Genetics Calculator
Build Punnett squares for monohybrid, dihybrid, and ABO blood-type crosses. See genotype and phenotype ratios with percentages under complete, incomplete, and codominant inheritance.
How geneticists use probability
Every Punnett square is a probability model. Mendel's law of segregation says the two alleles of a gene separate into different gametes, so a heterozygote (Aa) passes A or a to each child with 50% probability. His law of independent assortment says genes on different chromosomes are inherited independently — which is why the dihybrid 9:3:3:1 ratio is just two 3:1 ratios multiplied together.
When Punnett squares fail
The clean Mendelian ratios assume idealized conditions. Real inheritance often deviates:
- Linkage — genes close together on the same chromosome are inherited as a block, breaking independent assortment and distorting dihybrid ratios.
- Polygenic traits — height, skin color, and eye color involve many loci; their variation is continuous, not 3:1.
- Epistasis — one gene masks another's expression (e.g. albinism overriding pigment genes), producing modified ratios like 9:7 or 12:3:1.
- Incomplete penetrance — some individuals with a disease genotype never show the disease, so genotype and phenotype probabilities diverge.
Master the baseline first
Those complications are exceptions layered on top of the Mendelian baseline — and you can't spot a distorted ratio until you know the undistorted one. Use the calculator above to drill monohybrid, dihybrid, and blood-type crosses until the ratios are second nature, then the exceptions become easy to recognize in exam problems and pedigree analysis.
| Pattern | Heterozygote look | Classic ratio | Example |
|---|---|---|---|
| Complete dominance | Same as dominant homozygote | 3:1 | Pea flower color (purple × white) |
| Incomplete dominance | Intermediate blend | 1:2:1 | Snapdragons (red × white → pink) |
| Codominance | Both traits fully expressed | 1:2:1 | ABO and MN blood groups |
| Dihybrid (2 genes) | A_B_ shows both dominants | 9:3:3:1 | Pea seed shape × color |
| X-linked recessive | Carrier females usually unaffected | ~50% of sons affected | Color blindness, hemophilia |
| Multiple alleles (ABO) | IᴬIᴮ shows both (AB) | Varies by cross | A × B parents → A, B, AB, or O |
Frequently Asked Questions
What is a Punnett square?
A grid listing one parent's gametes along the top and the other's along the side. Each cell shows an offspring genotype, turning Mendelian segregation into countable probabilities — every cell is equally likely per child.
What is the difference between genotype and phenotype?
Genotype is the allele combination (AA, Aa, aa); phenotype is the observable trait. Under complete dominance, AA and Aa share a phenotype, so the 1:2:1 genotype ratio collapses to 3:1.
What does 9:3:3:1 mean?
In an AaBb × AaBb cross with complete dominance and independent assortment, phenotypes split 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb — the product of two independent 3:1 ratios.
What are the odds of a child having blue eyes?
Eye color is polygenic, so a single-gene Punnett square oversimplifies it. Two brown-eyed parents can have a blue-eyed child if both carry reduced-melanin alleles, but real probabilities involve multiple loci.
What is codominance, and how do ABO blood types work?
Codominance means both alleles are fully expressed. Iᴬ and Iᴮ are codominant and both dominant to i: IᴬIᴮ is type AB, ii is type O. A type A × type B couple can produce A, B, AB, or O children.
Related Calculators
Sources & Methodology
Punnett squares enumerate equally likely gamete combinations under Mendelian segregation and independent assortment; phenotype ratios follow from the selected dominance pattern (complete, incomplete, or codominant).
- MedlinePlus Genetics — U.S. National Library of Medicine
- Khan Academy — Classical genetics
- Nature Scitable — Genetics topic library
Standards and figures reviewed 2026.