round, yellow
56.3%
9/16 or 9/16
Dihybrid Cross Probability Calculator
Calculate exact offspring probabilities for two-gene crosses such as AaBb × AaBb, AaBb × aabb, and AaBb × Aabb. The tool builds gametes, displays a live Punnett grid, and reports genotype and phenotype probabilities for specific trait combinations.
Live dihybrid offspring probability calculator
Enter both parent genotypes and choose the phenotype you want to solve. Results update on every input change.
Choose a dihybrid cross preset
Start with a common cross, then edit the parent genotypes or phenotype target.
Parent 1 genotype
Use four alleles in AABB order, such as AaBb, Aabb, aaBb, or aabb.
Gametes from Parent 1
ab 25%aB 25%Ab 25%AB 25%
Parent 2 genotype
The first two letters represent locus A. The last two letters represent locus B.
Gametes from Parent 2
ab 25%aB 25%Ab 25%AB 25%
Phenotype labels
Rename the trait classes to match your organism or textbook problem.
Specific phenotype probability
Choose the phenotype combination you want to calculate.
Live probability result
round, yellow: 56.3%
In the cross AaBb × AaBb, the selected phenotype has a probability of 9/16 per offspring.
Target probability
56.3%
9/16
round, green
18.8%
3/16 or 3/16
wrinkled, yellow
18.8%
3/16 or 3/16
wrinkled, green
6.3%
1/16 or 1/16
Punnett grid
Each cell combines one gamete from each parent. Cell probability reflects the gamete probabilities, so homozygous parents create smaller grids.
Gametes
ab
aB
Ab
AB
ab
aabb
wrinkled, green
6.3%
aaBb
wrinkled, yellow
6.3%
Aabb
round, green
6.3%
AaBb
round, yellow
6.3%
aB
aaBb
wrinkled, yellow
6.3%
aaBB
wrinkled, yellow
6.3%
AaBb
round, yellow
6.3%
AaBB
round, yellow
6.3%
Ab
Aabb
round, green
6.3%
AaBb
round, yellow
6.3%
AAbb
round, green
6.3%
AABb
round, yellow
6.3%
AB
AaBb
round, yellow
6.3%
AaBB
round, yellow
6.3%
AABb
round, yellow
6.3%
AABB
round, yellow
6.3%
Genotype probabilities
Genotype classes collapse identical outcomes from the Punnett grid.
AaBb25.0% · 1/4
aaBb12.5% · 1/8
Aabb12.5% · 1/8
AaBB12.5% · 1/8
AABb12.5% · 1/8
aabb6.3% · 1/16
aaBB6.3% · 1/16
AAbb6.3% · 1/16
AABB6.3% · 1/16
Expected phenotype ratio
The ratio below uses sixteenths so it matches the standard dihybrid grid when both parents are heterozygous.
9:3:3:1
Most likely phenotype: round, yellow
round, yellow9/16
round, green3/16
wrinkled, yellow3/16
wrinkled, green1/16
What is a dihybrid cross?
A dihybrid cross follows two genes at the same time. Gregor Mendel used pea plants to study paired traits such as seed shape and seed colour, then described independent assortment from those patterns. His results showed that one trait can segregate without forcing the second trait to follow it.
Modern genetics connects that pattern to meiosis. Homologous chromosome pairs orient at metaphase I, and unlinked loci enter gametes in independent combinations. Nature Education describes independent assortment as the separation of different genes during reproductive-cell formation. Read the Nature Scitable definition.
In the classic AaBb × AaBb cross, each parent produces AB, Ab, aB, and ab gametes. Random fertilisation gives sixteen genotype cells. Complete dominance condenses those cells into four phenotype classes: A_B_, A_bb, aaB_, and aabb.
How to use this dihybrid probability tool
- 1
Enter both parental genotypes
Type each genotype in AABB order, such as AaBb, Aabb, aaBb, or aabb.
- 2
Rename phenotype classes
Use labels such as round, wrinkled, yellow, and green, or replace them with your organism’s traits.
- 3
Choose a target phenotype
Select A_B_, A_bb, aaB_, aabb, or a custom dominant and recessive combination.
- 4
Read the live probability result
Review the fraction, percentage, Punnett grid, genotype list, and expected phenotype ratio.
The calculator reads the first two letters as the A locus and the last two letters as the B locus. Use uppercase letters for dominant alleles and lowercase letters for recessive alleles.
How dihybrid probability works
Dihybrid probability uses the product rule when two loci assort independently. If Aa × Aa gives a 3/4 chance of A_ and Bb × Bb gives a 3/4 chance of B_, the joint A_B_ probability equals 9/16. That same multiplication creates the largest class in the 9:3:3:1 ratio.
A Punnett grid shows the same calculation visually. Four gametes from one heterozygous parent cross with four gametes from the other parent. The sixteen cells show genotype combinations, while phenotype grouping turns those cells into ratios.
OpenStax Biology explains that the 9:3:3:1 dihybrid ratio can collapse into two separate 3:1 monohybrid ratios when dominance and independent assortment both apply. Review the OpenStax inheritance chapter.
| Cross | Gametes | Common ratio | Use case |
|---|---|---|---|
| AaBb × AaBb | AB, Ab, aB, ab from each parent | 9:3:3:1 | Classic F₂ dihybrid cross |
| AaBb × aabb | Four gametes from one parent | 1:1:1:1 | Dihybrid test cross |
| AaBb × Aabb | Four gametes by two gametes | 3:3:1:1 | Specific phenotype probability problems |
Worked examples
Example 1: AaBb × AaBb
Each parent forms four gametes: AB, Ab, aB, and ab. Each gamete has a 25% probability. The A_B_ phenotype needs at least one dominant A allele and at least one dominant B allele.
