Mendel Begins Plant Experiments

When Mendel returned to Brno from Vienna in 1853, he had already made plans for a research program on heredity. As he wrote later, "The selection of the plant group which is to serve for experiments of this kind must be made with all possible care if it be desired to avoid from the outset every risk of questionable results."

In his research, Mendel used a discovery that had been made not long before: Plants have sex. It had long been assumed that plant flowering and reproduction simply happened. The first and most important step toward reversing that view came when an 18th-century scientist, Carolus Linnaeus, devised a new system of plant species classification that made plant sex the basis of species determination. Linnaeus also described hybrids. (When Mendel told his students about plant reproduction, often using plain street terms, some of the students would titter. "Don't be stupid! These are natural things," Mendel would say.)

Starting in the late 1700s, a German scientist named Joseph Koelreuter conducted a series of experiments showing that plant reproduction required that a grain of pollen, the male element in plant fertilization, must fertilize the plant

Joseph Koelreuter, pioneer in plant hybridization experiments, was the first to recognize the importance of insects and the wind in pollinating flowers.

equivalent of an egg to produce a seed that will grow into an adult plant. Plants can be self-pollinating, Koelreuter showed, but they also can be fertilized by pollen from other plants.

Koelreuter also conducted a long series of experiments in plant hybridization. When he started, only two well-known plant hybrids had been produced by artificial cross-pollination. His first hybrid was sterile; the plants did not produce seeds and the pollen grains were shrunken and sterile. But when he back-crossed these plants with pollen from plants of the previous generation, he did get fertile hybrids.

Over several years, Koelreuter produced the first accurate record of plant hybrids. The first-generation (Fl) hybrids, he wrote, had characters that were mostly intermediate between those of the parent plants. The F2 and back-crossed hybrids were all different, with characteristics that resembled those of the parent plants in varying ways. Koelreuter's explanation: The bewildering variety of characteristics was the result of combining plant materials "not intended for each other by the wise Creator"—hardly a scientific explanation. But he did note that in some hybrid generations, the plants had characteristics of the grandmother generation, the grandfather species, and the Fl parents, in the ratio of 1:2:1. He concluded that hybrid species sooner or later revert to the traits of one or the other original species.

Decades later, in the mid-19th century, a German botanist, Carl Gaertner, repeated some of Koelreuter's hybrid experiments, and did a large number of his own. The plants that he crossed included peas, tobacco, and corn. He observed that some traits were dominant, appearing in generation after generation, even as others disappeared in some generations and re-emerged later. He could not explain his results in a coherent way; he simply said that "the total nature of the species" determines the direction and form of hybrids.

Pifum

A 1791 drawing by J. Gaertner of the garden pea Pisum sativum, with which Mendel began his experiments in plant hybridization.

So when Mendel began his work with peas, he was doing the same sort of work that others had done. The difference— a huge difference—was in the conclusions he drew from his observations.

First he had to select the goal of his research. As he explained later, it was to determine exactly how traits are passed from generation to generation—"a question whose significance for the developmental history of organic forms must not be underestimated."

The plants he chose for his first experiments were varieties of peas belonging to the genus Pisum. The reasons, he later explained, were that Pisum produces fertile hybrids that can reliably be distinguished from one another, it can easily

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