Punnett Square Calculator

Background.

"We are both carriers. What are the chances our child has it?" For a condition inherited in the simplest recessive way, the answer is the bottom-right cell of a four-cell grid: a 25 percent chance with each pregnancy that the child inherits the variant from both parents, a 50 percent chance the child is an unaffected carrier, and a 25 percent chance of inheriting neither copy. Those are the figures StatPearls gives for autosomal recessive inheritance, and they fall straight out of a Punnett square.

The square works because of one fact about meiosis. A parent carries two copies of each gene and passes exactly one, chosen at random, to each egg or sperm. Write one parent's two possible contributions down the side and the other's across the top, fill in the four pairings, and each cell has the same one-in-four chance. Every answer is a count of cells, so the only percentages a single-gene cross can ever give are 0, 25, 50, 75 and 100.

Type each parent's genotype as two letters, capital for the dominant allele and lower case for the recessive one, and choose how the heterozygote shows up. That choice changes the visible classes but never the genotype odds. Under complete dominance an Aa offspring looks just like an AA one, so a 1 : 2 : 1 genotype split appears as 3 : 1. Under incomplete dominance or codominance the heterozygote is visibly different and the 1 : 2 : 1 stays.

The arithmetic is exact for the idealised cross: one gene on a non-sex chromosome, two alleles, every genotype equally likely to survive, and every carrier of a genotype showing its trait. It is not a personal risk figure. Real families involve uncertain genotypes, partial penetrance and tests with their own error rates, and a clinical geneticist or genetic counsellor is the right source for a number a decision depends on.

What is punnett square calculator?

A Punnett square is a table of every equally likely way two parents' gametes can combine at one or more genes, named after the British geneticist Reginald Punnett, who introduced it in the early 1900s while working with William Bateson on the newly rediscovered Mendelian rules. For one gene it has two rows and two columns. The rows are the two alleles one parent can pass on, the columns are the two the other can pass on, and each of the four cells is a possible offspring genotype. An allele is one version of a gene; a genotype is the pair an individual carries; homozygous means the pair matches (AA or aa) and heterozygous means it differs (Aa). The phenotype is what can be observed, and how genotypes map onto phenotypes depends on dominance, which describes the heterozygote: hidden behind the dominant allele, intermediate between the two homozygotes, or showing both alleles' products at once.

How to use this calculator.

1. 01Enter parent 1's genotype as two letters of the same gene: AA, Aa or aa. Any letter works, so Tt for tall and short peas or Rr for red and white flowers, but both parents must use the same one.
2. 02Enter parent 2's genotype. The cross is symmetric, so which parent goes in which box changes only how the grid is drawn, never the percentages.
3. 03Choose the dominance model. Complete dominance fits a classic dominant/recessive trait such as most recessive genetic conditions; incomplete dominance fits a blended heterozygote such as a pink snapdragon; codominance fits a heterozygote showing both products, such as the A and B antigens together.
4. 04Read the phenotype percentages for what offspring will look like and the genotype percentages for what they will carry. For a recessive condition the heterozygous figure is the carrier figure.
5. 05Copy the genotype and phenotype ratios for homework: they are reduced to the smallest whole numbers, and classes that cannot occur are left out. The grid shows each row in square brackets so you can check it against your own drawing.

The formula.

Two assumptions produce every number. The first is Mendel's law of segregation: a parent's two alleles separate during meiosis and each gamete receives one of them, each with probability ½. An Aa parent therefore makes A and a gametes in equal numbers, an AA parent makes only A, and an aa parent only a. The second is that the egg and the sperm are independent draws, so the four pairings in the grid are equally likely:

P(genotype) = number of cells containing it ÷ 4

For Aa × Aa the cells are AA, Aa, Aa and aa, giving P(AA) = ¼, P(Aa) = ½ and P(aa) = ¼. The same answer comes from the product rule without drawing anything: P(AA) = ½ × ½ = ¼, and the heterozygote can form two ways, A from the first parent with a from the second or the reverse, so P(Aa) = ¼ + ¼ = ½.

Dominance then decides which genotypes share a visible class. Under complete dominance AA and Aa look alike, so the dominant phenotype is P(AA) + P(Aa), the sum rule at work, and the recessive phenotype is P(aa) alone. Under incomplete dominance and codominance each genotype is its own phenotype, and the three phenotype figures equal the three genotype figures. Only the label on the middle class differs: a blend in one case, both allele products present in the other.

The grid grows quickly with more genes. Mendel's paper gives the general pattern: when both parents are heterozygous at n genes, each makes 2ⁿ kinds of gamete, and the offspring fall into 3ⁿ genotype classes spread over 4ⁿ combinations. One gene gives a 4-cell square, two genes a 16-cell square and three genes 64 cells. That is why this tool takes a single gene, and why crosses involving several genes are usually worked by multiplying the single-gene probabilities instead.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Abbott S. & Fairbanks D. J. (2016). Experiments on Plant Hybrids by Gregor Mendel. Genetics 204(2):407–422. doi:10.1534/genetics.116.195198. A peer-reviewed modern English translation of Mendel's 1866 Versuche über Pflanzen-Hybriden, published by the Genetics Society of America. Contains Mendel's single-character series (the 1:2:1 genotype series) and the statement that for n differing characters there are 2ⁿ gamete types, 3ⁿ genotype classes and 4ⁿ combinations. Open access, not paywalled. Retrieved 2026-07-29.
2. [2]Gulani A. & Weiler T. Genetics, Autosomal Recessive. StatPearls, NCBI Bookshelf ID NBK546620, last update 1 May 2023. States the carrier × carrier outcome verbatim — 'a 25% chance that the child will be affected, a 50% chance that the child will be a carrier, and a 25% chance that the child will be homozygous dominant and unaffected' — and derives it as 50% × 50% = 25%. Free full text. Retrieved 2026-07-29.
3. [3]Lewis R. G. & Simpson B. Genetics, Autosomal Dominant. StatPearls, NCBI Bookshelf ID NBK557512, last update 1 May 2023. Source for the 50 % transmission probability from a heterozygous affected parent, and for the definition of penetrance as 'the percentage of individuals who inherit a disorder allele AND display the phenotype' — the assumption this calculator makes explicit. Free full text. Retrieved 2026-07-29.
4. [4]MedlinePlus Genetics, US National Library of Medicine. Inheritance Patterns (formerly Genetics Home Reference), page last updated 19 April 2021. Source for the definitions of autosomal dominant, autosomal recessive and codominant inheritance used on this page, including 'Two different versions (alleles) of a gene are expressed, and each version makes a slightly different protein' for codominance. Free. Retrieved 2026-07-29.
5. [5]OpenStax, Biology 2e, section 12.3 'Laws of Inheritance'. Rice University, 2018, CC BY 4.0. Independent derivation of the same numbers via the product and sum rules: 'the probability of a homozygous dominant at A is 1/4 and the probability of a heterozygote at A is 1/2. The probability of the homozygote or the heterozygote is 1/4 + 1/2 = 3/4 using the sum rule.' Free, openly licensed. Retrieved 2026-07-29.
6. [6]Genetic Alliance & District of Columbia Department of Health. Understanding Genetics: A Guide for Patients and Health Professionals, appendix 'Inheritance Patterns' / 'Classic Mendelian Genetics'. NCBI Bookshelf IDs NBK115561 and NBK132145, 2009–2010. Plain-language statement of the five classical inheritance modes used for the scope statements on this page. Free full text. Retrieved 2026-07-29.