A trihybrid cross tracks three genes at once instead of two, turning a 4x4 dihybrid Punnett square into an 8x8 grid of 64 possible offspring. This calculator skips drawing that full grid — it uses the forked-line method to multiply each gene's independent probability directly, giving the same 27:9:9:9:3:3:3:1 ratio (and the probability of any specific offspring) much faster.

How 8 gametes per parent form

Each parent's three-gene genotype splits into gametes during meiosis. Under independent assortment, a heterozygous AaBbCc parent produces 2 x 2 x 2 = 8 gamete types: ABC, ABc, AbC, Abc, aBC, aBc, abC, and abc — one for every combination of an allele from each of the three genes. This is the forked-line method: instead of the FOIL mnemonic (which only expands two binomials), you branch out gene by gene, doubling the gamete count at each of the three genes.

Genotype ratio vs. phenotype ratio

The genotype ratio counts every exact allele combination across the 64-cell grid — for AaBbCc x AaBbCc that's 27 distinct genotype classes. The phenotype ratio groups those genotypes by which traits are actually visible: since a single dominant allele at each gene is enough to mask the recessive one, the 27 genotype classes collapse down to just 8 phenotype classes.

Why three independent 3:1 splits give 27:9:9:9:3:3:3:1

A trihybrid cross between two triple heterozygotes is really three independent monohybrid crosses happening at once. Each gene alone gives a 3:1 dominant-to-recessive split. Because the genes assort independently, you multiply all three ratios together: 3x3x3=27 (dominant at all three), 3x3x1=9 (dominant at two, recessive at one — three ways this can happen depending on which gene is recessive), 3x1x1=3 (dominant at one, recessive at two — again three ways), and 1x1x1=1 (recessive at all three) — the classic 27:9:9:9:3:3:3:1. This only holds when both parents are heterozygous at all three genes; if either parent is homozygous at one gene, that gene stops segregating and the ratio collapses to something simpler, like the 3:1 result of a cross where both parents are AABBCc instead of AaBbCc.