Punnett Square – Mendelian Genetics (One and Two Trait Pairs)
Integrated ScienceGenetics & EvolutionAges 14–15
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Genetics & Evolution
Choose the genotypes of the parents (P) with one or two gene pairs (pea plants: yellow/green seeds, round/wrinkled; purple/white flowers, tall/short stem). The Punnett square automatically builds the gametes and their combinations; tap a cell to see its genotype and phenotype. Click Simulate N offspring to run a random simulation and compare the experimental ratio with the theoretical 3:1 or 9:3:3:1 ratio on a bar chart.
Lesson: Mendel's Laws of Inheritance
What it shows
Mendel's pea crosses are modelled with the logic of a Punnett square. Each parent passes on one allele of every gene; during gamete formation the two alleles of a pair separate (law of segregation), and in a dihybrid cross the two gene pairs sort independently (law of independent assortment). Random fusion of gametes gives every cell of the square an equal probability, so Aa × Aa yields a 1 : 2 : 1 genotype ratio and a 3 : 1 phenotype ratio, and AaBb × AaBb gives 9 : 3 : 3 : 1. Simulated offspring show how real counts scatter around these expected ratios by chance.
How to use
Choose the Number of trait pairs, then set the Father plant's genotype and the Mother plant's genotype. Read the gametes along the edges of the square and tap cells to check genotype and phenotype. Press Simulate N offspring with a small Number of offspring simulated each time, such as 20, and compare the experimental bars with the theoretical ones; then press Simulate N more offspring several times and watch the gap shrink. Also try a test cross such as Aa × aa.
Parameters you can change
- Number of trait pairs One pair (monohybrid cross), Two pairs (dihybrid cross)
- Father plant's genotype AA, Aa, aa, AABB, AaBb, aabb, AAbb, aaBB, AaBB, AABb, Aabb, aaBb
- Mother plant's genotype AA, Aa, aa, AABB, AaBb, aabb, AAbb, aaBB, AaBB, AABb, Aabb, aaBb
- Number of offspring simulated each time 20–400 plants
- Illustrated trait Seed: A yellow/a green, B round/b wrinkled, Flower: A purple/a white, B tall stem/b short stem
Questions to explore
- What phenotype ratio does Aa × aa give, and how can this cross reveal an unknown genotype?
- How many different types of gametes can an AaBb plant produce, and why?
- Why does a sample of 20 offspring rarely show exactly a 3 : 1 ratio?