The A locus gives A_ with probability 3/4. The B locus gives B_ with probability 3/4. Multiplying the two values gives 9/16, or 56.25%, for the double-dominant phenotype.
Example 2: AaBb × Aabb
Parent 1 produces AB, Ab, aB, and ab gametes. Parent 2 produces Ab and ab gametes. The A locus acts like Aa × Aa, while the B locus acts like Bb × bb.
The probability of A_ equals 3/4. The probability of B_ equals 1/2. A round yellow offspring under this label set has probability 3/4 × 1/2 = 3/8, or 37.5%.
Molecular genetics behind Mendel’s pea traits
Classical symbols such as A and B hide real genes. In pea seed shape, Bhattacharyya and colleagues showed that the wrinkled phenotype comes from a transposon-like insertion in a gene encoding starch-branching enzyme I. That paper linked Mendel’s seed texture trait to starch biosynthesis. View the PubMed record.
Seed colour also has a molecular explanation. Sato and colleagues reported that Mendel’s green cotyledon phenotype involves a stay-green gene that affects chlorophyll degradation. The visible yellow-versus-green trait therefore connects a classroom ratio with plastid pigment metabolism. Read the stay-green gene study.
Practical use in coursework, breeding, and experiment planning
Students use dihybrid probability to solve textbook crosses without drawing every cell by hand. Teachers use the same calculations to explain why phenotype ratios need large sample sizes. A class may observe 8:4:3:1 in a small vial and still study the 9:3:3:1 expectation.
Breeders use two-locus reasoning when they track visible traits in plants, animals, or microbes. The model works best when loci assort independently, dominance stays complete, and every genotype has similar viability. When those assumptions fail, the observed data can point toward linkage, epistasis, selection, or incomplete penetrance.
Limitations of this calculator
This calculator assumes complete dominance at both loci. It does not model incomplete dominance, codominance, epistasis, sex linkage, cytoplasmic inheritance, or lethal genotype classes. Those patterns change phenotype ratios and need different rules.
The tool also assumes independent assortment. Linked loci can produce parental gametes more often than recombinant gametes. Use recombination frequency when a problem gives map distance in centimorgans.
This calculator supports genetics education and experimental planning. It does not provide clinical genetic counselling, diagnosis, or medical risk prediction.
Frequently asked questions
What does a dihybrid cross probability calculator do?
A dihybrid cross probability calculator predicts offspring probabilities for two genes at the same time. It takes parental genotypes such as AaBb × AaBb and builds gametes for each parent. The calculator then combines those gametes into genotype and phenotype probabilities. It also gives the chance of a specific phenotype, such as round yellow offspring from AaBb × Aabb.
Why does AaBb × AaBb produce a 9:3:3:1 ratio?
AaBb × AaBb produces a 9:3:3:1 phenotype ratio when both genes assort independently and show complete dominance. Each parent makes AB, Ab, aB, and ab gametes at 25% frequency. Sixteen fertilisation combinations collapse into four visible phenotype classes. Those classes equal 9 A_B_, 3 A_bb, 3 aaB_, and 1 aabb offspring out of 16.
How do I calculate the probability of a round yellow offspring?
First decide which alleles produce round and yellow phenotypes. In the common pea example, A_ represents round and B_ represents yellow. For AaBb × Aabb, the A locus gives a 3/4 chance of A_, while the B locus gives a 1/2 chance of B_. The product rule gives 3/4 × 1/2 = 3/8, or 37.5%.
What is the difference between genotype probability and phenotype probability?
Genotype probability tracks allele combinations such as AaBb, AAbb, or aabb. Phenotype probability groups genotypes that look the same under the inheritance model. In complete dominance, AABB, AABb, AaBB, and AaBb all show the A_B_ phenotype. That grouping explains why nine different genotype cells can form one visible class in a standard 9:3:3:1 cross.
Can I use this calculator for a dihybrid test cross?
Yes. Enter AaBb × aabb to model a dihybrid test cross. If the genes assort independently, the expected phenotype ratio equals 1:1:1:1. Each class has a 25% probability because the heterozygous parent makes four gamete types equally. A strong deviation from 1:1:1:1 can suggest linkage, viability differences, or scoring problems.
Does the calculator handle linked genes?
This calculator assumes independent assortment. That means it treats the A and B loci as unlinked or far enough apart that recombination randomises allele combinations. Linked genes on the same chromosome can produce parental gametes more often than recombinant gametes. Use a genetic linkage calculator when a problem gives recombination frequency in centimorgans.
Can a dihybrid calculator predict real family outcomes?
A dihybrid calculator predicts probabilities for a genetic model, not guaranteed outcomes for a family. Each child represents a new fertilisation event. A 9/16 probability does not mean the next sixteen children will show exactly nine affected or visible offspring. Human traits also often involve penetrance, variable expressivity, environmental effects, or many genes.
Why does the product rule work for dihybrid probability?
The product rule works when the two events behave independently. In a simple AaBb × AaBb cross, the chance of A_ equals 3/4 and the chance of B_ equals 3/4. Multiplying those values gives 9/16 for the A_B_ phenotype. This multiplication mirrors Mendel’s law of independent assortment during gamete formation.
Related genetics calculators
Use these tools to connect predicted dihybrid probabilities with observed offspring data.
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