The tall and short cross

The first experiment to follow is the one about stem height. It is the simplest of the seven, and everything else in this chapter is built on the shape of its answer.

What Mendel did

He took a pure tall pea plant and a pure dwarf pea plant — lines that had bred true for generations.
He removed the anthers from one flower before its pollen ripened, so it could not pollinate itself.
He dusted pollen from the other plant onto that flower by hand, and covered it.
He collected the seeds, grew them, and then allowed those plants to self-pollinate and grew their seeds in turn.

What came up

Parent generation (P)

pure tall × pure dwarf

↓

First filial generation (F₁)

every plant tall — not one dwarf, and not one of middling height

↓

Second filial generation (F₂)

tall and dwarf both present, in the ratio 3 tall : 1 dwarf

Figure to be added

Figure 20.19a — the cross drawn as a tree: the tall and dwarf parents at the top, the uniformly tall F1 below them, and the F2 beneath that showing three tall plants to every dwarf one.

The numbers

Mendel did not stop at "mostly tall". He counted thousands of F2 plants and found the tall ones outnumbered the dwarf ones three to one, close enough and consistently enough across all seven characters that it could not be coincidence.

Two surprises, not one

The F₁ plants were not of middling height — crossing tall with dwarf did not average them out, which is what most people expected. And then the dwarf form, absent from the whole F₁, came back in the F₂. Whatever makes a plant dwarf had survived a generation in which no plant was dwarf.

Something was hidden, not lost

A characteristic that disappears for one generation and returns in the next cannot have been destroyed. It was carried through the F₁ without showing. The next page gives that behaviour its name.