Dihybrid Cross Calculator
Calculate the genotype and phenotype ratios of a dihybrid cross (9:3:3:1).
This calculator models a standard cross between two heterozygous parents (AaBb × AaBb) for two independently-assorting traits. Optionally label the two traits and their dominant/recessive forms below.
Trait Labels (optional)
Trait 1 (gene A)
Trait 2 (gene B)
Cross: AaBb × AaBb (both parents heterozygous for both traits)
Phenotype Ratio
9 : 3 : 3 : 1
out of 16 offspring
Phenotype Distribution
Genotype Distribution (out of 16)
4×4 Punnett Grid (gametes: AB, Ab, aB, ab)
What Is a Dihybrid Cross?
A dihybrid cross examines the inheritance of two different traits at the same time. In the classic case, both parents are heterozygous for both genes (AaBb × AaBb). Each parent produces four kinds of gametes — AB, Ab, aB, and ab — in equal proportions, because the two genes assort independently. Combining the four gametes from each parent fills a 4×4 Punnett square with 16 equally likely offspring boxes.
This is the cross Gregor Mendel used to study seed shape and seed color in pea plants. Round (R) seeds are dominant over wrinkled (r), and yellow (Y) seeds are dominant over green (y). Crossing RrYy × RrYy gives the famous 9:3:3:1 phenotype ratio among the offspring.
The Law of Independent Assortment
Mendel's Law of Independent Assortment states that alleles of different genes are distributed to gametes independently of one another, provided the genes are on different chromosomes (or far apart on the same chromosome). The inheritance of seed shape does not influence the inheritance of seed color.
Because each gene follows a 3:1 ratio on its own, the two traits combine multiplicatively. The probability of showing both dominant phenotypes is 3/4 × 3/4 = 9/16, dominant/recessive is 3/4 × 1/4 = 3/16, recessive/dominant is 1/4 × 3/4 = 3/16, and both recessive is 1/4 × 1/4 = 1/16. These four fractions are exactly the 9:3:3:1 ratio.
Understanding the 9:3:3:1 Ratio
The 9:3:3:1 phenotype ratio is the signature outcome of a dihybrid cross between two heterozygotes. Out of every 16 offspring:
- 9/16 (56.25%) show both dominant traits
- 3/16 (18.75%) show the first dominant, second recessive
- 3/16 (18.75%) show the first recessive, second dominant
- 1/16 (6.25%) show both recessive traits
The accompanying genotype distribution sums to 16 boxes and is more diverse: 1 AABB, 2 AABb, 2 AaBB, 4 AaBb, 1 AAbb, 2 Aabb, 1 aaBB, 2 aaBb, and 1 aabb — nine distinct genotypes in a 1:2:2:4:1:2:1:2:1 ratio.
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Educational Disclaimer: This dihybrid cross calculator assumes simple Mendelian inheritance with complete dominance and independent assortment between two heterozygous parents (AaBb × AaBb). Real inheritance can involve gene linkage, incomplete dominance, codominance, epistasis, and environmental effects. Results are for educational purposes only.