Test cross Punnett square (1:1)
A genetic test cross — crossing an organism of unknown genotype against a homozygous-recessive parent (Bb × bb) to reveal whether it is heterozygous. The engine computes the 1:1 phenotype ratio that signals a heterozygous parent.
For the geneticist or student determining an unknown genotype
What this shows
A test cross answers a practical genetics question: an organism shows the dominant trait, but is it BB or Bb? Cross it against a homozygous-recessive partner (bb) and read the offspring. You write only the cross; the engine computes the outcome that gives the answer.
The recessive bb parent contributes only b gametes, so the offspring genotypes come straight from the unknown parent's gametes. Crossing Bb × bb yields a 1:1 phenotype ratio — half Bb (dominant phenotype), half bb (recessive) — the tell-tale signature of a heterozygous parent. (A BB parent would instead give all dominant offspring.) The engine fills the 2×1 grid, tints the two phenotype classes apart, and reports the 1:1 ratio, so the diagnostic result is immediate.