Free Punnett Square calculator for genetics
Select monohybrid or dihybrid mode, then enter each parent's genotype using standard allele notation (e.g., Aa or AaBb). The calculator constructs a grid placing one parent's possible gametes along the top and the other's down the side, then fills every cell with the resulting offspring genotype to map out all possible genetic combinations from the cross.
Predicting offspring genotypes is essential in selective breeding programs, genetic counseling for prospective parents, and agricultural science. By visualizing every possible allele combination before a cross occurs, breeders can maximize desirable traits and counselors can communicate the probability of inherited conditions, turning abstract genetics into clear, actionable information for decision-making.
A monohybrid cross between two heterozygotes (Aa x Aa) yields the classic 1:2:1 genotypic ratio — one homozygous dominant, two heterozygous, and one homozygous recessive. Dihybrid crosses produce a 9:3:3:1 phenotypic ratio when both parents are heterozygous for two independent genes, reflecting the combinatorial nature of independent assortment during meiosis.
For dihybrid crosses, ensure you use exactly four characters per genotype with capital letters for dominant and lowercase for recessive alleles (e.g., AaBb). Remember that Punnett squares assume independent assortment — linked genes on the same chromosome will deviate from expected ratios. For traits with incomplete dominance or codominance, interpret heterozygous phenotypes accordingly rather than applying simple dominance rules.
A Punnett Square is a diagram used in genetics to predict the genotype and phenotype combinations of offspring from a particular cross or breeding experiment. Named after British geneticist Reginald Punnett, who developed the method in 1905, this tool is fundamental to understanding Mendelian inheritance patterns.
The Punnett Square works by arranging the possible gametes (sex cells) from each parent along the top and side of a grid. Each box within the grid represents a possible genetic combination that could occur when the gametes combine during fertilization.
Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa)
Phenotype: The physical expression of genes (e.g., purple flowers, tall plant)
Allele: Different versions of a gene (dominant = uppercase, recessive = lowercase)
Heterozygous: Having two different alleles (Aa)
Homozygous: Having two identical alleles (AA or aa)
All offspring will be Aa (heterozygous). This is a test cross used to determine if a parent showing the dominant phenotype is homozygous or heterozygous.
Classic Mendelian ratio: 1 AA : 2 Aa : 1 aa (genotypic) and 3 dominant : 1 recessive (phenotypic). This is the most common cross in genetics studies.
1:1 ratio of Aa to aa offspring. Used to determine if a parent with dominant phenotype is heterozygous. If any offspring show recessive trait, parent must be Aa.
9:3:3:1 phenotypic ratio (if genes assort independently). This demonstrates Mendel's Law of Independent Assortment.
Gregor Mendel, the "father of modern genetics," established three fundamental laws through his experiments with pea plants:
Each organism carries two alleles for each trait, but passes only one allele to each offspring. During gamete formation, the two alleles separate (segregate) so that each gamete receives only one allele.
Genes for different traits are inherited independently of each other. The inheritance of one trait does not affect the inheritance of another. This applies to genes on different chromosomes or far apart on the same chromosome.
When two different alleles are present in a heterozygote (Aa), the dominant allele (A) masks the expression of the recessive allele (a). The recessive trait only appears when an organism is homozygous recessive (aa).
Trait: Flower color
Alleles: P (purple, dominant) and p (white, recessive)
Cross: Pp (purple) × Pp (purple)
Punnett Square Results:
• 25% PP (purple)
• 50% Pp (purple)
• 25% pp (white)
Phenotypic Ratio: 3 purple : 1 white
Genotypic Ratio: 1 PP : 2 Pp : 1 pp
While Punnett Squares are powerful tools, they have limitations:
Punnett Squares are used in various fields:
When solving genetics problems, always write out the parent genotypes clearly, determine possible gametes, construct the Punnett Square, and then calculate ratios. Practice with different crosses to build intuition for inheritance patterns!
A Punnett square is a diagram used in genetics to predict the possible genotypes and phenotypes of offspring from a specific cross between two parents. Developed by Reginald Punnett in 1905, it arranges one parent's possible gametes along the top and the other's down the side of a grid. Each cell in the grid represents a possible offspring genotype. For a monohybrid cross (one gene), the grid is 2x2 with 4 outcomes. For a dihybrid cross (two genes), it is 4x4 with 16 outcomes.
A dominant trait is expressed when at least one dominant allele is present (genotype AA or Aa), while a recessive trait is only expressed when two copies of the recessive allele are present (genotype aa). In standard notation, dominant alleles are written as uppercase letters and recessive alleles as lowercase. For example, if purple flower color (P) is dominant over white (p), both PP and Pp plants will have purple flowers, and only pp plants will have white flowers.
For a dihybrid cross, each parent has two genes with two alleles each (e.g., AaBb). First, determine all possible gamete combinations for each parent. An AaBb parent produces four gamete types: AB, Ab, aB, and ab. Create a 4x4 grid with one parent's gametes across the top and the other's down the side, yielding 16 offspring combinations. The classic phenotypic ratio for AaBb x AaBb is 9:3:3:1 (9 showing both dominant traits, 3 and 3 showing one dominant and one recessive, and 1 showing both recessive traits).