The Punnett square
Lines joining gametes get tangled once there is more than one of each. A grid does the same job and cannot be miscounted: one gamete per row, one per column, and every box is one possible offspring.
The F1 plants crossed with each other
Every F1 plant is Tt. Allowed to self-pollinate, each one makes T gametes and t gametes in equal numbers. Put those along the top and down the side.
| Tt × Tt | T | t |
| T | TT | Tt |
| t | Tt | tt |
The Punnett square for the F1 cross. Each box is one combination of two gametes.
Reading the four boxes
- One box is TT — a tall plant, pure for tallness.
- Two boxes are Tt — tall plants carrying a hidden dwarf factor.
- One box is tt — a dwarf plant, the only box with no T in it.
Three of the four boxes are tall
Three boxes contain at least one T and one contains none. Since T is dominant, those three plants are tall and the fourth is dwarf — which is exactly the 3 : 1 Mendel counted in the field, arrived at without planting anything.
The grid explains the disappearance
The dwarf form was never lost. It sat in the two Tt boxes all through the F1, unable to show because T was there too — and it became visible again the moment two t factors met in the same plant.