Dihybrid Cross Calculator
Calculate offspring ratios for two-trait crosses using the 9:3:3:1 ratio. Find probabilities of specific genotypes and phenotypes using the product rule and independent assortment.
🧬 Genetics📐 P(AaBb) = P(Aa) × P(Bb)🧬 Biology
Probability of first trait dominant phenotype
Probability of second trait dominant phenotype
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Dihybrid Cross Calculator | — | P(AaBb) = P(Aa) × P(Bb) | probability |
Step-by-Step Examples
Example 1
AaBb x AaBb
Standard dihybrid cross. p1=0.75 (Aa x Aa gives 3/4 dominant), p2=0.75.
- P(both dominant) = 0.75 x 0.75 = 0.5625 = 9/16
- P(dom A, rec b) = 0.75 x 0.25 = 0.1875 = 3/16
- P(rec a, dom B) = 0.25 x 0.75 = 0.1875 = 3/16
- P(both recessive) = 0.25 x 0.25 = 0.0625 = 1/16
✓ Classic 9:3:3:1 ratio (9/16 : 3/16 : 3/16 : 1/16)
Example 2
AaBB x AaBb
P(dominant A from AaxAa) = 3/4. P(dominant B from BBxBb) = 1.
- P(A_B_) = 0.75 x 1.00 = 0.75
- P(A_bb) = 0.75 x 0 = 0
- P(aaB_) = 0.25 x 1.00 = 0.25
- P(aabb) = 0
✓ 3:0:1:0 ratio (only B dominant offspring)
Example 3
AABb x aabb
p1 = P(dominant from AA x aa) = 1.0 (all Aa). p2 = P(dominant from Bb x bb) = 0.5.
- P(A_B_) = 1.0 x 0.5 = 0.5
- P(A_bb) = 1.0 x 0.5 = 0.5
- P(aaB_) = 0
- P(aabb) = 0
✓ 1:1:0:0 ratio
Real-World Applications
Plant Genetics
Mendel used dihybrid pea crosses to discover independent assortment law.
Genetic Counseling
Calculate probability child inherits two different recessive disease alleles from carrier parents.
Animal Breeding
Predict coat color and pattern combinations in dogs, cats, and livestock.
Education
Demonstrates product rule and independent assortment in a concrete way.
Common Mistakes to Avoid
⚠️
Multiply independent probabilities
Product rule: P(A and B) = P(A) x P(B) only if A and B are INDEPENDENT (genes on different chromosomes).
⚠️
Check for linkage first
If genes are linked (same chromosome), probabilities do not multiply independently. Recombination frequency needed.
⚠️
9:3:3:1 is for Aa x Aa for BOTH traits
The classic ratio assumes both parents are diheterozygous (AaBb x AaBb) and genes assort independently.
Frequently Asked Questions
What is independent assortment? ▾
Mendel second law: alleles at one locus assort independently of alleles at another locus (when genes are on different chromosomes). Gives 9:3:3:1 ratio.
What is the product rule in genetics? ▾
P(A and B) = P(A) x P(B) for independent events. Use to find probability of specific offspring genotype from two independent loci.
What is the sum rule? ▾
P(A or B) = P(A) + P(B) for mutually exclusive events. Use when multiple genotypes show same phenotype.
How does linkage change the ratio? ▾
Linked genes produce offspring in parental and recombinant classes. Recombinant frequency (map distance) predicts deviation from 9:3:3:1.
What is a testcross? ▾
Cross unknown genotype with homozygous recessive (aabb). Offspring phenotype ratios directly reveal gamete frequencies of unknown parent.