The four crosses and what each one gives
A monohybrid cross tracks one gene with a dominant allele (A) and a recessive one (a). Each parent passes one allele, so the grid has four equally likely boxes. The parents' genotypes decide the ratio.
| Cross | Genotype ratio | Phenotype ratio |
|---|---|---|
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dominant : 1 recessive |
| Aa × aa | 1 Aa : 1 aa | 1 dominant : 1 recessive |
| AA × aa | all Aa | all dominant |
| AA × Aa | 1 AA : 1 Aa | all dominant |
| aa × aa | all aa | all recessive |
The Aa × aa cross is the classic "test cross" — a 1:1 result tells you the dominant-looking parent was a heterozygote, not AA.
The 2×2 grid for a heterozygous cross
For Aa × Aa, each parent contributes A or a with equal chance. Filling the four boxes gives one AA, two Aa and one aa.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Where the ratio breaks down
- Incomplete dominance. If the heterozygote shows a blended trait (red × white = pink), the phenotype ratio matches the genotype ratio: 1:2:1, not 3:1.
- Codominance. Both alleles show at once (like blood type AB), so again all three genotypes look different.
- Linked genes. A single Punnett square assumes independent assortment; genes close together on the same chromosome do not sort freely.
Common questions
Why does Aa x Aa give a 3:1 ratio?
The 2x2 grid produces one AA, two Aa and one aa. Since A is dominant, the three offspring carrying at least one A all show the dominant trait, and only aa shows the recessive one. That is 3 dominant to 1 recessive.
What is the difference between genotype and phenotype ratio?
Genotype is the allele combination, phenotype is the visible trait. Aa x Aa gives a 1:2:1 genotype ratio (AA:Aa:aa) but a 3:1 phenotype ratio, because AA and Aa look the same when A is dominant.
Do the ratios guarantee the outcome in a real litter?
No. A 3:1 ratio is the probability per offspring, not a fixed count. In a litter of four you might see 4:0 or 2:2 by chance. The predicted ratio only shows up reliably across large numbers.
