🎓 Prepared by students from Boğaziçi University

What is a Punnett Square?

A Punnett square is a diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross. Named after Reginald Punnett, it's the go-to tool for visualizing Mendelian inheritance.

Short answer

A Punnett square is a grid that lines up one parent's possible gametes across the top and the other's down the side, so each box shows a possible offspring genotype and the overall probability of each outcome.

Building a Punnett Square

1
1. Identify genotypes
Write each parent's genotype, e.g., Aa × Aa.
↓
2
2. List gametes
Split each parent's alleles into possible gametes: A, a for each.
↓
3
3. Draw the grid
Place one parent's gametes across the top, the other's down the side.
↓
4
4. Fill the boxes
Combine row and column alleles into each box's offspring genotype.
↓
5
5. Read the ratios
Count genotypes/phenotypes across all boxes to get the ratio, e.g., 3:1.

Try it: interactive calculator

Probability offspring is homozygous recessive

25%

= (50/100)*(50/100)*100

Parent 1: chance of passing recessive allele50 %Parent 2: chance of passing recessive allele50 %

Step-by-step worked examples

Cross Aa × Aa using a Punnett square. What fraction of offspring are homozygous recessive (aa)?

Gametes from each parent: A, a.
Grid boxes: AA, Aa, Aa, aa (4 total).
aa appears in 1 of 4 boxes.
Fraction homozygous recessive = 1/4 = 25%.

Cross Aa × aa using a Punnett square. What genotype ratio results?

Parent 1 gametes: A, a. Parent 2 gametes: a, a.
Grid boxes: Aa, Aa, aa, aa.
Ratio = 2 Aa : 2 aa = 1:1.

Using the multiplication rule, what is the probability that two Aa × Aa parents produce a child that is homozygous recessive (aa)?

P(a from parent 1) = 1/2 = 50%.
P(a from parent 2) = 1/2 = 50%.
P(aa) = 0.5 × 0.5 = 0.25 = 25%, matching the Punnett square result.

Flashcards

Quick quiz

Q1.How many boxes are in a standard monohybrid Punnett square?

Correct answer: B. A single-gene cross uses a 2×2 grid = 4 boxes.

Q2.In an Aa × Aa Punnett square, how many boxes show the genotype Aa?

Correct answer: C. 2 of the 4 boxes are Aa (one from each parent's dominant/recessive combination).

Q3.A Punnett square for Aa × aa produces which genotype ratio?

Correct answer: A. Aa × aa gives 2 Aa : 2 aa, simplified to 1:1.

Q4.What does each box in a Punnett square represent?

Correct answer: C. Each box shows one possible combination of parental alleles, i.e., one possible offspring genotype.

📄Download this topic as a printable worksheet (PDF)Summary + 10 questions + answer key — print it, share it in class.↓

Study better with Bounlu apps

Notek

The full card deck, worked steps and AI-tutor support for “What is a Punnett Square?” are in Notek — study by hand before your exam.

Get it free

PromyNever miss an exam or homework deadline with Promy: AI turns the dates in your inbox into reminders automatically.

Common mistakes

✗

Filling a Punnett square with phenotypes instead of alleles. — Correct: Boxes should contain allele combinations (genotypes, e.g., Aa); phenotypes are interpreted afterward.

✗

Assuming every box represents an equal 100% chance. — Correct: Each box represents an equal probability out of the total number of boxes (e.g., 1/4 for a 2×2 grid).

✗

Mixing up rows and columns with different parents. — Correct: Keep one parent's gametes consistently on top and the other's on the side to avoid errors.

✗

Forgetting to simplify the resulting ratio. — Correct: Always reduce ratios like 2:2 to 1:1 for a clear result.

FAQ

What is a Punnett square used for?

It predicts the possible genotypes and phenotypes — and their probabilities — resulting from a genetic cross.

How do you calculate probabilities with a Punnett square?

Count how many boxes show the genotype/phenotype you want, then divide by the total number of boxes.

What is an example of a Punnett square?

An Aa × Aa cross gives a 4-box grid with genotypes AA, Aa, Aa, aa — a 1:2:1 genotype ratio.

What is the formula behind a Punnett square?

It's based on the multiplication rule of probability: P(offspring genotype) = P(allele from parent 1) × P(allele from parent 2).