. Every gamete carries just one allele. Parent 2 (Tt) makes gametes that carry T or t, half of each. Every gamete carries just one allele.
- Fill in the Punnett square. Put parent 1's gametes across the top and parent 2's down the side. Each box takes the allele from the top of its column and the allele from the side of its row, with the capital letter written first, so a box with t and T is Tt. Reading the boxes row by row, the four boxes are TT, Tt, Tt and tt.
- Count the genotypes. That gives 1 box of TT, 2 boxes of Tt and 1 box of tt.
- Work out the phenotypes. A box with at least one T shows the dominant characteristic (tall). Only tt shows the recessive characteristic (short). That gives 3 boxes showing tall and 1 box showing short.
- Turn the counts into probabilities. The four boxes are four equally likely outcomes, so each box is a 1 in 4 chance for one offspring. The probability of tall is 3 boxes out of 4 = 3/4, which is 75% or 3 in 4. The probability of short is 1 box out of 4 = 1/4, which is 25% or 1 in 4. The phenotype ratio is 3 : 1, and the genotype ratio is 1 : 2 : 1. Check: the probabilities add up to 1 (3 + 1 = 4 boxes out of 4).
Your cross is saved only in this browser, on this device, so it is still here next time. Nothing is sent to us.
Four steps
- Type the genotypes. Two letters for each parent, with the same letter in both: a capital for the dominant allele and a small letter for the recessive one, such as Tt and tt.
- Name the traits (optional). Say what the dominant and recessive alleles give, such as tall and short, to see your own words in the results.
- Read the square. Parent 1’s gametes are across the top and parent 2’s down the side. Each box is one equally likely outcome.
- Check the working. The step-by-step panel shows the gametes, the boxes and the counts that give each fraction, so you can compare it with your own method.
Two crosses with invented traits
The traits below are invented for the example. In a made-up kind of pea plant the allele for tall (T) is dominant to the allele for short (t).
Example 1: two heterozygous plants (Tt × Tt)
| Parent 2 gametes are the row headings | T | t |
|---|---|---|
| T | TT tall | Tt tall |
| t | Tt tall | tt short |
The genotypes are TT with probability 1/4, Tt with probability 1/2 and tt with probability 1/4. The phenotypes are tall with probability 3/4 (75%, 3 in 4) and short with probability 1/4 (25%, 1 in 4), a ratio of 3 : 1 (tall to short). The genotype ratio is 1 : 2 : 1.
Example 2: a heterozygous plant with a recessive one (Pp × pp)
In another made-up plant, purple flowers (P) are dominant to white flowers (p). The pp parent can only pass on p, so the four boxes are Pp, pp, Pp, pp. Half of the boxes show purple flowers (probability 1/2, 50%) and half show white flowers (probability 1/2), a 1 : 1 ratio. Type these into the calculator to see the full working.
All the single-gene crosses
With one letter there are only six different crosses. The fractions below were calculated by the same code as the calculator.
| Cross | Genotype probabilities | Phenotype probabilities | Phenotype ratio |
|---|---|---|---|
| RR × RR | RR 1 | dominant 1 | all the same |
| RR × Rr | RR 1/2, Rr 1/2 | dominant 1 | all the same |
| RR × rr | Rr 1 | dominant 1 | all the same |
| Rr × Rr | RR 1/4, Rr 1/2, rr 1/4 | dominant 3/4, recessive 1/4 | 3 : 1 |
| Rr × rr | Rr 1/2, rr 1/2 | dominant 1/2, recessive 1/2 | 1 : 1 |
| rr × rr | rr 1 | recessive 1 | all the same |
R is the dominant allele and r the recessive allele. “Dominant” means the dominant characteristic is shown and “recessive” the recessive one. The order of the parents does not change a result, so Rr × RR is the same cross as RR × Rr.
Foundation and Higher tier
This summary is from the GCSE Biology specifications of AQA (8461), Pearson Edexcel (1BI0) and OCR Gateway (J247). The specification is the exam board’s list of what can be examined. If you take Combined Science, or a different board, read the genetics points in your own specification, because they may be worded differently.
Foundation tier
- AQA: complete a Punnett square diagram, and extract and interpret information from genetic crosses and family trees.
- AQA: understand the concept of probability in predicting the results of a single-gene cross, and use direct proportion and simple ratios to express the outcome of a genetic cross.
- Edexcel (3.14, 3.16) and OCR (B5.1i, B5.1j): explain single-gene or monohybrid inheritance with Punnett squares, and use probabilities, ratios and percentages from the crosses. These points are not marked as Higher only, so they apply to both tiers.
Use the calculator to check a square you have completed yourself, and read the working panel to see where a step went differently.
Higher tier
- Everything in Foundation.
- AQA only: construct a genetic cross by Punnett square diagram and use it to make predictions using the theory of probability. AQA marks this point Higher tier only, so on AQA expect to build the square from the parents’ genotypes yourself and give the probability.
- Edexcel and OCR do not mark the Punnett square points as Higher only, so there is no extra Higher point to learn on their specifications.
Cover the square, work out a cross on paper from the two genotypes, then type them in to check your boxes and your fraction.
