The count is the argument
Show on a big screenA batch is a group of plants counted together. One batch of 8 F2 plants has five tall, three short. Another batch of 8 has all tall. Neither batch shows 3 tall to 1 short clearly.
- A small batch: 8 F2 plants. 5 tall, 3 short.
- Another small batch: 8 plants. 8 tall, 0 short.
- A big batch: 400 F2 plants, counted one by one.
- About 300 tall, 100 short.
- Big counts come close to three tall to one short.
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Slide to 8, then to 400. Count new batches. Watch the short share.
The rule
Two Tt plants are crossed. Each seed gets T or t from each parent. So the four pairs TT, Tt, Tt and tt are equally likely. Three of them give tall plants. One gives a short plant. We write this as 3 : 1, three tall to one short.
Take 400 plants. One pair in four gives short, so about a quarter are short. A quarter of 400 is 100. So about 100 grow short. The other three quarters are tall: 3 × 100 = 300. About 300 grow tall.
A batch of 8 can be far from 3 : 1. A batch of 8 could even be all tall, by chance. In a big count, chance moves the share of short plants less. So a big count tests the ratio better.
Try the lab to finish this lesson.