Counting the ways alleles combine
A Punnett square is bookkeeping for genetics. Each parent contributes one allele per gene to each offspring, and the square lists every combination those contributions can make, each one equally likely.
For a single gene, a parent carrying Aa passes on A half the time and a the other half. Line up two such parents and the four cells of the grid fall out as AA, Aa, Aa, aa.
The monohybrid result
| Genotype ratio | 1 AA : 2 Aa : 1 aa |
|---|---|
| As percentages | 25% : 50% : 25% |
| Phenotype ratio | 3 dominant : 1 recessive |
The jump from 1:2:1 to 3:1 is the key move: dominance hides the difference between AA and Aa, so three of the four outcomes look the same and only the aa quarter shows the recessive trait.
From one gene to two
Adding a second independent gene multiplies the possibilities rather than adding them. A dihybrid cross has sixteen cells and a 9:3:3:1 phenotype ratio, but each gene on its own still follows the same 3:1 pattern. The square scales up cleanly because the genes assort independently — which is exactly the assumption to check before trusting the ratio.
Frequently asked questions
What is a Punnett square?
A grid that lays out every way two parents' alleles can combine in their offspring. Each parent's possible gametes label the rows and columns, and each cell is one equally likely offspring genotype. It turns a genetics question into simple counting.
What does Aa × Aa give?
The classic monohybrid result: a genotype ratio of 1 AA : 2 Aa : 1 aa, which is 25% : 50% : 25%. Because A is dominant, the three genotypes collapse into a phenotype ratio of 3 showing the dominant trait to 1 showing the recessive — the famous 3:1.
What is the difference between genotype and phenotype ratios?
Genotype is the exact allele pairing (AA, Aa or aa); phenotype is the visible trait it produces. Since AA and Aa both show the dominant trait, the 1:2:1 genotype ratio becomes a 3:1 phenotype ratio. The calculator reports both, because they answer different questions.
What is a dihybrid cross?
A cross tracking two genes at once, such as AaBb × AaBb. Each parent makes four gamete types, so the grid is 4×4 with sixteen cells, and independent assortment gives the well-known 9:3:3:1 phenotype ratio when both genes show simple dominance.
Are these ratios exact predictions?
They are probabilities, not guarantees. A 3:1 ratio is what you expect on average over many offspring; a single litter of four will not always split cleanly into three and one, just as four coin flips are not always two heads and two tails. Larger numbers track the ratio more closely.