Punnett Square Calculator
2×2 grid, 4 cells — enter 2-character genotypes (e.g., Aa)
Uppercase = dominant, lowercase = recessive
Same letter as Parent 1 at each position
Allele Notation Guide
Uppercase letter (A, B, C) = Dominant allele
Lowercase letter (a, b, c) = Recessive allele
Example: Aa = one dominant + one recessive (heterozygous / carrier)
Enter Parent Genotypes
Select a cross type, enter genotypes for both parents (or load a preset), and the Punnett square grid and probability ratios will appear here instantly.
How to Use the Punnett Square Calculator
Choose a Cross Type
Select Monohybrid (1 trait pair), Dihybrid (2 trait pairs), or Trihybrid (3 trait pairs) from the cross type buttons. Monohybrid is best for learning the basics, producing a simple 2×2 grid. Dihybrid creates a 4×4 grid and demonstrates Mendel's Law of Independent Assortment with the classic 9:3:3:1 ratio.
Enter or Load Parent Genotypes
Type each parent's genotype using uppercase for dominant alleles and lowercase for recessive. For example, 'Aa' means one dominant and one recessive allele at that locus. Or click one of the preset buttons to auto-fill a well-known cross like the Mendel dihybrid pea experiment or a cystic fibrosis carrier cross.
Review the Punnett Square Grid
The color-coded grid appears instantly showing every possible offspring genotype. Each row header shows a gamete from Parent 1, each column header shows a gamete from Parent 2, and each cell shows the offspring genotype that results. Toggle between Genotype View and Phenotype View to see either the genetic makeup or the observable trait expression.
Read Ratios, Probabilities, and Export
Below the grid, find the classic Mendelian ratio, individual genotype and phenotype frequencies with percentages, and the zygosity label for each genotype. Use the Export CSV button to download results for class assignments or genetic counseling notes. Use Print Results to get a clean printed copy. Expand the Step-by-Step panel to see a full explanation of how the grid was constructed.
Frequently Asked Questions
What is the difference between genotype and phenotype in a Punnett square?
Genotype refers to the actual genetic makeup of an organism — the specific alleles it carries at each locus. For example, 'Aa' is a genotype. Phenotype refers to the observable physical characteristic that results from those alleles. In a simple dominant-recessive system, both 'AA' and 'Aa' genotypes produce the dominant phenotype because the dominant allele masks the recessive one. Only the 'aa' genotype produces the recessive phenotype. In a monohybrid cross of Aa × Aa, the genotypic ratio is 1 AA : 2 Aa : 1 aa (1:2:1), while the phenotypic ratio is 3 dominant : 1 recessive (3:1), because both AA and Aa show the dominant trait.
What do the classic Mendelian ratios mean?
The classic ratios describe how frequently each phenotype appears among offspring. For a monohybrid cross between two heterozygotes (Aa × Aa), the phenotypic ratio is 3:1 — three offspring show the dominant phenotype for every one that shows the recessive phenotype. For a dihybrid cross (AaBb × AaBb), the ratio is 9:3:3:1 — nine show both dominant traits, three show dominant A with recessive b, three show recessive a with dominant B, and one shows both recessive traits. For a trihybrid cross, the ratio is 27:9:9:9:3:3:3:1. These ratios are theoretical predictions that hold exactly only over a very large number of offspring due to random chance in fertilization.
What does 'carrier' or 'heterozygous' mean in genetics?
A carrier is an individual who has one dominant allele and one recessive allele at a gene locus — they are heterozygous. Carriers express the dominant phenotype (they appear unaffected), but they carry and can pass on the recessive allele to their children. For example, in cystic fibrosis inheritance, 'Ff' individuals are carriers: they do not have the disease (because F is dominant) but have a 50% chance of passing the recessive 'f' allele to each child. If two carriers have children (Ff × Ff), there is a 25% chance of an affected child (ff), 50% chance of carrier children (Ff), and 25% chance of homozygous dominant children (FF) who are neither affected nor carriers.
Can this calculator handle X-linked or sex-linked traits?
This calculator handles autosomal Mendelian inheritance — traits carried on non-sex chromosomes with clear dominant-recessive relationships. X-linked inheritance (such as color blindness, hemophilia, or Duchenne muscular dystrophy) follows different rules because males have only one X chromosome (XY) while females have two (XX). In X-linked recessive traits, a single copy of the recessive allele causes the condition in males, while females need two copies to be affected. This requires specialized X^A/X^a/Y notation. For clinical questions about X-linked traits, consult a genetic counselor who uses specialized tools for sex-linked pedigree analysis.
Why does a dihybrid cross produce 16 cells instead of 4?
In a monohybrid cross, each parent produces 2 types of gametes (e.g., A and a for parent Aa), creating a 2×2 = 4 cell grid. In a dihybrid cross, each parent produces 4 types of gametes (e.g., AB, Ab, aB, ab for parent AaBb), creating a 4×4 = 16 cell grid. This exponential growth follows the formula 2^n gametes per parent and 4^n total cells, where n is the number of trait pairs. A trihybrid cross (n=3) gives 8 gametes per parent and 64 cells. This is why higher-order crosses become complex quickly — a pentahybrid cross would have 32 gametes per parent and 1,024 cells in the grid.
How accurate are Punnett square probability predictions?
Punnett square predictions are theoretically exact probabilities for simple Mendelian traits, but real offspring ratios vary due to random chance. Just as flipping a fair coin 4 times does not always give exactly 2 heads and 2 tails, a cross of Aa × Aa producing 4 children will not always give exactly 3 dominant and 1 recessive phenotype. The predicted ratios become more accurate as the number of offspring increases. With 100 or more offspring, actual ratios tend to converge toward the theoretical values. Additionally, the predictions assume equal viability of all genotypes, no mutation, random mating, and simple dominance — conditions that may not hold for all traits or species.