Who Mendel was
Everything in the rest of this chapter came out of one man's vegetable garden, and out of his willingness to count what grew in it.
Figure to be added
Gregor Mendel
Gregor Mendel was an Austrian priest who had also taken a degree in science. Working in the garden of his monastery, he carried out a long series of breeding experiments on the pea plant and worked out the rules by which characteristics pass from parents to offspring. He is known today as the father of genetics.
What he actually did
Mendel crossed pea plants, grew the offspring, and then did the thing that made the difference: he sorted the offspring into groups and counted how many fell into each. Nobody before him had treated inheritance as something you could put numbers on.
Why counting mattered
Looking at one plant tells you what that plant is like. Counting a thousand tells you what the rule is. The ratios that come out of Mendel's counts — three to one, one to two to one — are the whole evidence for everything this chapter claims.
He had no idea what a gene was
Chromosomes had not been seen, DNA was unknown, and Mendel never used the word gene. He deduced that something came in pairs and separated when gametes formed, purely from the numbers. The physical thing he was describing was only identified half a century later.