The man who counted
Show on a big screenA seed-saver saves seeds only from plants that give one kind. Then she knows what will grow. Crossing two plants means moving pollen from one to the other. Plants and seeds that come from a cross are its offspring.
- Start with a line that breeds true.
- Cross two lines by hand, not left to chance.
- Count every offspring, season after season.
- A ratio that repeats is a trustworthy result.
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Pick a tall parent. Cross and count. Then reveal its pair.
The rule
Mendel worked with garden peas in the 1850s and 1860s. He asked what decides a child's traits. A true-breeding line makes the same trait each time it is bred.
Mendel used true-breeding lines, crossed them by hand, and counted every offspring. Counts turn a guess into a pattern you can test.
A ratio compares two counts, like 3 tall to 1 short.
A true tall line is TT. T is a tall copy and t is a short copy. T hides t, so TT and Tt plants are tall. Only tt is short.
The other parent is a true short plant, tt. So a true tall line gives only tall seedlings. The cause is clear.
A mystery tall plant is TT or Tt. Its pair is hidden. A short seedling is tt. It got a t from each parent. So the mystery plant must be Tt. If all are tall, the plant may be TT. It may be Tt by chance.
Try the lab to finish this lesson.