Punnett Squares, Made Visual
Click any parent allele to flip it, and watch the cross fill in — genotype and phenotype ratios update live. Monohybrid and dihybrid inheritance, finally clear.
A Punnett square is the single most useful tool in genetics: a little grid that predicts what happens when two organisms breed. Once you can read one, inheritance stops feeling like guesswork and starts looking like simple counting.
What a Punnett square shows
Every organism carries two copies (alleles) of each gene — one from each parent. A capital letter marks a dominant allele, a lower-case letter a recessive one. When an organism makes sex cells (gametes), each gamete gets just one of the two alleles, chosen at random.
A Punnett square lists one parent’s possible gametes across the top and the other parent’s down the side, then fills each cell by combining them. The finished grid shows every equally-likely combination the offspring could inherit.
A Punnett square doesn’t tell you what will happen — it tells you the odds.
See it: cross the alleles
The cross below is Bb × Bb— two heterozygous parents. Click any allele on a parent to flip it between dominant (B) and recessive (b), and watch the grid and ratios update instantly.
Text description ↓Hide text description ↑
An interactive Punnett square. Each parent’s two alleles are shown as buttons you can toggle between a dominant capital letter and a recessive lower-case letter. For the default cross Bb × Bb, the two gametes B and b are written across the top and down the side, giving a 2×2 grid with cells BB, Bb, Bb and bb.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Three of the four cells (BB, Bb, Bb) show the dominant phenotype and one (bb) shows the recessive phenotype, a 3:1 phenotype ratio. The genotype ratio is 1 BB : 2 Bb : 1 bb.
Genotype vs phenotype ratio
Two words that trip students up. The genotype is the pair of alleles (BB, Bb or bb). The phenotypeis the characteristic you actually see — and because a single dominant allele is enough to be expressed, both BB and Bb give the same dominant phenotype.
Dihybrid crosses (9:3:3:1)
Follow two genes at once and the grid grows to 4×4. Each parent now makes four kinds of gamete, and the classic cross of two double-heterozygotes (BbEe × BbEe) gives the famous 9 : 3 : 3 : 1phenotype ratio — provided the two genes assort independently.
Text description ↓Hide text description ↑
A 4×4 Punnett square for the dihybrid cross BbEe × BbEe. Each parent produces four gametes (BE, Be, bE, be), giving 16 offspring cells. The phenotype ratio is 9 showing both dominant traits, 3 showing the first dominant and second recessive, 3 the reverse, and 1 showing both recessive traits — the 9:3:3:1 ratio.
A brown-eyed parent with genotype Bb is crossed with a blue-eyed parent (bb). What fraction of children are expected to have blue eyes?
The Bb parent makes gametes B and b; the bb parent makes only b. The offspring are Bb, Bb, bb, bb — a 1 : 1 ratio of brown to blue. So half the children are expected to have blue eyes, even though one parent has brown.
Common mistakes
Practice
A pea plant heterozygous for tallness (Tt) is crossed with a short plant (tt). What is the expected phenotype ratio of the offspring?
Show the answer ↓Hide the answer ↑
The Tt parent gives T and t; the tt parent gives only t. Offspring are Tt, Tt, tt, tt — a 1 tall : 1 short ratio.
Where next?
Once a population is involved rather than a single cross, the same allele frequencies can be tracked with the Hardy–Weinberg principle — the natural sequel to this guide.
Frequently asked questions
What is a Punnett square used for?+
What is the difference between genotype and phenotype?+
Why do two heterozygous parents give a 3:1 ratio?+
What ratio does a dihybrid cross give?+
The ScholarsGate Biology Team
Oxbridge & Russell Group biology tutors
Written and reviewed by ScholarsGate tutors who teach GCSE, A-Level and undergraduate biology. Every explainer is checked against the AQA, Edexcel, OCR and CIE specifications.
Keep exploring
Want a tutor to walk you through it?
Book a DBS-checked GCSE Biology tutor for a 1-on-1 lesson — online or in person.
Find a GCSE Biology tutor