Math July 13, 2026 · 8 Min Read

Punnett Square Calculator – Guide & Formulas

Generate Punnett squares for monohybrid and dihybrid genetic crosses. Calculate genotype and phenotype ratios with interactive grid visualization.

Generate interactive Punnett squares for monohybrid and dihybrid crosses. Calculate expected genotype and phenotype ratios for Mendelian genetics.

Key Takeaway

Use the free Punnett Square Calculator to generate punnett squares for monohybrid and dihybrid genetic crosses. calculate genotype and phenotype ratios with interactive grid visualization. Get instant results with step-by-step explanations.

How to Use the Punnett Square Calculator

  1. Select cross type: Monohybrid (1 trait) or Dihybrid (2 traits).
  2. Enter parental genotypes for each parent (e.g., Aa x Aa for monohybrid, AaBb x AaBb for dihybrid).
  3. Use letters to represent alleles: uppercase for dominant, lowercase for recessive.
  4. Review the generated Punnett square grid showing all offspring combinations.
  5. Analyze the genotype ratio, phenotype ratio, and probability percentages.

The Formula

For a monohybrid cross Aa x Aa: genotypic ratio = 1 AA : 2 Aa : 1 aa, phenotypic ratio = 3 dominant : 1 recessive. For a dihybrid cross AaBb x AaBb: phenotypic ratio = 9:3:3:1. Total combinations = 4^n where n = number of heterozygous loci.

Variable Definitions

  • AA: Homozygous dominant genotype
  • Aa: Heterozygous genotype
  • aa: Homozygous recessive genotype
  • P: Probability of a given genotype or phenotype
  • 9:3:3:1: Classic dihybrid phenotypic ratio for two independent traits

Monohybrid Cross: Heterozygous Tall x Heterozygous Tall

Cross two heterozygous pea plants (Tt x Tt) for plant height.

  1. Step 1: Identify parental genotypes: Parent 1 = Tt, Parent 2 = Tt.
  2. Step 2: Determine gametes: Both parents produce T and t gametes (50% each).
  3. Step 3: Fill Punnett square: TT (top-left), Tt (top-right), Tt (bottom-left), tt (bottom-right).
  4. Step 4: Count genotypes: 1 TT : 2 Tt : 1 tt (1:2:1 ratio).
  5. Step 5: Determine phenotypes: 3 Tall (TT, Tt, Tt) : 1 short (tt) = 75% tall, 25% short.

Frequently Asked Questions

How do I write genotypes for a Punnett square?

Use a letter for each gene (e.g., T for tall, t for short). Write both alleles: TT = homozygous dominant, Tt = heterozygous, tt = homozygous recessive. For dihybrid crosses, use two letters (e.g., TtYy).

What is the difference between genotype and phenotype?

Genotype is the genetic makeup (e.g., Tt). Phenotype is the observable trait (e.g., tall plant). Multiple genotypes can produce the same phenotype (e.g., TT and Tt both produce tall).

How do I handle sex-linked traits in a Punnett square?

For X-linked traits, males are XY (hemizygous) and females are XX. Write the allele on the X chromosome (e.g., X^A for dominant, X^a for recessive). Males only need one copy to express the trait.

What is incomplete dominance and how does it change the ratio?

In incomplete dominance, heterozygotes show an intermediate phenotype (e.g., red x white = pink flowers). The phenotypic ratio becomes 1:2:1 (same as genotypic ratio) instead of 3:1.

Can I use this for codominance crosses?

Yes. In codominance, both alleles are fully expressed (e.g., AB blood type). The Punnett square is the same, but heterozygotes show both phenotypes simultaneously rather than blending.

How do dihybrid cross ratios change with linked genes?

The classic 9:3:3:1 ratio assumes independent assortment (genes on different chromosomes). Linked genes on the same chromosome do not assort independently and produce modified ratios with more parental-type offspring.