Preset buttons
These buttons load common classroom crosses. They help you compare 3:1, 1:2:1, codominant, and dihybrid outcomes without typing each example.
Phenotype Probability Calculator for Genetic Crosses
Calculate the probability of a specific offspring phenotype from parent genotypes. The tool supports complete dominance, recessive traits, incomplete dominance, codominance, and custom targets such as A_B_, A_bb, aa, and Aa.
Live Phenotype Probability Calculator
Enter parent genotypes, choose the inheritance rule, and define the phenotype you want. The result updates instantly as a percentage, fraction, distribution chart, and Punnett grid.
Load a phenotype probability example
Start with a textbook cross, then edit the genotypes and target phenotype pattern.
Parent genotype inputs
Type one, two, or three loci. Use paired alleles such as Aa, AaBb, or AaBbCc.
Phenotype rule and target
Choose how genotypes map to phenotypes, then enter the phenotype you want to calculate.
Pattern guide
Use A_ for at least one dominant allele, aa for homozygous recessive, and Aa for an exact heterozygote.
Live phenotype result
A_B_ probability = 56.25%
This equals 9/16 of the possible offspring outcomes under the selected inheritance rule.
Target chance
56.25%
Phenotype probability distribution
Each bar shows the chance of a phenotype class after combining all gamete outcomes.
A_B_56.25%
A_bb18.75%
aaB_18.75%
aabb6.25%
Parent gametes
Parent 1 gametes
AB 25%Ab 25%aB 25%ab 25%
Parent 2 gametes
AB 25%Ab 25%aB 25%ab 25%
Target interpretation
Cross: AaBb × AaBb
Rule: complete
Matching outcomes: 9 of 16 Punnett cells before probability weighting.
Most common class: A_B_ at 56.25%
Punnett grid behind the probability
Highlighted cells match your target pattern. Each cell shows genotype and phenotype class.
Gametes
AB
Ab
aB
ab
AB
AABB
A_B_
AABb
A_B_
AaBB
A_B_
AaBb
A_B_
Ab
AABb
A_B_
AAbb
A_bb
AaBb
A_B_
Aabb
A_bb
aB
AaBB
A_B_
AaBb
A_B_
aaBB
aaB_
aaBb
aaB_
ab
AaBb
A_B_
Aabb
A_bb
aaBb
aaB_
aabb
aabb
What questions does Phenotype Probability Calculator answer?
A student often knows the parent genotypes but needs one specific outcome. For example, AaBb × AaBb can produce A_B_, A_bb, aaB_, and aabb phenotypes. This calculator answers the direct question: what is the chance of the phenotype I want?
The tool separates genotype from phenotype. That distinction matters because AA and Aa can show the same dominant phenotype, while Aa forms a separate class in incomplete dominance and codominance. OpenStax describes how Mendel's dominance rule works and why other inheritance patterns extend that rule. Read the OpenStax inheritance overview.
You can use the calculator for single-locus crosses, dihybrid crosses, and three-locus practice questions. It works best when each gene uses two alleles and the genes assort independently.
How to use Phenotype Probability Calculator
- 1
Enter the parent genotypes
Type each parent genotype using paired alleles, such as Aa, AaBb, or Aabb.
- 2
Choose the phenotype rule
Select complete dominance, incomplete dominance, codominance, or custom target pattern logic.
- 3
Set the target phenotype
Use patterns such as A_, aa, A_B_, A_bb, or Aa to define the offspring class you want.
- 4
Read the live probability
Review the percentage, fraction, gamete list, phenotype distribution, and highlighted Punnett grid cells.
Use capital letters for dominant alleles and lowercase letters for recessive alleles. Keep the same gene order in both parents. Write AaBb × Aabb, not AaBb × BbAa.
What each part of Phenotype Probability Calculator does
Parent genotype cards
Each card stores one parent's alleles. The calculator converts those alleles into gametes before it builds offspring combinations.
Inheritance rule selector
This control tells the calculator how genotype becomes phenotype. Complete dominance combines AA and Aa, while incomplete dominance and codominance keep Aa separate.
Target phenotype input
This field defines the offspring class you want. A_B_ means at least one dominant allele at both loci, while aa means homozygous recessive at one locus.
Result banner, bars, and Punnett grid
The banner gives the direct answer. The bars show all phenotype classes. The grid lets you see which gamete combinations match the target and which combinations produce other outcomes.
Phenotype rules used by the calculator
Complete dominance maps AA and Aa to the same dominant phenotype. Recessive phenotypes need aa, because one dominant allele masks the recessive allele in the heterozygote.
Incomplete dominance gives the heterozygote an intermediate phenotype. Aa × Aa therefore gives 25% homozygous dominant, 50% intermediate, and 25% homozygous recessive offspring. Nature Education explains this genotype-phenotype relationship and contrasts incomplete dominance with codominance. Review the Nature Education dominance article.
Codominance lets both alleles express in the heterozygote. A codominant Aa offspring does not look like AA or aa. The calculator treats that heterozygote as its own phenotype class.
