This tool shows what happens when two parents with a chosen genotype have offspring, for one gene with two alleles and complete dominance. It lists the gametes each parent can make, fills a Punnett square, and gives the genotype and phenotype probabilities.
It can also simulate a batch of offspring, so you can see the difference between a probability and what a small sample actually produces.
How do I use it?
- Choose a fictional trait. There are three: a fern with green or silver leaves, a beetle with round or notched wings, and a moss with gold or pale spores. In each, A is the dominant allele and a is the recessive one.
- Choose the genotype of parent 1 and parent 2 from AA, Aa or aa.
- Press Show model to see the gametes, the Punnett square, the genotype probabilities and the phenotype probabilities.
- Type a number of offspring from 1 to 1000 and press Simulate offspring to compare expected and observed counts.
- Press Reset to return to Aa × Aa with 8 offspring.
Changing a parent clears the simulated sample, because it no longer matches the cross.
Example walk-through
The tool opens with Aa × Aa. Each parent makes gametes A and a. The square has four boxes: AA, Aa, Aa and aa.
That gives AA 1/4 = 25%, Aa 2/4 = 50% and aa 1/4 = 25%, so the genotype ratio is 1 : 2 : 1. Green is dominant, so green leaves appear with probability 75% (AA plus Aa) and silver with 25%.
Press Simulate offspring with 8. The expected counts are 2.0, 4.0 and 2.0.
The observed counts will usually differ, for example 3, 3 and 2, because each offspring is an independent chance. Change the number to 1000 and simulate again.
The observed counts now sit much closer to 250, 500 and 250, although they still will not match exactly.
Now try Aa × aa. The gametes are A, a and a, a, so the square shows Aa twice and aa twice. The ratio is 0 : 1 : 1 and the phenotype split is 50% dominant, 50% recessive.
How do I read the result?
- Gametes line: each parent passes on one allele per gamete, so an Aa parent makes A or a with equal chance.
- Punnett square: rows are the gametes of parent 1 and columns are the gametes of parent 2. Each box is equally likely.
- Genotype probabilities: the share of boxes with each genotype, written as a fraction and a percentage.
- Ratio line: the smallest whole-number form of the genotype counts.
- Phenotype line: the dominant and recessive phenotype chances added up from the genotypes.
- Observed table: appears after a simulation. Compare the expected column with the observed column.
What are the assumptions and limits?
The model assumes one gene, complete dominance, equally likely gametes, random fertilisation and independent offspring. The organisms are invented.
It is not a prediction for any real family and not a personal genetic interpretation. Questions about a real person’s health belong with a doctor or genetic counsellor, not a revision tool.
Which lessons explain the output?
Start with distinguishing gene, allele and chromosome, then constructing a simple inheritance grid.
The genotype and phenotype lines are covered in separating genotype from phenotype, and the simulation is explained in probability across repeated offspring. The whole topic sits under cell division and inheritance and test it with the mixed practice set.
Explaining a source of genetic variation follows naturally. See the tools page for the other interactive tools.
If square-filling is fine but the written explanation still loses marks, our team can go through your answers with you. See online one-to-one Biology tuition for how that works.