Punnett Square Activity
Goal: Predict genotype and phenotype ratios for monohybrid, dihybrid and blood-type crosses, and explain why a Punnett square gives probabilities rather than guarantees.
What to do
- On the Monohybrid tab, press “Aa × Aa”. Record the genotype ratio, the phenotype ratio and the chance of the recessive phenotype. Repeat with “Aa × aa”.
- Type AA for Parent 1 and aa for Parent 2, and record the results.
- Switch to the Dihybrid tab and press “AaBb × AaBb”. Record the results.
- Switch to the Blood type tab and press “AO × BO”. Record the genotype ratio, the blood-type ratio and the chance of type O.
Record your results
| Cross | Genotype ratio | Phenotype ratio | Chance of recessive (type O) |
|---|---|---|---|
| Aa × Aa | |||
| Aa × aa | |||
| AA × aa | |||
| AaBb × AaBb | |||
| AO × BO (blood type) |
Questions
Two Aa parents have three children who all show the dominant trait. What is the chance that their fourth child shows the recessive trait? Explain.
Why does the test cross Aa × aa give a 1 : 1 ratio, and how could a breeder use a test cross to tell an AA parent from an Aa parent?
For AaBb × AaBb, what fraction of the 16 boxes shows both recessive traits? Write it as a fraction and as a percentage, and give the phenotype ratio.
Check your dihybrid answer with the rule of multiplication: what is the chance of aa, of bb, and of both together?
A type A parent (AO) and a type B parent (BO) have children. Which blood types can the children have, and with what probability?
Challenge: from Aa × Aa crosses Mendel counted 705 purple and 224 white flowers. Is that close to 3 : 1? Check it with the chi-square calculator.
Free from Biology Lab · thebiologylab.org · MS-LS3-2: Model why asexual reproduction gives genetically identical offspring and sexual reproduction gives genetic variation. HS-LS3-3: Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population.