The hidden one comes back
A true tall pea line is crossed with a true short line. Every seed grows tall. These first seeds are F1. Cross two F1 plants. Their seeds are F2. Some F2 plants are short.
- A true tall plant, TT, crossed with a true short plant, tt.
- F1: every offspring is Tt, and every one grows tall.
- Cross two F1 plants: T or t from each parent.
- The grid fills: TT, Tt, Tt, tt.
- Three plants grow tall for every one that grows short.
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Tap the four boxes. Count the short plants.
The rule
Each plant holds a pair of letters. Each letter is a gene copy from one parent. T stands for tall. t stands for short.
Capital T is the dominant copy. It shows in the plant. Small t is the recessive copy. It hides when T is there.
A gene pair has two copies. A dominant copy shows in the plant. A recessive copy shows only when both copies are recessive.
TT and Tt plants are tall. Only tt is short. A tall plant needs just one T, because T hides t. The book calls the first cross's offspring F1. It calls their offspring F2.
So the short trait hides in every F1 plant. It returns in F2 as tt. That plant got t from both parents.
The grid in the steps is a Punnett square. It lists every way two parents' copies can pair.