How to Calculate a Punnett Square

Updated 4 November 2026 · Standard formula, shown below

This guide explains how to calculate a Punnett square, a simple grid used to predict the possible genotypes and phenotypes of offspring from two parents. You’ll learn to set up the square, fill it in, and interpret the results as probabilities. We’ll cover monohybrid crosses (one gene) and touch on dihybrid crosses (two genes).

What You’ll Need

Pen and paper, or a digital drawing tool
Knowledge of the parents’ genotypes (e.g., Aa, Bb, etc.)
Understanding of dominant and recessive alleles (uppercase = dominant, lowercase = recessive)

Step-by-Step

Determine the parents’ genotypes. You need the allele combination for each parent for the trait you’re studying. For example, if both parents are heterozygous for a trait, their genotype is Aa. If one is homozygous dominant, it’s AA; if homozygous recessive, aa.
List the possible gametes (sex cells) for each parent. Each gamete carries one allele from each gene. For a single gene, a parent with genotype Aa can produce two types of gametes: A and a. A parent with AA produces only A. Write these along the top and left side of the square.
Draw a grid. For a monohybrid cross (one gene), draw a 2×2 square. For a dihybrid cross (two genes), draw a 4×4 square. The number of rows and columns equals the number of possible gamete types from each parent.
Label the top and side. Place the gametes from one parent along the top (one per column) and the gametes from the other parent along the left side (one per row).
Fill in the boxes. For each box, combine the allele from the top column and the allele from the left row. Write both alleles together, typically with the dominant allele first (e.g., Aa, not aA). This gives the genotype of the potential offspring.
Count the genotypes. After filling all boxes, count how many times each genotype appears. For a 2×2 square, there are 4 boxes total.
Calculate genotype ratios. Express each genotype count as a fraction or percentage of the total. For example, if 2 out of 4 boxes are Aa, the genotype probability is 2/4 = 50%.
Determine phenotypes. Apply the rules of dominance: if an allele is dominant (uppercase), it masks the recessive (lowercase). For example, if A is dominant and a is recessive, then both AA and Aa show the dominant phenotype, while aa shows the recessive phenotype.
Calculate phenotype ratios. Add up the genotype probabilities that result in the same phenotype. For a monohybrid cross of two heterozygotes (Aa x Aa), the phenotype ratio is typically 3 dominant : 1 recessive.

Common Mistakes

Forgetting to separate alleles correctly. Each gamete must contain only one allele per gene. For a parent with genotype Aa, the gametes are A and a, not Aa.
Writing both alleles in the wrong order. While order doesn’t affect the genotype, it’s conventional to write the dominant allele first (e.g., Aa not aA).
Confusing genotype and phenotype. Genotype is the genetic makeup (e.g., Aa), phenotype is the physical trait (e.g., brown eyes).
Using the wrong number of boxes. For a monohybrid cross, it’s 2×2; for a dihybrid cross, it’s 4×4. Don’t use a 4×4 for a single gene unless you’re tracking two genes.
Forgetting that probabilities are per offspring. Each box represents an equally likely outcome for a single offspring, not a guarantee for a litter or multiple children.

Tips

Always write down the parents’ genotypes first. It’s easy to make a mistake if you try to do it in your head.
Use a ruler for neatness. A clean grid reduces errors when filling in boxes.
Double-check your gametes. For each parent, list all possible combinations of alleles. For a dihybrid cross, use the FOIL method (First, Outer, Inner, Last) to get all four gamete types.
Practice with a known example. The classic monohybrid cross of two heterozygotes (Aa x Aa) should always yield a 1:2:1 genotype ratio (AA:Aa:aa) and a 3:1 phenotype ratio.

Troubleshooting

If your results don’t seem right, check the following:

Re-verify the parents’ genotypes. Are you sure the parents are heterozygous or homozygous? A common error is assuming a parent is heterozygous when it’s actually homozygous dominant.
Check your gamete list. For a dihybrid cross, you should have four gametes per parent. If you have fewer, you may have missed a combination.
Ensure you’re combining alleles correctly. Each box should have two alleles, one from each parent. If a box has only one allele, you skipped a step.
Recalculate percentages. If your percentages don’t add up to 100%, you may have miscounted. For a 2×2 square, each box is 25%; for a 4×4, each is 6.25%.
Consider the possibility of sex-linked traits. If the gene is on the X or Y chromosome, the square may need to include sex chromosomes, and the ratios will differ between males and females.

Frequently Asked Questions

Q: What is a Punnett square?
A: A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. It shows all possible combinations of parental alleles.

Q: How do I know which allele is dominant?
A: Dominance is usually indicated by an uppercase letter for the dominant allele and a lowercase letter for the recessive allele. In a problem, it may be stated explicitly (e.g., “brown eyes (B) are dominant to blue eyes (b)”).

Q: Can a Punnett square predict the exact outcome for a specific child?
A: No, it only gives probabilities. Each box represents an equal chance for a single offspring. The actual outcome for any one child is random.

Q: What is the difference between a monohybrid and a dihybrid cross?
A: A monohybrid cross tracks one gene (e.g., seed color), while a dihybrid cross tracks two genes simultaneously (e.g., seed color and seed shape). Dihybrid crosses use a 4×4 square and involve 16 possible combinations.

Q: How do I handle codominance or incomplete dominance?
A: In these cases, the heterozygous genotype produces a distinct phenotype (e.g., a blend or both traits expressed). You still use the same square, but the phenotype ratio will differ. For incomplete dominance, Aa might be a mix, so the phenotype ratio is 1:2:1 instead of 3:1.

Q: What if the parents have multiple alleles for a gene?
A: For genes with more than two alleles (e.g., ABO blood type), you still use the same method, but you may need a larger square if both parents are heterozygous for different alleles. For example, a cross between IAi and IBi would use a 2×2 square with gametes IA, i and IB, i.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.