What Is Genotype Probability?
A genotype is the pair of alleles an individual carries for a gene, such as Aa. A Punnett square pairs every gamete from one parent with every gamete from the other and counts the results.
Each parent heterozygous for n genes makes 2 to the power n kinds of gametes. A cross of AaBb with AaBb therefore has a 4 by 4 square of 16 boxes and gives the classic 9 : 3 : 3 : 1 phenotype ratio.
Write each parent with two letters per gene, up to three genes. The calculator assumes complete dominance and independent assortment.
How to Use the Genotype Probability Calculator
- Write each parent with two letters per gene, such as Aa or AaBb.
- Use the same letter for the same gene in both parents.
- Read the genotype and phenotype probabilities, and check the chart.
Genotype Probability Formula
Probability of a genotype
Gametes from n heterozygous genes
Genotype Probability Variables Explained
| Symbol | Meaning | Notes |
|---|---|---|
| A, a | Dominant and recessive alleles of one gene | Capital is dominant |
| Gamete | The single allele per gene a parent passes on | |
| Box | One gamete pairing in the Punnett square | |
| P | Probability of an outcome | Boxes with the outcome ÷ total boxes |
Genotype Probability Worked Example
With the example values loaded in the calculator above, the steps are:
- Parent 1 = AaBb, Parent 2 = AaBb.
- Parent 1 makes 4 gametes: AB, Ab, aB, ab. Parent 2 makes 4: AB, Ab, aB, ab.
- Cross every gamete of one parent with every gamete of the other to fill 16 Punnett boxes.
- Count each genotype and divide by 16. A phenotype is dominant if at least one dominant allele is present.
Result: Offspring genotypes = AABB: 1/16 = 6.25%; AABb: 2/16 = 12.5%; AAbb: 1/16 = 6.25%; AaBB: 2/16 = 12.5%; AaBb: 4/16 = 25%; Aabb: 2/16 = 12.5%; aaBB: 1/16 = 6.25%; aaBb: 2/16 = 12.5%; aabb: 1/16 = 6.25%; Offspring phenotypes = A_ (dominant), B_ (dominant): 9/16 = 56.25%; A_ (dominant), bb (recessive): 3/16 = 18.75%; aa (recessive), B_ (dominant): 3/16 = 18.75%; aa (recessive), bb (recessive): 1/16 = 6.25%.
Worked example: Aa × Aa
Each parent makes gametes A and a. The four boxes are AA, Aa, Aa and aa.
So P(AA) = 1/4, P(Aa) = 1/2 and P(aa) = 1/4. With complete dominance, the dominant phenotype has probability 3/4 and the recessive 1/4.
Two genes: AaBb × AaBb
Each parent makes four gametes (AB, Ab, aB, ab), so the square has 16 boxes. The phenotypes come out in the classic 9 : 3 : 3 : 1 ratio.
Where Genotype Probability Is Used
Biology homework
Check monohybrid and dihybrid cross answers.
Predicting traits
Find the chance of a recessive phenotype in offspring.
Teaching Punnett squares
See genotype and phenotype counts for any cross of up to three genes.
Common Genotype Probability Mistakes to Avoid
- Cross two genes by multiplying the probabilities of each gene, if they assort independently.
- Check your phenotype probabilities add to 1.
Genotype Probability Terms and Definitions
- Genotype
- The allele pair an individual carries, such as Aa.
- Phenotype
- The visible trait that results from the genotype.
Genotype Probability FAQs
How do you calculate genotype probability?
Fill a Punnett square with every gametes pairing and divide the count of each genotype by the total number of boxes.
What is the genotype ratio of Aa × Aa?
1 AA : 2 Aa : 1 aa.
What is the phenotype ratio of a dihybrid cross?
AaBb × AaBb gives 9 : 3 : 3 : 1 with complete dominance.
What if the genes are linked?
Linked genes do not assort independently, so the ratios change. This calculator assumes independent assortment.