Back in the nineteenth century, an Austrian monk called Gregor Mendel did experiments in his monastery garden, breeding peas in large quantities. He counted the numbers of plants that had flowers of various colours, or that had peas that were wrinkly or smooth, as the generations went by. Mendel never saw or touched a gene. All he saw were peas and flowers, and he could use his eyes to count different types. He invented a model, which involved what we would now call genes (though Mendel didn’t call them that), and he calculated that, if his model were correct, in a particular breeding
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