Punnett Square Calculator - Monohybrid Cross Ratios

A Punnett square calculator is a quick way to predict the genotypes and phenotypes of children from the parents' two-letter genotypes. The grid is a 2x2 matrix where each cell has a 25% chance of being the actual child. This monohybrid tool covers one autosomal gene with two alleles, so it returns the probability of AA, Aa, and aa children, the genotypic ratio, and the phenotypic ratio in one click. Use the result panel to read the per-genotype probabilities and the dominant versus recessive probability in a single view.

• • Working genetics homework: Pick the parents' genotypes from the AA, Aa, and aa selectors and read the 2x2 grid, the 1:2:1 genotypic ratio, and the 3:1 phenotypic ratio for a textbook monohybrid cross without redrawing the diagram by hand.
• • Carrier-screening math for recessive disease: Set both parents to Aa (carriers) to model a child inheriting an autosomal recessive disorder, then read the 25% affected, 50% carrier, 25% unaffected split that counselors use to explain recurrence risk.

A monohybrid Punnett square covers one gene with two alleles, with a capital letter for the dominant allele and a lowercase letter for the recessive one. This calculator renders the same 2x2 grid Reginald Punnett described in 1905, with the genotypic ratio as the count of AA, Aa, and aa cells and the phenotypic ratio grouping AA and Aa as dominant.

The result panel reports the genotypic and phenotypic ratios in their reduced whole-number form plus the dominant and recessive probabilities. The grid dovetails with the allele frequency calculator when the same cross is interpreted at the population level using p and q allele frequencies, and with a goodness-of-fit test when the cross is checked against observed offspring counts.

The tool takes the mother's and father's two-letter genotype, splits each into its two gamete alleles, builds a 2x2 grid, and counts the four cells into AA, Aa, and aa buckets to produce the genotypic ratio, the phenotypic ratio, and the dominant versus recessive probabilities.

• motherGenotype: Mother's two-letter genotype (AA, Aa, or aa) for the autosomal trait.
• fatherGenotype: Father's two-letter genotype (AA, Aa, or aa) for the same trait.
• countAA, countAa, countaa: Number of the four grid cells that contain AA, Aa, and aa respectively.

Each grid cell has a 25% chance of being the actual child, because each parent passes one of its two alleles at random. The phenotypic ratio collapses the heterozygous genotype into the dominant phenotype, so a 1 AA : 2 Aa : 1 aa grid becomes 3 dominant : 1 recessive. For two unlinked genes, the binomial distribution calculator covers the 4x4 grid and the 9:3:3:1 ratio on this site.

According to Wikipedia: Punnett square, a Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles, and the standard monohybrid cross between two heterozygotes gives a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1.

Punnett squares assume independent assortment of alleles and only apply to one gene at a time. For a dihybrid cross the grid grows to 4x4, and the permutation and combination calculator counts the 16 distinct genotype outcomes a 4x4 dihybrid grid produces on this site.

Four short concepts explain every result on the page.

These four ideas form the foundation of a monohybrid Punnett square. The same dominant and recessive logic drives the Hardy-Weinberg equilibrium equation p^2 + 2pq + q^2 = 1, which is the population-level extension of a single Punnett square. The bacteria growth calculator applies the same generational growth math to a microbial culture, so it is a useful companion when you are tracking how a beneficial allele spreads through a colony over many generations.

Four short steps are enough to read a complete 2x2 grid, genotypic ratio, and phenotypic ratio off the page.

1. 1 Choose the mother's genotype: Select AA, Aa, or aa in the Mother's Genotype dropdown. AA means homozygous dominant, Aa means heterozygous (carrier), and aa means homozygous recessive.
2. 2 Choose the father's genotype: Repeat the selection for the Father's Genotype dropdown. The two selections together fully determine the 2x2 monohybrid cross.
3. 3 Read the 2x2 Punnett grid: The result panel renders the four-cell grid with the mother's alleles on the rows, the father's alleles on the columns, and the offspring genotype inside each cell. Each cell is 25% likely.
4. 4 Read the ratios and probabilities: The result panel reports the AA : Aa : aa genotypic ratio, the dominant : recessive phenotypic ratio, and the per-genotype and per-phenotype probabilities in their smallest whole-number form.

To check the recurrence risk for two cystic fibrosis carriers (Aa x Aa), leave both parents set to Aa and read the grid: 1 AA : 2 Aa : 1 aa with a 3:1 phenotype split. 25% of the children will be affected (aa), 50% will be unaffected carriers (Aa), and 25% will be neither (AA), so the standard 1-in-4 recurrence quote matches the tool exactly. For a class assignment or a lab report, paste the resulting counts into the chi-square goodness-of-fit test on this site to see whether the observed offspring match the expected Mendelian ratio.

A purpose-built monohybrid tool removes the manual 2x2 grid drawing and the ratio reduction in one step.

• • One-click 2x2 grid and ratios: The Punnett square calculator returns the 2x2 grid, the AA : Aa : aa genotypic ratio, the dominant : recessive phenotypic ratio, and the dominant versus recessive probability in a single step, so you never redraw the diagram by hand.
• • Six parent combinations covered: Handles every one of the six valid mother x father combinations including the homozygous-dominant, homozygous-recessive, and carrier edge cases.
• • Reduced ratios you can quote: Reduces the genotypic and phenotypic ratios to their smallest whole-number form using the greatest common divisor, so the result panel shows the textbook 1:2:1 and 3:1 ratios.
• • Carrier risk calculation: Quotes the 25% affected / 50% carrier / 25% unaffected split for two Aa parents, the standard recurrence risk for autosomal recessive disorders like cystic fibrosis and Tay-Sachs.

Two variables drive the result, and two limitations tell you when to step outside the 2x2 grid.

• • The tool covers one autosomal gene at a time. Sex-linked traits like hemophilia or color blindness need a different grid because the father passes his Y chromosome to sons and his X chromosome to daughters, which the standard 2x2 grid does not model.
• • Real crosses can deviate from the 3:1 ratio when there is incomplete penetrance, lethal alleles, or epistasis. The tool assumes Mendelian segregation with two equally-likely alleles, so use the 1:2:1 or 3:1 ratio as the null hypothesis and check observed counts with a goodness-of-fit test before quoting a non-Mendelian conclusion.

According to Omni Calculator: Punnett Square, a Punnett square for a single autosomal gene is a 2x2 grid of allele combinations where each cell has a one-in-four (25%) probability, and a cross between two carriers such as Aa x Aa gives a 25% chance of an affected child with an autosomal recessive disorder. Use this Punnett square calculator to verify those numbers against your own cross.

As published by Biology Online: Punnett Square Definition, the calculator-style use of capital and lowercase letters to mark dominant and recessive alleles is a long-standing convention, and the underlying predictions depend on the assumptions of Mendelian segregation and independent assortment of alleles.

Wikipedia notes that a Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles, and that phenotypes predicted by a Punnett square can be modified by polygenic inheritance, epistasis, and other effects. For a single-offspring probability check before scaling up to a family, the probability calculator returns the exact per-pregnancy chance of each outcome the 2x2 grid implies.