This Dihybrid Cross Calculator predicts the offspring genotype and phenotype ratios for two traits in a Mendelian dihybrid cross. Enter the parental genotypes (e.g., AaBb × AaBb) and the calculator returns the expected counts and percentages for each outcome.
It uses standard Mendelian assumptions: independent assortment, complete dominance, and no linkage. Use it to check homework answers, plan breeding experiments, or quickly visualize classic genetics results.
What a dihybrid cross tests
A dihybrid cross studies inheritance of two traits at the same time. Each trait has two alleles (for example, A/a for one trait and B/b for another). The classic example is crossing individuals that are heterozygous for both traits.
In Mendelian genetics, the key idea is that the alleles for one gene assort independently of the alleles for the other gene, producing predictable ratios.
Core concepts and variables
To compute expected offspring, the calculator treats each trait separately and then combines the results.
- Trait 1 alleles: A and a (inputs like AA, Aa, or aa).
- Trait 2 alleles: B and b (inputs like BB, Bb, or bb).
- Dominance: A and B alleles are assumed dominant over a and b.
- Independent assortment: gametes are formed independently for each trait.
- Total offspring (N): used to convert ratios into expected counts.
How the calculator works (the formulas)
The calculator computes genotype probabilities for each trait, then multiplies them to get combined two-trait probabilities.
Step 1: compute gamete probabilities for each parent
For a single trait, the gamete allele probabilities depend on the parent genotype.
| Parent genotype | Gamete outcomes | Probabilities |
|---|---|---|
| AA | A | 1.0 |
| Aa | A or a | 0.5 each |
| aa | a | 1.0 |
The same logic applies to trait 2 using B/b alleles.
Step 2: compute offspring genotype probabilities for each trait
For trait 1, the calculator multiplies gamete probabilities from Parent 1 and Parent 2. For example, if Parent 1 is Aa and Parent 2 is Aa, then:
- P(AA) = 0.5 × 0.5 = 0.25
- P(Aa) = (0.5 × 0.5) + (0.5 × 0.5) = 0.5
- P(aa) = 0.5 × 0.5 = 0.25
The calculator does this automatically for both traits.
Step 3: combine trait 1 and trait 2 probabilities
Because of independent assortment, the probability of a combined genotype is the product of the trait probabilities:
- P(AxBy) = P(trait 1 genotype Ax) × P(trait 2 genotype By)
Then the calculator converts combined genotypes into phenotypes using dominance rules:
- A_ means trait 1 phenotype shows the dominant trait (A).
- aa means trait 1 shows the recessive trait (a).
- B_ means trait 2 phenotype shows the dominant trait (B).
- bb means trait 2 shows the recessive trait (b).
Outputs: genotypes and phenotypes
The calculator returns two types of results:
- Genotype ratio: expected proportions for each combined genotype (like AABb).
- Phenotype ratio: expected proportions for each trait expression (like A_B_).
It also shows expected counts by multiplying each probability by your chosen total offspring number.
How to use the Dihybrid Cross Calculator
- Select each parent’s genotype for Trait 1 (AA, Aa, or aa).
- Select each parent’s genotype for Trait 2 (BB, Bb, or bb).
- Enter the total offspring you want to model.
- Click Calculate to view expected genotype and phenotype ratios and counts.
Practical examples
Example 1: Classic dihybrid cross (AaBb × AaBb)
In the classic Mendelian dihybrid cross, both parents are heterozygous for both traits: AaBb × AaBb. The expected phenotype ratio is 9:3:3:1.
That corresponds to:
- 9 dominant for both traits (A_B_)
- 3 dominant for trait 1 only (A_bb)
- 3 dominant for trait 2 only (aaB_)
- 1 recessive for both (aabb)
Use the calculator to confirm these ratios and to get expected counts for any total offspring number.
Example 2: Test cross (AaBb × aabb)
A test cross uses an individual with unknown genotype crossed to a homozygous recessive (like aabb). If you cross AaBb × aabb, the phenotype ratio becomes 1:1:1:1.
This is useful in breeding and lab work because it reveals hidden (heterozygous) alleles by observing offspring phenotypes.
Common mistakes to avoid
- Mixing up genotype and phenotype: genotypes include allele combinations; phenotypes show trait expression.
- Forgetting dominance: A_ and B_ produce dominant phenotypes; aa and bb produce recessive phenotypes.
- Assuming linkage: this calculator assumes genes assort independently. Linked genes can change ratios.
- Using it for non-Mendelian cases: incomplete dominance, codominance, epistasis, or multiple alleles require different models.
Frequently Asked Questions
What is a dihybrid cross ratio?
A dihybrid cross ratio describes the expected proportions of offspring phenotypes or genotypes when two traits are crossed at once. For the classic heterozygous cross AaBb × AaBb, the phenotype ratio is 9:3:3:1, and the genotype distribution follows predictable Mendelian probabilities.
Does a dihybrid cross assume independent assortment?
Yes. Standard dihybrid cross calculations assume independent assortment, meaning the allele combinations for trait 1 do not affect those for trait 2. This is why you can multiply probabilities for each trait. If genes are linked on the same chromosome, real outcomes can deviate from 9:3:3:1.
How do I interpret results like A_B_ or A_bb?
Notation like A_B_ means trait 1 shows the dominant phenotype (A allele present) and trait 2 also shows the dominant phenotype (B allele present). A_bb means trait 1 is dominant while trait 2 is recessive. The underscore indicates either heterozygous or homozygous.
What counts as a genotype in a dihybrid cross?
A genotype in a dihybrid cross includes both traits’ allele pairs together, such as AABb or AaBb. Trait 1 contributes two alleles (AA, Aa, or aa) and trait 2 contributes two alleles (BB, Bb, or bb). Combined genotypes determine exact expected probabilities.
Why don’t real offspring always match the predicted ratios?
Real populations show variation because of randomness in inheritance. Small sample sizes can produce ratios that differ from theory even when assumptions are correct. Also, deviations occur if dominance is incomplete, genes are linked, or there are selection effects, mutations, or non-Mendelian inheritance.