Dihybrid Cross Calculator - Punnett Square

A dihybrid cross tracks two separate genes, each with two different alleles, located on non-homologous chromosomes. According to Gregor Mendel's Law of Independent Assortment, allele pairs separate independently during the formation of gametes. For two heterozygous parents (e.g., AaBb), each parent can produce four unique gamete types: AB, Ab, aB, and ab. Crossing these gametes requires a 16-box Punnett square. The calculator determines the frequency of every resulting offspring genotype—ranging from homozygous dominant (AABB) to homozygous recessive (aabb)—by multiplying individual independent probabilities or charting the full grid distribution.

Let us trace a classic dihybrid cross where both parents are heterozygous for seed shape and seed color. Parent 1 is AaBb (Round and Yellow seeds) and Parent 2 is AaBb (Round and Yellow seeds). First, we determine the four gamete combinations for each parent: AB, Ab, aB, and ab. When mapped across a 16-square grid, we tally the offspring genotypes. For instance, the probability of obtaining offspring that are homozygous recessive for both traits (aabb, wrinkled and green seeds) is calculated by multiplying the independent recessive probabilities: 1/4 chance for 'aa' multiplied by 1/4 chance for 'bb', yielding a 1/16 (or 6.25%) chance. The classic phenotypic ratio for such a cross will always result in a predictable 9:3:3:1 distribution.

Always assign clear, consistent letters for alleles, using uppercase for dominant traits and lowercase for recessive traits. When setting up your crosses manually on paper, remember to use the FOIL method (First, Outer, Inner, Last) to derive all four possible gametes from each parent genotype. Double-check that your final phenotypic and genotypic fractions add up to a total of 1 (or 100%) to ensure your probability calculations are mathematically sound.