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Classic Genetics: Mendel And Earlier Works

• Mendelian Genetics
• Consequences of MENDEL's Discoveries
• Literature

Even before MENDEL, it was already known that plants have a sexuality. It was also understood that both parents contribute equally to the procreation of their offspring. Experiments with hybrids became common around the turn of the 19th century and led to several new insights. In the 20th century, genetics developed into one of the fastest growing fields of modern biology. In the second half of this century, the molecular approach to genetics gained an enormous importance.

Ever since people began to wonder at the world around them they began to ask questions. How, for example, could it be that a horse always gives birth to a horse? And why do wheat seeds always produce new wheat plants and not rye or barley? How can a baby come from its mother's womb and why does it often resemble its parents, or sometimes even its grandparents or its uncle or aunt? The first theories that tried to explain these riddles came down to us through the thoughts of the early Greek philosophers also called the natural philosophers. Before they tried to explain the world in rational terms, these questions had been answered by myths. They may have been old even then.

Although the underlying principles were not understood for the longest part of human history, the control of heredity or, in other words, the cultivation of certain species has a long and largely successful tradition. It seems to have been common knowledge that the rules of heredity apply to both plants and animals. We know nowadays that they are shared by every living thing. And what we know about them is based directly on the principles discovered by GREGOR MENDEL. Though he never had knowledge about the molecular basis of heredity's principles, he found out about genes, chromosomes and the process of mitosis.

In this chapter we will discuss the ideas and observations that preceded Mendel's work; we will get to know his research, the resulting Mendelian laws and further consequences and we will give a short outlook on following important discoveries.

The recently published excellent essay: "Heredity before Mendel" by V. OREL is available at:

Early Experiences

Wherever there have been found traces of human cultures have there also been cultured plants. Some very early findings stem from the period of the quaternary Ice Age, the Pleistocene or the older stone age. The spectrum of cultivated species is astonishingly small, when compared to the number of wild-growing plants. Early cultivation in the Euro-Asiatic area concerned especially cereals: emmer (Triticum dicoccum), one-grained spelt (Triticum monococcum), seed wheat (Triticum aestivum), spelt (Triticum spelta and, very similar, Triticum aestivum), heard wheat (Triticum durum), T. dicoccoides, compactum and aegilopoides, barley (Hordeum vulgare, Hordeum spontanaeum) and several Leguminosae. Rye and oats were begun to be cultivated much later. It is striking that nearly all these species belong to only one genus, namely Triticum. Until today the number of cultivated species of economical importance is relatively small.

Further details about a wide range of cultivated plants can be found in

Everybody, who takes an interest in cultivated plants will sooner or later have to face two basic questions: How did the cultivated plants develop from the respective wild plants and how did the new types spread? The answers are given by cereals found in excavations (that is, how we know about the species mentioned above) and in pictorial and written documents. There exist nearly no hints, at best speculations, on how efficient the selection procedures used to stabilize newly developed cultivated species, to optimize the yield and to protect them from other influences, were.

Early ideas about the mechanisms of heredity were probably based on observations man made of himself. It was obvious that certain features were transferred from parent to child. Most likely inbreeding occured in all early human cultures, as it does also with many Mammalia-species. This was often linked to the isolation of partial populations, whereby the prerequisites for the accumulation of (mostly unfavourable) features were given. The cause for this was at first searched for in religious dogmata and myths, since it was not believed in the heredity of deformities or conspicuous negative features. The experience gained through thousands of years that inbreeding is linked to more disadvantages than advantages constitutes therefore a decisive progress. The ban of inbreeding was the consequence of this observation and can be regarded as the first example of applied genetics.

The understanding of the mechanisms of heredity demand a high degree of abstraction. First contributions to this topic were therefore made very late in the course of botany. Plant genetics established only after it was recognized that sexuality exists also with plants. This discovery was made by R. J. CAMERARIUS (1665-1721), who was a professor of medicine at Tübingen and the director of the botanical garden. He published his work "De sexu plantarum epistola" in 1694. He wrote:

The meaning and the advantages of the sexual reproduction of humans and animals were mostly recognized in the 17th and 18th century. The philosopher J. G. HERDER (1744-1803) wrote in his work "Ideas about The History of Mankind" ("Ideen zur Philosophie der Geschichte der Menschheit"; 1785-1792):

"The finest way, finally, in which nature combines variety and continuance of forms in its genera is the creation and mating of two sexes. How wonderfully fine and spiritual do the features of both parents blend in the faces and physique of their children. As if their souls had been poured into them at varying ratios and nature's thousand-fold forces of organization had expressed themselves in them. That illnesses and properties of education, that even inclinations and dispositions are inherited is world-wide known; yes, sometimes even the features of long gone ancestors do emerge again from the stream of generations. Just as undeniable, although hard to explain, is the influence of the motherly frames of mind or body on the unborn, whose result many a sad example displays his whole life."

