Test Cross Simulator
Run the Test Cross Simulator MicroSim Fullscreen
About This MicroSim
This MicroSim simulates the test cross procedure used to determine whether an organism showing a dominant phenotype is homozygous dominant (AA) or heterozygous (Aa). Students cross the mystery parent with a homozygous recessive tester and generate offspring one at a time, observing the statistical evidence accumulate. A confidence meter shows how certain the determination is based on the offspring data so far.
How to Use
- Select a trait from the dropdown (Seed Shape, Flower Color, or Plant Height)
- Click "Generate Offspring" to produce one offspring at a time — observe whether it shows the dominant or recessive phenotype
- Watch the confidence meter — it calculates the statistical likelihood of each genotype based on accumulated evidence
- After 20+ offspring, the "Reveal Genotype" button appears — but try to predict the answer first!
- Click "New Mystery" to start fresh with a randomly assigned genotype
Lesson Plan
Grade Level
9-12 (college placement Biology)
Duration
10-15 minutes
Prerequisites
- Understanding of dominant and recessive alleles
- Knowledge of homozygous vs. heterozygous genotypes
- Familiarity with Punnett squares
Activities
- Exploration (5 min): Generate offspring for several mystery parents. Notice how quickly you can determine heterozygous parents vs. how long it takes to be confident about homozygous parents.
- Guided Practice (5 min): Before revealing, predict the genotype based on the ratio. Explain your reasoning using probability.
- Assessment (5 min): Why can't a test cross ever prove with 100% certainty that a parent is homozygous dominant? Discuss the role of sample size in statistical confidence.
Assessment
- Can the student explain why recessive offspring prove heterozygosity?
- Can the student explain why all-dominant offspring don't guarantee homozygosity?
- Can the student describe the expected offspring ratios for AA x aa vs. Aa x aa?