Simple inheritance cross explorer

This tool builds a Punnett grid for a cross between two parents and reads off the genotype and phenotype ratios. It helps students who want to check a diagram they drew, and anyone revising the terms allele, genotype and phenotype.

How do I use it?

1. Choose the genotype of parent 1 and parent 2 from the lists: AA, Aa or aa.
2. Choose a dominance model. Complete dominance is the usual textbook case, where A hides a.
3. Read the model assumptions first, then the gametes and the grid.
4. Compare the genotype ratio with the phenotype ratio, and read the probability sentence underneath.
5. Press reset to return to Aa × Aa with complete dominance.

How does it work?

Each parent has two alleles, and each gamete carries one of them. The tool lists the two gametes for each parent, then fills a four-box grid by pairing one gamete from each side.

Every box has the same chance, 1 in 4. The tool counts how many boxes show AA, Aa and aa, then divides all counts by their highest common factor to give the ratio. An impossible genotype is rejected instead of guessed.

The phenotype step needs a stated model. Under complete dominance, AA and Aa show the dominant trait and only aa shows the recessive trait. Under incomplete dominance, Aa shows a third, in-between trait.

What does a worked example look like?

This is a worked example. Cross Aa with Aa. Each parent passes on A or a, so the grid holds AA, Aa, Aa and aa.

The genotype ratio is AA : Aa : aa = 1 : 2 : 1. Under complete dominance, three boxes show the dominant trait and one shows the recessive trait, so the phenotype ratio is 3 : 1. Under incomplete dominance the same grid gives 1 : 2 : 1 for the phenotypes too.

Now try AA × aa. Every box is Aa, so all offspring are Aa. The tool says so directly instead of writing a ratio with a single term.

What are the limits?

The tool covers one gene with two alleles on a non-sex chromosome. It does not cover sex-linked traits, blood groups, linked genes or pedigrees.

A probability here is a textbook expectation over a large number of offspring. It is not a prediction about any real person or family, and it gives no health or counselling advice. Your school syllabus decides which models you need, so check your notes for the exact wording your teacher expects.

Where does this fit in my revision?

Start with the Biology inheritance hub for the ideas behind the grid. The lesson on using allele, genotype and phenotype terminology fixes the wording, and completing basic inheritance diagrams shows the layout step by step.

When you are ready to test yourself, use the inheritance practice set. For lessons with a teacher, see Biology tuition or the one-hour trial class (from RM50).