Punnett Square Calculator

Use a single letter for each gene (e.g., T for tall, t for short). Write both alleles: TT = homozygous dominant, Tt = heterozygous, tt = homozygous recessive. For dihybrid crosses, use two letters per parent (e.g., TtYy).

Genotype is the genetic makeup of an organism (e.g., Tt). Phenotype is the observable trait or physical expression (e.g., tall plant). Multiple genotypes can produce the same phenotype — both TT and Tt produce the tall phenotype.

For X-linked traits, males are XY (hemizygous — only one copy of the X chromosome) and females are XX. Write the allele superscript on the X chromosome (e.g., X^A for dominant, X^a for recessive). Males express the trait with just one copy.

In incomplete dominance, heterozygotes show an intermediate phenotype that blends both alleles (e.g., red × white = pink flowers). The phenotypic ratio becomes 1:2:1 (same as genotypic ratio) instead of the classic 3:1.

Yes. In codominance, both alleles are fully and simultaneously expressed in heterozygotes (e.g., AB blood type). The Punnett square grid is identical, but heterozygotes display both phenotypes rather than blending them.

The classic 9:3:3:1 ratio assumes independent assortment (genes on different chromosomes or far apart on the same chromosome). Linked genes on the same chromosome do not assort independently and produce modified ratios with more parental-type offspring and fewer recombinants.

In a dihybrid cross between two double heterozygotes (AaBb × AaBb) with independent assortment, 9/16 show both dominant traits, 3/16 show the first dominant and second recessive, 3/16 show the first recessive and second dominant, and 1/16 shows both recessive traits.

A monohybrid cross uses a 2×2 grid (4 squares). A dihybrid cross uses a 4×4 grid (16 squares). A trihybrid cross uses an 8×8 grid (64 squares). The number of squares equals 4^n where n is the number of independently assorting gene pairs.

Absolutely. While the classic textbook cross uses identical genotypes (e.g., Aa × Aa), you can input any combination such as Aa × aa (test cross) or AA × aa. The calculator generates the correct grid for any parental genotype pair.

A test cross mates an individual with a dominant phenotype but unknown genotype (e.g., A?) with a homozygous recessive (aa). The Punnett square shows that if all offspring show the dominant trait, the parent is likely AA; if approximately 50% show the recessive trait, the parent is Aa.

Count how many squares contain that genotype and divide by the total number of squares. For example, in Aa × Aa, the probability of AA is 1/4 = 25%, the probability of Aa is 2/4 = 50%, and the probability of aa is 1/4 = 25%.

A Punnett square refers to the equal-sided grid used when both parents produce the same number of gamete types. When parents produce different numbers of gametes (e.g., AAa × Aa), the grid becomes a Punnett rectangle — it still works the same way but is not square.

Punnett squares are designed for simple Mendelian inheritance involving one or a few genes. Polygenic traits (controlled by many genes, like height or skin color) follow continuous distributions and are better analyzed with quantitative genetics methods, not Punnett squares.

Epistasis occurs when one gene masks or modifies the expression of another gene. This can alter expected ratios — for example, the 9:3:3:1 dihybrid ratio might become 9:3:4, 12:3:1, or 9:7 depending on the type of epistatic interaction between the genes.

Blood type (ABO system) involves three alleles: I^A, I^B, and i. I^A and I^B are codominant, while i is recessive. Create a Punnett square with the parental genotypes (e.g., I^A i × I^B i) to predict the possible blood types of offspring (A, B, AB, or O).

Yes. Males produce X and Y sperm in equal proportions, while females always contribute an X egg. A simple 2×2 Punnett square shows that each child has a 50% chance of being XX (female) or XY (male), regardless of the parental genotypes.

Punnett squares assume complete dominance, independent assortment, equal gamete viability, and random fertilization. They do not account for linked genes, epistasis, incomplete penetrance, environmental influences, lethal alleles, or polygenic inheritance.

A trihybrid cross involves three independent gene pairs (e.g., AaBbCc × AaBbCc) and produces a 64-square grid (8×8). The classic phenotypic ratio is 27:9:9:9:9:3:3:3:1, reflecting all combinations of dominant and recessive phenotypes across three traits.

The Punnett square is a fundamental tool for teaching Mendelian genetics because it provides a visual representation of probability in inheritance. It helps students understand allele segregation, independent assortment, and how genotype ratios translate to phenotype ratios.

A test cross between a heterozygote (Aa) and a homozygous recessive (aa) produces offspring in a 1:1 genotypic ratio — 50% Aa and 50% aa. This corresponds to a 1:1 phenotypic ratio (50% dominant, 50% recessive) and is used to determine an unknown genotype.

For n independently assorting traits, each parent produces 2^n gamete types, resulting in a 2^n × 2^n grid with 4^n total squares. For example, 3 traits give an 8×8 grid (64 squares) and 4 traits give a 16×16 grid (256 squares).

Heterozygous means having two different alleles for a gene (e.g., Aa). In a Punnett square, heterozygous parents each contribute two different gamete types, creating a mix of homozygous dominant, heterozygous, and homozygous recessive offspring.

Linked genes are located on the same chromosome and tend to be inherited together, violating independent assortment. This produces a surplus of parental-type offspring and fewer recombinant types compared to the expected Punnett square ratios.

The Punnett Square Calculator uses standard mathematical formulas validated against authoritative references. The specific formula is displayed in the calculator interface with a detailed explanation of each variable.

Each calculator includes a step-by-step worked example showing exactly how the formula is applied. You can follow these steps with pen and paper to verify any result.

The Punnett Square Calculator accepts numeric inputs including integers and decimals. Invalid inputs (letters, special characters) are rejected with clear error messages.