Mendel's search for true-breeding hybrids.

It has been widely believed that Gregor Mendel’s experiments with peas (genus Pisum) were designed to discover the basic principles of heredity (genetics). Those more familiar with the history of genetics know that his work with peas sought to explain the laws governing the origin and development of hybrids (Hartl and Orel 1992, p. 245; Monaghan and Corcos 1985, p. 49–50; Stern and Sherwood 1966, p. 2). His subsequent work with hawkweeds (genus Hieracium) was an attempt to find an explanation for the apparent constancy of some hybrid forms in this and other species (Orel 1984, p. 63). One of his main motives was to see if hybrids between different varieties of a species or between different species could breed true, that is, hybrids produce only more hybrids. In those days, the difference between varieties and species was not always well defined. True-breeding hybrids behaved like new species. Mendel published 2 botanical papers in German: his Pisum paper was published in 1866, the one on Hieracium was published in 1870. Both of these papers and his letters to his friend and advisor Carl Nägeli have been translated into English in Stern and Sherwood (1966). In the text of the present paper, numbers following Mendel’s name refer to pages in the Stern and Sherwood book. A complete English translation of Mendel’s 1866 paper by William Bateson can be found on the internet (see Mendelweb). The concept of reproductive isolation as a hallmark of a biological species did not become popular until well into the 20th century (Mayr 1982, p. 270–275). Mendel found that his intervarietal pea hybrids were phenotypically identical but did not breed true when self fertilized, segregating pure breeding parental types (homozygotes) as well as hybrids (heterozygotes) in the now familiar ratio 1:2:1. [N.B. Modern terms are used here and elsewhere in this paper instead of Mendel’s terms that have become passé.] Mendel was aware that Charles Darwin’s book On the Origin of Species was published in 1859 before he began his pea experiments in 1862. Darwin did not really shed much light on how new species originate by natural selection. And the role of hybridization in that process was begging for experimental insights, especially for how an ancestral species can split into 2 or more daughter species that coexist in time (horizontal evolution or speciation, cladogenesis). After Mendel found some principles of hybrid behavior in his pea varieties, he hoped to find the same regularities operative in other plant species. In his famous 1866 paper (published in The Proceedings of the Brünn Natural Science Society), Mendel also reported his interspecific hybridizations with another species of plant (beans, genus Phaseolus). He hybridized bean species differing in plant height and shape of ripe pods and found that they followed the same laws of development as Pisum. The colors of both flowers and unripe pods, however, seemed to be more complex. Crosses between plants with white flowers and plants with crimson blossoms produced hybrids with flowers less intensely crimson than the parental variety. The progeny of hybrids had flowers varying from crimson to pale violet and white. The unripe seed pods also had various shades of coloring from dark green to yellow. Because white flowered plants seemed to appear in about 1/16 of the progeny of hybrids (F2), Mendel proposed that there were 2 color elements (genes) A1 and A2 that together in homozygotes contributed to crimson color. Plants with the recessive alleles a1 and a2, when homozygous together, produced white flowers. Other gene combinations contributed to the range of colors between crimson and violet. Mendel seems to be the first to conceive of multiple factors for a single character.