Calculate phenotype ratios for three-trait genetic crosses. Predict outcomes when tracking three independent genes simultaneously.
Last updated: March 2026
A trihybrid cross is a genetic cross between two individuals that differ in three traits, each controlled by a different gene. This extends the principles of Mendelian genetics to analyze the simultaneous inheritance of three independent characteristics.
While a monohybrid cross tracks one gene (producing 4 possible offspring in a 2×2 grid) and a dihybrid cross tracks two genes (16 offspring in a 4×4 grid), a trihybrid cross tracks three genes and produces 64 possible offspring combinations in an 8×8×8 conceptual space.
The classic trihybrid cross between two triple heterozygotes (AaBbCc × AaBbCc) produces the famous 27:9:9:9:3:3:3:1 phenotypic ratio, representing all eight possible phenotype combinations. This demonstrates the law of independent assortment across three genes.
Classic Trihybrid Cross: AaBbCc × AaBbCc
This is the hallmark ratio of a trihybrid cross between two triple heterozygotes. It represents the eight possible phenotype combinations from three independently assorting genes.
The famous 27:9:9:9:3:3:3:1 phenotypic ratio is not memorized—it is derived by combining three independent monohybrid crosses. Each gene follows the classic Mendelian ratio of 3 dominant : 1 recessive.
Because the three genes assort independently, the probabilities are multiplied together.
These probabilities simplify to the classic 27:9:9:9:3:3:3:1 phenotypic ratio for a cross between two AaBbCc parents.
During meiosis, each gamete receives one allele from every gene. A parent with genotype AaBbCc has two possible alleles at each of three loci, producing 2³ = 8 unique gamete combinations.
Each gamete is produced with an equal probability of 1/8. Crossing two heterozygous parents therefore produces 8 × 8 = 64 possible offspring genotype combinations.
This calculator models a classical Mendelian trihybrid cross and assumes the following:
If these assumptions are violated, observed offspring ratios may differ substantially from the expected Mendelian predictions.
This calculator reports phenotype ratios, which describe the observable traits of the offspring. Multiple genotypes can produce the same phenotype when complete dominance is present.
Count every unique allele combination. A trihybrid cross can generate many distinct genotypes because each gene has three possible genotype classes (AA, Aa, aa).
Group genotypes according to their observable traits. Under complete dominance, the numerous genotypes collapse into just eight phenotype classes, producing the familiar 27:9:9:9:3:3:3:1 ratio.
Real biological systems do not always follow classical Mendelian inheritance. The expected trihybrid ratios may change under several conditions.
This calculator assumes classical Mendelian inheritance and therefore does not model these more complex genetic mechanisms.
Each parent can produce 8 different gamete types (2³ = 8 combinations of alleles). When crossed, 8 × 8 = 64 possible offspring combinations. This assumes independent assortment of all three genes.
No, this calculator assumes independent assortment (genes on different chromosomes or far apart on the same chromosome). Linked genes don't follow these ratios and require different analysis methods.
This is the phenotypic ratio for AaBbCc × AaBbCc. 27/64 show all dominant traits, 9/64 each show two dominant + one recessive, 3/64 each show one dominant + two recessive, and 1/64 shows all recessive.
Yes! A test cross would be something like AaBbCc × aabbcc. This produces simpler ratios (1:1:1:1:1:1:1:1 for all eight phenotypes) useful for determining unknown genotypes.
A dihybrid cross tracks two genes (16 offspring, 9:3:3:1 ratio), while a trihybrid tracks three genes (64 offspring, 27:9:9:9:3:3:3:1 ratio). The principles are the same, just more complex.
They're less common than monohybrid or dihybrid crosses in controlled breeding but do occur. They're important in plant and animal breeding programs tracking multiple desirable traits simultaneously.
This calculator assumes complete dominance. With incomplete dominance or codominance, you'd see more phenotype categories (27 genotypes would produce 27 different phenotypes instead of 8).
Yes, but it gets very complex (256 offspring combinations!). For four or more genes, computer programs or simplified approaches are typically used instead of manual calculation.
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