Punnett Square Generator
Enter parent genotypes to generate Punnett squares for monohybrid and dihybrid crosses.
| A | a | |
|---|---|---|
| A | AA | aA |
| a | aA | aa |
Genotype Ratio
Phenotype Ratio
What is a Punnett square?
A Punnett square is a diagram used in genetics to predict the possible genotypes and phenotypes of offspring from a particular cross (mating). Developed by Reginald Punnett in the early 1900s, it provides a simple visual method to calculate the probability of inheriting specific trait combinations based on parental alleles.
Each parent contributes one allele per gene to their offspring. A Punnett square shows all possible combinations of parental alleles. For a single gene with two alleles (e.g., dominant B and recessive b), a monohybrid cross produces a 2x2 grid with 4 possible combinations. Dihybrid crosses (two genes) produce a 4x4 grid with 16 combinations.
How to use this tool
Enter the genotypes of both parents (e.g., Bb x Bb for a monohybrid cross). The tool generates the Punnett square showing all possible offspring genotypes, their ratios, and phenotype predictions assuming complete dominance. For dihybrid crosses, enter two-gene genotypes (e.g., BbRr x BbRr).
Key genetics concepts
- Genotype — the genetic makeup (e.g., BB, Bb, or bb). Phenotype — the observable trait.
- Dominant allele (capital letter) — expressed when one or two copies present. Recessive allele (lowercase) — only expressed when two copies present.
- Homozygous — two identical alleles (BB or bb). Heterozygous — two different alleles (Bb).
- The classic monohybrid ratio from Bb x Bb is 1:2:1 (BB:Bb:bb) genotypically, or 3:1 (dominant:recessive) phenotypically.
Classic examples
Mendel's pea experiments demonstrated the 3:1 ratio with traits like flower color (purple vs white) and seed shape (round vs wrinkled). In humans, examples include earlobe attachment (free vs attached), widow's peak hairline, and tongue rolling ability. However, most human traits are polygenic (controlled by multiple genes) and do not follow simple Mendelian patterns.
Example in practice
Cross two heterozygous parents, Bb × Bb, where B (brown) is dominant over b (blue). The Punnett square gives BB, Bb, Bb, bb — a 3:1 phenotype ratio of brown to blue and a 1:2:1 genotype ratio. So each child has a 75% chance of brown eyes and a 25% chance of blue, with a 50% chance of being a carrier like the parents.
Common mistakes
A common mistake is confusing genotype ratios with phenotype ratios; the classic 3:1 is what you see, while the underlying genotypes split 1:2:1. Another is treating probabilities as guarantees — a 3:1 ratio is expected across many offspring, not four fixed outcomes in one small litter. Assuming simple dominance also misleads when traits are co-dominant or polygenic.
Frequently asked questions
Do Punnett squares work for all traits?
Punnett squares work well for traits controlled by one or two genes with simple dominance. They become complex for polygenic traits (height, skin color), codominant alleles (blood types), sex-linked traits (color blindness), and traits influenced by environmental factors. For these cases, more advanced genetic models are needed, though the basic principle of allele combination still applies.
What is the probability of two carriers having an affected child?
If both parents are carriers (heterozygous, Aa) for an autosomal recessive condition, the Punnett square shows: AA (25%), Aa (50%), aa (25%). There is a 25% chance the child will be affected (aa), a 50% chance of being a carrier (Aa), and a 25% chance of being unaffected and not a carrier (AA). This is the basis for genetic counseling in conditions like cystic fibrosis and sickle cell anemia.
Does a 3:1 ratio mean exactly three of four offspring show the trait?
No. The 3:1 ratio is a probability that holds on average over many offspring, not a fixed count. With only four children each still has an independent 75% chance of the dominant trait, so you might see all four brown-eyed or, less often, all blue — the ratio emerges only across large numbers.