Read from the exam boards’ own specifications on 20 October 2026: AQA GCSE Biology (8461): 4.6.1.6 Genetic inheritance ; Pearson Edexcel GCSE (9-1) Biology (1BI0) specification, Topic 3 Genetics ; OCR GCSE (9-1) Biology A Gateway Science (J247) specification, B5.1 . Exam boards update their specifications, so check the current version for your course and tier. This is a summary, not official material.
Test yourself on genetics
Once you can draw a square from two genotypes, try the free Genetics and Punnett Squares Quiz: terms, Punnett squares, family trees and sex determination, with an explanation for every answer.
What a Punnett square shows
A Punnett square is a grid that shows every way two parents’ alleles can combine in their offspring. Each parent has two alleles of a gene. When a parent makes gametes (sex cells), the two alleles are separated, so each gamete carries just one. The square puts one parent’s gametes across the top and the other’s down the side, and each box shows the genotype of one possible offspring.
For one gene there are four boxes, and they are four equally likely outcomes, so each box is worth a probability of 1/4, or 25%. To answer a question, count the boxes that match what it asks for, divide by four and simplify the fraction.
Genotype, phenotype, dominant and recessive
The genotype is the alleles an organism has, written as letters such as Rr. The phenotype is the characteristic that results, such as tall or short. A dominant allele, written as a capital letter, is expressed even when there is only one copy. A recessive allele, written as a small letter, shows only when there are two copies.
An organism is homozygous when its two alleles are the same (RR or rr) and heterozygous when they are different (Rr). That is why RR and Rr are different genotypes but give the same phenotype, and why a recessive characteristic can skip a generation: two parents who show the dominant characteristic can both carry the recessive allele.
Ratios, fractions and “in 4”
One square can be read in three ways. For two heterozygous parents (Rr and Rr) the boxes are RR, Rr, Rr and rr, so the genotype ratio is 1 : 2 : 1 and the phenotype ratio is 3 : 1, three showing the dominant characteristic to one showing the recessive. The probability of the recessive characteristic is 1/4, which is 25% or 1 in 4.
A ratio compares the groups with each other, and a probability compares one group with the whole. A 3 : 1 ratio does not give a probability of 1/3: the same cross gives 3/4 for the dominant characteristic and 1/4 for the recessive one. The calculator shows both so you can see how they fit together.
What this calculator leaves out
It handles one gene with two alleles, written with a single letter. It does not do dihybrid crosses (two genes at once), codominance, multiple alleles or sex-linked genes, and it keeps the letters X and Y free because they name the sex chromosomes. Sex determination uses the same idea of a square, with the sex chromosomes in the gametes: for XX and XY each child has a probability of 1/2 of being a girl and 1/2 of being a boy.
Real characteristics are rarely as simple as the examples here. Most depend on several genes working together, and a single-gene square only suits single-gene questions.
Common mistakes
- Writing both alleles of a parent in one header box. The gametes go across the top and down the side one allele at a time, so Rr becomes R and r.
- Treating Rr and rR as different genotypes. They are the same, so the two boxes are added together: 2 boxes out of 4, which is 1/2.
- Giving a genotype when the question asks for a phenotype, or the other way round. Rr is a genotype and tall is a phenotype, so say which one you are giving.
- Turning a 3 : 1 ratio into “1 in 3”. The ratio compares the two groups. As probabilities the same cross gives 3/4 and 1/4.
- Reading the ratio as a head count. Four offspring from two heterozygous parents need not be exactly three dominant and one recessive, because each offspring has the same chance whatever the others were.
- Using two different letters for one gene. Both parents of a single-gene cross use the same letter, so Tt and Rr would be two different genes.
- Forgetting the check at the end: the four boxes must add up to 4 out of 4, which is a total probability of 1.
Questions about this tool
Is the Punnett square Higher tier only at GCSE?
It depends on the exam board and on what you are asked to do. AQA’s GCSE Biology specification asks every student to complete a Punnett square, to interpret genetic crosses, to understand probability for a single-gene cross and to use simple ratios. Only constructing a genetic cross by Punnett square and using it to make predictions with probability is marked Higher tier only. Edexcel’s and OCR’s Biology specifications do not mark these points as Higher only. Check your own specification and tier: the box above has the details.
What do monohybrid and dihybrid mean?
A monohybrid cross follows one gene, so the Punnett square has four boxes. A dihybrid cross follows two genes at once and needs a 16-box square. None of the three GCSE Biology specifications we read (AQA 8461, Pearson Edexcel 1BI0 and OCR Gateway J247) mentions dihybrid crosses, so this calculator does not do them. If your teacher has set one, ask which method they expect.
Why does the calculator say 1/2 when I counted 2 boxes out of 4?
They are the same number. Two boxes out of four is 2/4, which simplifies to 1/2, and that is also 50% or 1 in 2. The tables show the fraction, the percentage and the “in 4” form side by side, and the working panel shows the simplifying step. Give the form your question asks for.
Are my crosses stored anywhere?
Only in your own browser, on your own device, so your last cross is still there next time you visit. Nothing is sent to us, and the button under the calculator resets it. We only count anonymous, cookie-free visits and whether the calculator was used, never the genotypes you type.
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These practice questions were written by Cloudia Tutoring. They are not official AQA, Pearson Edexcel or OCR material, and Cloudia Tutoring is not affiliated with or endorsed by any exam board or the Department for Education. Always check your own exam board's specification.
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