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Inheritance and chance
Choose the parents’ alleles, predict the offspring and see why a small sample can differ from the expected ratio.
Change the model
Change a value, then use Update model or leave the field to redraw. The table gives the same quantities as the graph. Your entries are not saved to progress; changing the model resets the check question.
Try these comparisons
- Compare AA × aa with Aa × Aa. Which cross always gives Aa in this model?
- Try Aa × aa. Predict the probability of the recessive phenotype before adding offspring.
- Compare several samples of 20 with a sample of 200. Look at proportions rather than just counts.
Read the alleles
An allele is a version of a gene. In this model, each parent has two alleles: AA, Aa or aa. A gamete carries one allele from the pair. Each offspring receives one allele from each parent.
The genotype is the allele combination. The phenotype is the feature it produces. Here A is dominant: AA and Aa produce the dominant phenotype, while aa produces the recessive phenotype. Dominant does not mean more common or better.
Use the four combinations
The model lists four equally likely ways to combine the parents’ alleles. Some combinations give the same genotype. With Aa × Aa, the possibilities are AA, Aa, Aa and aa. The chance of aa is one in four, or 25%.
The four boxes are possibilities, not a promise about the next four offspring. Each offspring is a new independent event. Having one aa offspring does not force the next three to have the dominant phenotype.
Compare expected and observed counts
Expected count = probability × number of offspring. In a sample of 20 from Aa × Aa, the expected aa count is 0.25 × 20 = 5. The observed count might be 3, 5 or 8. That difference can arise by chance.
Add groups of 20 and compare the observed proportions with the probabilities. Larger samples usually give more stable proportions, but a larger sample need not be closer on every run. Start a new sample to compare runs.
Worked example
For Aa × aa, the four combinations are Aa, Aa, aa and aa. Two out of four are aa, giving a 50% probability. In 20 offspring the expected aa count is 10; an actual sample need not contain exactly 10.
What the model assumes
One gene with two alleles, complete dominance and equally likely gametes. Offspring survive equally and events are independent. Many real traits involve several genes, environmental effects or different inheritance patterns.