What is a monohybrid cross?

A monohybrid cross is a genetic cross between two parents that differ in a single trait or gene. It produces a 2x2 Punnett square with four possible offspring genotype combinations.

What notation should I use for alleles?

Use a single letter for each allele. Uppercase letters (A, B, T…) represent dominant alleles; lowercase letters (a, b, t…) represent recessive alleles. Both alleles for one parent must use the same letter (e.g., A and a, not A and b).

What does "dominant" mean in this tool?

The tool assumes complete dominance: any genotype with at least one uppercase (dominant) allele shows the dominant phenotype. Only the homozygous recessive genotype (e.g., aa) shows the recessive phenotype.

What is a test cross?

A test cross is a cross between an individual with an unknown genotype and a homozygous recessive individual (e.g., aa). If half the offspring show the recessive phenotype, the unknown parent is heterozygous (Aa).

Why are there only 4 boxes in the square?

A monohybrid cross involves a single gene with two alleles per parent, producing 2 × 2 = 4 possible allele combinations. Dihybrid crosses (two genes) produce a 4x4 square with 16 combinations.

Is this accurate for all inheritance patterns?

This tool models complete dominance only. It does not cover codominance, incomplete dominance, sex-linked traits, or polygenic inheritance, which require different models.

Punnett Square Monohybrid

Enter two parent genotypes for a single gene, and instantly see the 2x2 Punnett square with genotype and phenotype ratios.

Parent Genotypes

Parent 1

Parent 2

Examples:

Punnett Square

Top row = Parent 2 alleles | Left column = Parent 1 alleles

Genotype Ratios

Phenotype Ratios (complete dominance assumed)

Enter parent alleles on the left to generate the Punnett square.

Summary

The Punnett Square Monohybrid tool models a single-gene (one-trait) genetic cross using Gregor Mendel's laws of segregation. Enter one allele pair for each parent (e.g., Aa x Aa), and the tool generates the classic 2x2 Punnett square, tallies all four offspring genotype combinations, and calculates genotype ratios and phenotype ratios assuming complete dominance. All computation runs in your browser — no data is sent to any server.

How it works

Enter the two alleles for Parent 1 (e.g., A and a for heterozygous Aa).
Enter the two alleles for Parent 2 (e.g., A and a).
The tool fills the 2x2 Punnett square by pairing each parent 1 allele with each parent 2 allele.
Genotypes of all four cells are tallied and expressed as ratios (e.g., 1 AA : 2 Aa : 1 aa).
Phenotypes are derived from dominance: one uppercase allele produces the dominant phenotype.
Results display as a visual grid plus percentage bars for genotypes and phenotypes.

Use cases

Predict offspring genotype ratios for homework or exam preparation.
Model carrier status for autosomal recessive conditions.
Demonstrate Mendel's Law of Segregation in classroom settings.
Quickly check expected phenotype ratios for a test cross (Aa x aa).
Explore homozygous-dominant, heterozygous, and homozygous-recessive probabilities.
Assist breeders in estimating trait expression rates in offspring.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu