Mendel and His Experiments

Gregor Mendel selected the garden pea, Pisum sativum, as his experimental organism, and this choice was itself one of the most consequential decisions in the history of biology, because it is precisely what made his experiments interpretable in a way that experiments on many other organisms of his time were not. The pea plant is naturally self-pollinating — each flower's own pollen normally fertilises its own ovules, enclosed as they are within a keel-shaped petal structure that largely excludes foreign pollen — and this property let Mendel first obtain, simply by allowing several successive generations of uninterrupted self-fertilisation, pure-breeding (true-breeding) lines of pea plants in which a given character bred true, generation after generation, without any variation appearing. At the same time, the very same flower structure that promotes self-pollination can be deliberately overridden by the experimenter: Mendel could emasculate a flower by carefully removing its own anthers with forceps before they matured and released pollen, and then dust the exposed stigma with pollen collected by hand from a different, chosen plant, thereby performing a controlled cross-pollination whenever he needed one. This dual capability — natural self-pollination for maintaining pure lines, and easy artificial cross-pollination for performing planned hybridisation experiments — was rare among plants available to Mendel and was central to the reliability of his results.

Mendel further sharpened his experimental design by choosing to study seven pairs of contrasting (antagonistic) characters in the pea, each of which showed exactly two clearly distinguishable, non-overlapping forms with nothing intermediate between them: seed shape (round versus wrinkled), seed colour (yellow versus green), pod shape (inflated versus constricted), pod colour (green versus yellow), flower colour (violet versus white), flower position on the stem (axial versus terminal), and stem height (tall versus dwarf). Because each character had only two sharply distinct, unambiguous forms, Mendel could score every single plant in a cross without any doubt about which class it belonged to — a crucial precondition for the kind of large-scale, statistical counting of offspring classes on which his entire method depended. The garden pea additionally offered several purely practical advantages: it has a comparatively short life cycle, so several generations could be raised and scored within Mendel's working years; it produces a large number of seeds per pod and many pods per plant, giving Mendel the large sample sizes his statistical approach to biology required; and numerous already-established pure varieties, differing from each other in exactly these seven characters, were readily available to him from local seed merchants, sparing him the years of work it would otherwise have taken to develop pure-breeding lines from scratch. …