Dihybrid Cross Calculator and Punnett Square

A dihybrid cross follows the inheritance of two genes at the same time. In the simplest Mendelian model, each gene has two alleles, one written with an uppercase letter and one with the matching lowercase letter. The uppercase allele is treated as completely dominant. An organism with AA or Aa therefore shows the dominant phenotype for the first trait, while only aa shows the recessive phenotype. The same reasoning applies independently to BB, Bb, and bb for the second trait. This calculator accepts compact four-letter genotypes such as AaBb. The first two characters describe the first gene and the final two describe the second gene. It builds every distinct gamete each parent can form by choosing one allele from each gene. AaBb can produce AB, Ab, aB, and ab gametes. AABb produces only AB and Ab because every gamete receives A. The calculator pairs all parental gametes, evaluates the phenotype of each possible offspring, and reports the proportions in four phenotype classes. The classic cross AaBb by AaBb produces the well-known 9 to 3 to 3 to 1 phenotype ratio. Nine sixteenths show both dominant traits, three sixteenths show only the first dominant trait, three sixteenths show only the second dominant trait, and one sixteenth shows both recessive traits. A test cross between AaBb and aabb produces four phenotype classes in equal proportions. These probabilities describe expected frequencies over many offspring, not a guarantee for a family of four or even sixteen individuals. The simple Punnett model depends on several assumptions. The two genes should assort independently, which is most likely when they are on different chromosomes or far apart on the same chromosome. Dominance must be complete, allele transmission must be equally likely, and genotype should map directly to phenotype without environmental or viability effects. Linked genes, incomplete dominance, codominance, epistasis, sex linkage, lethal combinations, meiotic drive, and sampling variation can all produce different observations. Use the calculator to check homework, design teaching examples, or form an expected ratio before analyzing experimental offspring. Always write each gene's allele pair together and keep the same gene order for both parents. For observed biological data, compare counts with expected probabilities using an appropriate goodness-of-fit test and consider whether the model's assumptions are justified. A Punnett square is a model of allele combinations; careful interpretation connects that model to real inheritance.