LINNÉ's proposition of the constancy of species (see theme 1) seemed to inhibit the progress of genetics. But even in his own time several successful observations on the breeding of different plant species were made and he himself, too, succeeded in the cross-breeding of the two goats-beard-species Tragopogon pratensis x Tragopogon porrifolius. The study brought him the first prize in a competition of the Kaiserliche Akademie der Wissenschaften zu St. Petersburg in 1760. The result of this experiment caused a change in his original attitude. The proposition of the constancy of species was taken out of the last edition of "Systema naturae" that he edited himself. He thought from then on that new species can develop by the cross-breeding of already existing ones.

Consequently mother and father contribute equally and specifically to the procreation of the bastard. KÖLREUTER did also cultivate bastards of species of the genera Dianthus, Matthiola, Hyoscyamus, Verbascum, Hibiscus, Datura, Cucurbita, Aquilegia, Cheiranthus etc. But his studies left many questions unanswered. Cross-breedings were performed at the end of the 18th and the beginning of the 19th century with a whole range of more or less related plant species by a number of researchers with different success and not always clearly evaluable results. Most cultivators had practical aims. They were, for example, interested in a new colour of a certain ornamental plant, but took hardly any interest in the underlying processes.

A short notice of the French researcher and farmer M. SAGERET (1763-1851) deals with cross-breeding in the family of Curcubitaceae. For the first time in the history of plant hybrids the characters of the parental generation were grouped into opposing pairs. He found thus, when crossing two melon races of the species Curcumis melo L. the following segregation of characters:

These properties were not mingled in hybrids. They were therefore not intermediate, but did resemble one or the other parent. One feature was dominant, the other recessive and all properties were inherited distinctively.

A competition with the question "What does experience teach us concerning the production of new species by artificial insemination of one species with the pollen of another and which useful and ornamental plants can be produced and propagated by it?" was initiated by the Dutch Academy in Harlem. It was won by C. F. GAERTNER with his work "Experiments and Observations on the Production of Hybrids in the Plant Kingdom" ("Versuche und Beobachtungen über die Bastarderzeugung im Pflanzenreich") in 1837. An updated version of the work that was characterized by methodological progress, was published in 1849. 9000 experiments were analyzed and the following conclusions were drawn:

1. An exact classification of the species and their careful raising are the preconditions of all hybridization experiments. All harvested fruits have to be stored separately and all germinating plants have to be raised.

2. At simultaneous pollination with own and foreign pollens no mixture of the properties occurs in the products. Always pollination did take place by only one of the pollen species and this was always the species related closest to the ovaries (ovules). Successive pollination of a half stigma of Nicotiana rustica each with pollen of Nicotiana paniculata and Nicotiana rustica resulted either in homogeneous N. rustica plants or in N. paniculata hybrids. Each pollen grain does thus have its own effect, independent of the others. Never occurs a fusion of two or more paternal types with the maternal ones, nor are two embryos of different types produced by one egg.

3. In confirmation of KÖLREUTER's experiments it was found out that cross-breeding of the same, pure species results in always the same type of hybrid.

4. In a simple cross-breeding both paternal and maternal factors of two different plant species are at work.

5. In the production of hybrids the single properties are modified, mixed and crossed and do partially offset each other. Consequently it is a general law of cross-breeding for both animals and plants that the characteristics of the parents are never purely and unaltered passed down to the offspring.

6. It is still unknown, which rules govern the mixture of the parents' properties. It is equally unknown, why the whole habitus of one hybrid is altered, while in another only parts, like leaves, flowers, fruits or seeds have changed. The production of hybrids is therefore no chemical process as KÖLREUTER thought, but a process analogous to the begetting of animals, that results in both kingdoms in variants and varieties of further generations.

7. The creation of new forms from elements and characters of the parents after cross-breeding is of equal importance to plant physiology and systematics. The question arising for systematics is, whether stable species exist or whether they are an object of change and development.

MENDEL criticized GAERTNER's work, since he found thorough descriptions of the single experiments and satisfactory diagnoses of the different types of hybrids to be missing. In addition he did think that all details on the hybrids' features were too vague.

The 1861-competition of the Parisian Academy of Science was won by Ch. NAUDIN. The topic had been "The Study of Plant Hybrids under the Aspect of Fertility and Perseverance or Loss of Their Traits". NAUDIN's study with the title "Recent Studies on the Hybridization of Plants" was submitted in 1863. He worked on hybrids of the genera Papaver, Mirabilis, Primula, Datura, Nicotiana, Petunia. Digitalis, Linaria, Ribes, Luffa, Coccinia and Cucumis. But his studies suffered from too small amounts of material and unfavourable external factors (frost, drought, pests). He lacked also the space necessary to carry them out on a larger scale. He recognized the segregation of the hybrids in following generations, but did not notice ratios. He discussed the question, whether new species could develop from hybrids and came to the conclusion that this was not the case. He pointed out the difficulties of the term species and cited the fact that naturally occurring hybrids of willows (Salix) and blackberries (Rubus) do always segregate again and therefore lack the principal characteristic of the species, constancy.