This page opens the Punnett square calculator on a monohybrid cross, Aa × Aa, the one-gene cross that produces Mendel’s 3:1 ratio. Change either parent to AA or aa, switch the dominance mode, and the 2 × 2 square, the ratios and the gametes update as you type.
A monohybrid cross follows one gene with two alleles. Each parent contributes one allele to every offspring, so a heterozygous parent (Aa) makes two kinds of gametes, A and a, in equal numbers. Crossing two heterozygotes therefore gives four equally likely combinations: AA, Aa, aA and aa. Written as a ratio that is 1 AA : 2 Aa : 1 aa, the genotypic ratio of the cross. When A is completely dominant, AA and Aa cannot be told apart by looking, and the phenotypic ratio collapses to 3 dominant : 1 recessive.
Monohybrid ratios change once the heterozygote has a phenotype of its own. Set the mode to Incomplete and the square colours the Aa cells as a blend: red × white snapdragons give 1 red : 2 pink : 1 white. Set it to Codominant and both alleles show at the same time, the pattern behind the AB blood group or roan cattle, with the same 1:2:1 count. In both modes the genotypic and phenotypic ratios are identical, which is the quickest way to recognise them in exam data.
Open Analysis to see the genotypic and phenotypic ratios side by side, the two gametes of each parent with their 1/2 probabilities, and the zygosity bar, which for Aa × Aa shows half the offspring homozygous and half heterozygous. Tap any cell in the square to highlight every cell with the same outcome and read its probability, for instance Aa · 2/4 = 1/2 · 50%. If you need two genes at once, move the slider to a dihybrid cross; to find out whether a dominant parent is AA or Aa, use the test cross preset.
A monohybrid cross follows a single gene with two alleles, for example Aa × Aa. It is the simplest Punnett square, a 2 × 2 grid with four cells, and it is where Mendel's 3:1 ratio comes from.
A 1:2:1 genotypic ratio (1 AA : 2 Aa : 1 aa) and, under complete dominance, a 3:1 phenotypic ratio, because AA and Aa look the same.
All four offspring show the dominant trait, but half are AA and half are Aa, a 1:1 genotypic ratio. No recessive offspring are possible.
Because the heterozygote has its own intermediate phenotype, so nothing is hidden: 1 red : 2 pink : 1 white for snapdragons. Codominance gives the same 1:2:1 count with both traits shown at once.
Cross it with a homozygous recessive partner (aa). If any offspring show the recessive trait, the plant was Aa; if all offspring are dominant, it was almost certainly AA. Switch this calculator to the test cross preset to see both cases.