The classic Mendelian genetics tool — cross two parents’ alleles for a single gene to find the probability of each possible offspring outcome.
How it works
Every allele from parent 1 is paired with every allele from parent 2, and the resulting genotype and phenotype ratios are tallied across all four combinations.
What this does not include
This does not include multi-gene (dihybrid) crosses or genes with more than two alleles — this calculator specifically handles a single-gene, two-allele monohybrid cross.
How to use this calculator
- Enter the gene letter and each parent’s two alleles (uppercase for dominant, lowercase for recessive).
A worked example
Both parents Aa × Aa: the four offspring combinations are AA, Aa, Aa, aa — a genotype ratio of AA:1, Aa:2, aa:1, giving a phenotype ratio of 3 dominant : 1 recessive (75% show the dominant trait).
One parent Aa, the other aa: phenotype ratio becomes 2 dominant : 2 recessive — exactly 50% dominant.
What the terms mean
| Term | Meaning |
|---|---|
| Genotype | The actual allele pair an offspring inherits (AA, Aa, or aa) |
| Phenotype | The observable trait — dominant if at least one dominant allele is present |
Edge cases worth knowing
This predicts probabilities across many offspring, not any single one’s actual outcome. A specific offspring from an Aa × Aa cross could just as easily be aa (recessive) despite the 3:1 dominant-favored ratio.
This handles a single gene with two alleles only. A dihybrid cross (two genes at once) or a gene with more than two alleles needs a different, larger grid this calculator doesn’t build.
Frequently asked questions
Why is the classic Aa × Aa cross a 3:1 phenotype ratio?
Because three of the four possible offspring combinations (AA, Aa, Aa) carry at least one dominant allele and show the dominant trait, while only one (aa) shows the recessive trait.
What does uppercase versus lowercase mean?
By convention, the uppercase letter represents the dominant allele and lowercase the recessive allele for the same gene.
Does this predict an individual offspring’s traits exactly?
No — it gives the probability distribution across many offspring; any single offspring’s actual genotype is still a matter of chance within that distribution.
