56.25
%
18.75
%
18.75
%
6.25
%
75
%
75
%
16
16
56.25
%
18.75
%
18.75
%
6.25
%
75
%
75
%
16
16
The Dihybrid Cross Calculator predicts offspring phenotype ratios when two genes are considered simultaneously. A dihybrid cross involves parents that are heterozygous for two independently assorting genes, producing the famous 9:3:3:1 phenotype ratio in the F2 generation first observed by Mendel.
Enter the genotype of each parent for both genes (number of dominant alleles: 2=homozygous dominant, 1=heterozygous, 0=homozygous recessive) to see the expected offspring phenotype classes.
For two independently assorting genes, the probability of each phenotype class is the product of the individual gene probabilities. For the classic AaBb × AaBb cross:
Gene A alone: 3/4 dominant, 1/4 recessive. Gene B alone: 3/4 dominant, 1/4 recessive.
Both dominant (A_B_): 3/4 × 3/4 = 9/16
A dominant, B recessive (A_bb): 3/4 × 1/4 = 3/16
A recessive, B dominant (aaB_): 1/4 × 3/4 = 3/16
Both recessive (aabb): 1/4 × 1/4 = 1/16
Inputs
Results
The classic 9:3:3:1 ratio: 56.25% show both dominant traits, 18.75% each show one dominant and one recessive, and 6.25% show both recessive traits.
Inputs
Results
A test cross of AaBb × aabb produces four phenotype classes in equal proportions (1:1:1:1 or 25% each), confirming independent assortment.
The 9:3:3:1 ratio describes the expected phenotype proportions in the F2 generation of a dihybrid cross between two heterozygous parents (AaBb × AaBb). Out of 16 equally likely outcomes: 9 show both dominant phenotypes, 3 show dominant A with recessive B, 3 show recessive A with dominant B, and 1 shows both recessive phenotypes.
Independent assortment is Mendel's second law, stating that genes on different chromosomes are inherited independently. During meiosis, the segregation of alleles at one locus does not influence the segregation at another locus (provided they are on different chromosomes). This is what produces the 9:3:3:1 ratio in dihybrid crosses.
Deviations occur with: linked genes (on the same chromosome), epistasis (one gene masks another), lethal allele combinations, incomplete dominance, or codominance. For linked genes, the recombinant classes are underrepresented. Epistasis can produce modified ratios like 9:7, 12:3:1, or 9:3:4.
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