Mendel's Cross and Punnett Square Lab
Cross pea plants, build the Punnett square and grow hundreds of seeds to test Mendel's ratios.
- NCERT Figure 8.3 (inheritance over two generations)
- Figure 8.5 (independent inheritance)
Tap a parent's alleles to change them (TT → Tt → tt). Tap or hover over a box in the Punnett square to see which gametes made it.
Parent 1 (P)
Tall
Parent 2 (P)
Short
Tap or hover over a box to see which two gametes made it.
Phenotypes
- Tall4/4 (100%)
Ratio 1
Genotypes
- Tt × 4
Ratio 1
Parent 1
Parent 2
Quick crosses
About the Mendel's Cross and Punnett Square Lab
This Punnett square simulator brings Mendel's pea plant experiments from NCERT Class 10 Heredity to life. Cross tall and short plants, round and wrinkled seeds or yellow and green seeds, and the simulation works out the gametes, the Punnett square and the genotype and phenotype ratios for you.
Follow a monohybrid cross from the parental generation to the F1 and F2 generations to see the 3 : 1 ratio appear, then try a dihybrid cross for the 9 : 3 : 3 : 1 ratio. Grow hundreds of seeds at random and compare your results with Mendel's own counts.
Try this
- 1Cross TT with tt, then self-pollinate the F1 plants. What fraction of the F2 is short?
- 2Do a test cross (Tt × tt). How does it reveal the genotype of a tall plant?
- 3Switch to the dihybrid cross and grow 556 seeds. Compare with Mendel's 315 : 108 : 101 : 32.
What you will learn
- Work out gametes and offspring of a cross with a Punnett square.
- Explain the 3 : 1 phenotype and 1 : 2 : 1 genotype ratios of the F2 generation.
- Show that two traits are inherited independently in a dihybrid cross.
Key ideas
- Dominant and recessive
- One copy of a dominant allele (T) is enough to show the trait. A recessive trait (short) shows only in tt.
- Independent inheritance
- Genes for seed shape and seed colour separate independently, giving new combinations in the F2.
Board exam tip
Always show the gametes and the Punnett square when explaining a cross. Ratios alone do not earn full marks.
Frequently asked questions
What is the F2 ratio of a monohybrid cross?
Self-pollinating the Tt plants of the F1 generation gives a phenotype ratio of 3 tall : 1 short and a genotype ratio of 1 TT : 2 Tt : 1 tt.
What is the 9 : 3 : 3 : 1 ratio in a dihybrid cross?
When RrYy plants are crossed, 9 out of 16 offspring are round yellow, 3 are round green, 3 are wrinkled yellow and 1 is wrinkled green. This shows that seed shape and seed colour are inherited independently.
What is a test cross?
A test cross crosses a plant showing the dominant trait with a recessive tt plant. If any offspring are short, the tall parent must be Tt; if all of them are tall, it is TT.