Worked phenotype probability examples
Example 1: recessive phenotype from Aa × Aa
Two carriers produce four genotype outcomes: AA, Aa, Aa, and aa. Only aa shows the recessive phenotype under complete dominance. The phenotype probability equals 1/4, or 25%.
Example 2: A_bb phenotype from AaBb × Aabb
The target A_bb needs at least one dominant A allele and two recessive b alleles. Parent 1 makes AB, Ab, aB, and ab gametes. Parent 2 makes Ab and ab gametes. Four of eight weighted outcomes match A_bb, so the probability equals 50%.
Genotype probability vs phenotype probability
Genotype probability asks which allele pair an offspring inherits. Phenotype probability asks what trait class that genotype produces. A single phenotype can contain several genotypes.
Aa × Aa shows the difference clearly. The genotype ratio is 1 AA : 2 Aa : 1 aa. Under complete dominance, AA and Aa share the dominant phenotype, so the phenotype ratio becomes 3 dominant : 1 recessive.
Dihybrid crosses expand the same idea. A_B_ includes AABB, AABb, AaBB, and AaBb genotypes. That is why a phenotype class can be common even when any single genotype inside it has a smaller probability.
When phenotype probability needs extra biological context
This calculator assumes independent assortment unless you enter a single-locus cross. Linked genes can change gamete frequencies and shift phenotype probabilities away from simple Punnett square expectations.
Real traits can also involve penetrance, expressivity, environment, epistasis, or many genes of small effect. Human eye colour, height, and skin pigmentation do not follow a single Aa pattern.
Use this page for learning, homework checking, and planning simple genetic crosses. Do not use it as a clinical genetic risk assessment or diagnostic tool.
Phenotype Probability Calculator FAQs
What does a phenotype probability calculator do?
A phenotype probability calculator estimates the chance that offspring will show a selected visible trait. It starts with parent genotypes such as Aa, AaBb, or Aabb. The tool builds gametes, combines them, and maps each offspring genotype to a phenotype rule. It then reports the target phenotype as a percentage, fraction, and distribution chart.
How do I enter a target phenotype such as A_B_ or aa?
Use A_ when one dominant allele gives the phenotype. Use aa when only the homozygous recessive genotype gives the phenotype. For a two-gene target, combine both parts, such as A_B_ for dominant phenotypes at both loci or A_bb for dominant A with recessive b. The calculator also accepts exact genotype targets such as Aa when you want a heterozygote probability.
What is the phenotype probability for Aa × Aa under complete dominance?
Aa × Aa produces AA, Aa, Aa, and aa offspring genotypes. Under complete dominance, AA and Aa show the dominant phenotype because each genotype carries at least one A allele. That gives a dominant phenotype probability of 3/4, or 75%. The recessive phenotype aa appears in 1/4, or 25%, of offspring.
Why does AaBb × AaBb give 56.25% for A_B_?
A dihybrid AaBb × AaBb cross creates 16 equally likely Punnett square cells when the genes assort independently. Nine cells contain at least one dominant A allele and at least one dominant B allele. That makes A_B_ equal to 9/16. As a percentage, 9/16 equals 56.25%.
Can this calculator handle incomplete dominance?
Yes. In incomplete dominance, the heterozygote has its own phenotype instead of matching one homozygote. Aa × Aa therefore gives a 1:2:1 phenotype ratio, not a 3:1 ratio. The calculator labels AA, Aa, and aa as separate phenotype classes. This works well for teaching examples such as red, pink, and white flower colour.
Can this calculator handle codominance?
Yes. Codominance means the heterozygote expresses both alleles at the same time. A common classroom example uses blood-group style logic, where a heterozygote shows both allele products. The calculator separates AA, Aa, and aa as distinct phenotype classes under codominance. It gives the Aa codominant phenotype a 50% probability in an Aa × Aa cross.
Is phenotype probability the same as genotype probability?
No. Genotype probability counts allele combinations, while phenotype probability counts visible or measurable trait classes. Under complete dominance, AA and Aa differ as genotypes but share the same dominant phenotype. That is why Aa × Aa gives a 1:2:1 genotype ratio but a 3:1 phenotype ratio. In incomplete dominance and codominance, the heterozygote forms a separate phenotype, so genotype and phenotype ratios can match.
Why can real offspring counts differ from calculated phenotype probabilities?
Calculated probabilities describe expected outcomes across many offspring. A small family or classroom sample can differ because fertilisation includes chance. Biology can also change the expected pattern through linkage, selection, incomplete penetrance, or environmental effects. Use a chi-square test when you want to compare observed counts with the expected phenotype ratio.
Related phenotype and Punnett square calculators
Use these tools when you need the full cross table or want to compare observed offspring counts with an expected ratio.
Related Tools
Dihybrid Cross Probability Calculator
Calculate exact genotype and phenotype ratios for two-locus crosses such as AaBb × AaBb.
Open CalculatorMendelian Ratio Chi-Square Calculator
Compare observed offspring counts with expected ratios such as 3:1, 1:2:1, and 9:3:3:1.
Open Calculator