Punnett Square Calculator

Genetics guide

What is a Punnett Square and how does it work?

A Punnett Square is a grid diagram invented by the British geneticist Reginald Crundall Punnett in the early 1900s to predict the genotype and phenotype outcomes of a genetic cross. It is the most widely used teaching tool in classical genetics and provides a visual map of all possible ways alleles can combine in the offspring of two parents.

Key terms

• Gene: A specific segment of DNA that codes for a particular trait, such as flower color, eye color, or seed shape.
• Allele: One of two or more versions of a gene. In this calculator, each allele is represented by a single letter. Capital letters (B, T, R) represent dominant alleles; lowercase letters (b, t, r) represent recessive alleles.
• Genotype: The complete set of alleles an organism carries for a particular gene, written as a two-letter pair. For example, BB, Bb, or bb.
• Phenotype: The observable trait that results from the genotype. A dominant allele masks the effect of a recessive one, so BB and Bb both produce the dominant phenotype.
• Homozygous: Both alleles are the same (BB or bb).
• Heterozygous: The two alleles are different (Bb).

How to read a Punnett Square

The mother's alleles are placed along the top of the grid and the father's alleles along the left side. Each cell in the grid is filled by combining one allele from the column header with one from the row header. The result in each cell is one possible genotype for a child, and each cell represents an equal probability of occurring.

For a Bb x Bb cross, the four cells produce BB, Bb, Bb, and bb. That gives a 1:2:1 genotype ratio. Since BB and Bb both express the dominant phenotype, the dominant-to-recessive phenotype ratio is 3:1.

The monohybrid cross ratios

A monohybrid cross involves a single gene locus with two alleles. The classic Mendelian ratios are:

Mendel's Law of Segregation

Gregor Mendel's first law states that the two alleles for any gene separate from each other during the formation of gametes (egg and sperm cells), so each gamete receives exactly one allele. When fertilization occurs, the offspring receives one allele from each parent, restoring the two-allele genotype. The Punnett Square models this process exactly: each row header and column header represents the single allele a gamete can carry.

Dominant vs. recessive

Dominance refers to the relationship between two alleles at the same gene locus. A dominant allele masks the effect of a recessive allele when both are present. This means that a heterozygous individual (Bb) looks identical to a homozygous dominant individual (BB) in terms of observable traits. Only the homozygous recessive individual (bb) expresses the recessive phenotype.

This is why recessive traits can "skip" generations: two phenotypically dominant parents who are both carriers (Bb) have a 25% chance of producing a recessive (bb) child, even though neither parent shows the recessive trait.

The test cross

A test cross crosses an unknown genotype with a known homozygous recessive individual (bb). If any offspring show the recessive phenotype, the unknown parent must have been heterozygous (Bb), not homozygous dominant (BB). Mendel used test crosses to determine the genotypes of his pea plants.

Worked example: flower color in peas

In Mendel's original pea plant experiments, purple flower color (P) is dominant over white (p). Crossing two heterozygous purple plants:

Mendel observed this 3:1 phenotype ratio across thousands of plants and used it to deduce the laws of inheritance decades before DNA was discovered.

Limitations of this calculator

• Monohybrid only. This calculator handles one gene at a time. Dihybrid crosses (two genes) produce a 4x4 grid with 16 possible combinations.
• Simple dominance assumed. Incomplete dominance (where heterozygotes show an intermediate phenotype) and codominance (where both alleles are fully expressed, as in blood type AB) are not modeled here.
• Equal gamete probability assumed. The calculator assumes each allele is equally likely to be passed, which is Mendel's Law of Segregation. Meiotic drive, where one allele is preferentially transmitted, is not modeled.

Disclaimer

Punnett Square ratios represent statistical probabilities, not guarantees. Each individual offspring is the result of an independent fertilization event. A family with four children from a Bb x Bb cross has a 25% chance of each child being bb, but they will not necessarily produce exactly one bb child. The ratios describe the long-run frequencies across many offspring.