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How to do a Punnett square
A simple grid that predicts the chances of offspring inheriting each trait.
A Punnett square shows the possible offspring from a genetic cross. Write one parent's alleles along the top and the other's down the side, then fill each box with the pair of alleles it gives.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
The five steps
We'll use one gene in diploid pea plants with complete dominance: tall (T, dominant) and short (t, recessive). This simple model does not describe every inherited trait.
- 01
Write the genotypes
For example, two heterozygous parents: Tt × Tt.
- 02
Work out the gametes
Each parent gives one allele to each gamete. A Tt parent makes T and t gametes.
- 03
Draw the grid
Put one parent's gametes along the top, the other's down the side.
- 04
Fill in the boxes
Each box gets the allele from its column and the allele from its row. Write the capital letter first.
- 05
Read the results
Count the genotypes, then the phenotypes. Tt × Tt gives 1 TT : 2 Tt : 1 tt, or 3 tall : 1 short.
A 25% chance of a short plant
Try it: pick the parents
Choose a genotype for each parent and the grid works out the cross. T is dominant (tall), t is recessive (short).
- Genotypes
- 1 TT : 2 Tt : 1 tt
- Phenotypes
- 3 tall : 1 short
- Odds
- 25% chance of a short plant
Common crosses at a glance
| Cross | Genotype ratio | Phenotype ratio |
|---|---|---|
| TT × tt | All Tt | All dominant |
| Tt × Tt | 1 TT : 2 Tt : 1 tt | 3 dominant : 1 recessive |
| Tt × tt | 1 Tt : 1 tt | 1 dominant : 1 recessive |
| TT × Tt | 1 TT : 1 Tt | All dominant |
| tt × tt | All tt | All recessive |
Key terms
- Allele
- A version of a gene, such as T or t.
- Genotype
- The alleles an organism has, such as Tt.
- Phenotype
- The trait that shows, such as tall.
- Dominant
- An allele that shows in the phenotype even if only one copy is present.
- Recessive
- An allele that only shows if two copies are present (tt).
- Homozygous
- Two identical alleles (TT or tt).
- Heterozygous
- Two different alleles (Tt).
Common mistakes
A ratio is a probability, not a guarantee. A 3:1 ratio means each offspring has a 75% chance of being tall. It doesn't mean exactly 3 of every 4.
Genotype is not phenotype. Tt is a genotype. Tall is a phenotype.
A recessive trait needs two copies. Only tt plants are short. Tt plants are tall.
Use the same letter for both alleles of a gene. Capital for dominant, lowercase for recessive. Don't mix letters like T and S.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
A tall plant (Tt) is crossed with a short plant (tt). What is the probability that each offspring will be tall?
How to get there
- 1
The Tt parent makes T and t gametes. The tt parent makes only t.
- 2
The Punnett square gives Tt, Tt, tt and tt.
- 3
T is dominant, so 2 of the 4 equally likely boxes are tall: a 1/2 chance for each offspring. Actual numbers can vary.
Check yourself
What fraction of offspring from Tt × Tt are tt?
The cross gives TT, Tt, Tt and tt, so 1 in 4 is tt.
Which of these is a genotype?
A genotype is the combination of alleles, such as Tt. Tall is the phenotype.
A cross gives a 1:1 phenotype ratio. Which cross is it?
Tt × tt gives Tt, Tt, tt, tt, which is 1 dominant to 1 recessive.
FAQ
What does a Punnett square show?
The possible genotypes of offspring from a cross, and the probability of each. It's a prediction, not a guaranteed outcome.
How do you read a 3:1 ratio?
Three out of every four offspring are expected to show the dominant trait, and one the recessive. Each offspring has a 75% chance of the dominant trait.
How do you do a dihybrid cross?
A dihybrid cross follows two genes. Each parent such as RrYy makes four gamete types (RY, Ry, rY, ry), so you use a 4×4 grid. RrYy × RrYy gives an expected 9:3:3:1 phenotype ratio if the genes assort independently, each shows complete dominance, and there is no interaction that changes the phenotypes.
Can Punnett squares predict real families?
They predict probabilities when the inheritance pattern is known. This page uses a simple single-gene model. Punnett squares can also represent codominance, incomplete dominance and sex-linked traits, but linked genes need additional information and many traits involve several genes. See also how DNA is copied.
Sources and further reading
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