Preformation versus epigenesis

A question posed by Aristotle was whether the embryo is preformed and therefore only enlarges during development or whether it differentiates from an amorphous beginning. Two conflicting schools of thought had been based on that question: the preformation school maintained that the egg contains a miniature individual that develops into the adult stage in the proper environment; the epigenesis school believed that the egg is initially undifferentiated and that development occurs as a series of steps. Prominent supporters of the preformation doctrine, which was widely held until the 18th century, included Malpighi, Swammerdam, and Leeuwenhoek. In the 19th century, as criticism of preformation mounted, the Prussian Estonian embryologist Karl Ernst von Baer provided the final evidence against the theory. His discovery of the mammalian egg and his recognition of the formation of the germ layers out of which the embryonic organs develop laid the foundations of modern embryology.

The fertilization process

Despite the many early descriptions of spermatozoa, their essential role in fertilization was not proved until 1879, when the Swiss physician and zoologist Hermann Fol observed the penetration of a spermatozoon into an ovum. Prior to that discovery, during the period from 1823 to 1830, the existence of the sexual process in flowering plants had been demonstrated by the Italian astronomer and optician Giovanni Battista Amici and confirmed by others. The discovery of fertilization in plants was of great importance to the development of plant hybrids, which are produced by cross-pollination between different species; it was also of great significance to the studies of genetics and evolution.

The universal occurrence and remarkable similarity of the fertilization process, regardless of the organism in which it occurs, provoked many of the leading investigators of the time to search for the underlying mechanism. It was realized that there must be some way by which the number of chromosomes is reduced before fertilization; otherwise, the chromosome number would double every time a sperm fused with an egg. In 1883 the Belgian embryologist and cytologist Edouard van Beneden showed that the eggs and the sperm in the worm Ascaris contain half the number of chromosomes found in the body cells. To account for the halving of the chromosomes in the sex cells, a process known as meiosis, in 1887 the German biologist August Weismann suggested that there must be two different types of cell division, and by 1900 the details of meiosis had been elucidated.

Pre-Mendelian theories of heredity

The fundamental laws of heredity were discovered in 1865 by the Austrian botanist, teacher, and Augustinian prelate Gregor Mendel, though his work was ignored until its rediscovery in 1900. There were, however, a number of views on the subject that had been expressed long before Mendel. The Greek philosophers, for example, believed that the traits of individuals were acquired from contact with the environment and that such acquired characteristics could be inherited by offspring. Because Lamarck was the most famous proponent of the inheritance of acquired characteristics, the theory is called Lamarckism. This concept, which emphasized the use and disuse of organs as the significant factor in determining the characteristics of an individual, postulated that any alterations in the individual could be transmitted to the offspring through the gametes.

In 1885 Weismann suggested that hereditary characteristics were transmitted by what he called germ plasm—as distinguished from the somatoplasm (body cells)—which linked the generations by a continuous stream of dividing germ cells. In stating definitely seven years later that the material of heredity was in the chromosomes, Weismann anticipated the chromosomal basis of inheritance.

The English explorer, anthropologist, and eugenicist Francis Galton made a number of important contributions to genetics in the 19th century, one of which was a study of the hereditary nature of ability, from which he developed the concept that judicious breeding could improve the human race (eugenics). Galton’s most-significant work was the demonstration that each generation of ancestors makes a proportionate contribution to the total makeup of the individual. Thus, he suggested, if a tall man marries a short woman, each should contribute half of the total heritage, and the resultant offspring should be intermediate between the two parents.

Mendelian laws of heredity

The fame of Gregor Mendel, the father of genetics, rests on experiments he did with garden peas, which possess sharply contrasting characteristics—for example, tall versus short; round seed versus wrinkled seed. When Mendel fertilized short plants with pollen from tall plants, he found the offspring (first filial generation) to be uniformly tall. But if he allowed the plants of that generation to self-pollinate (fertilize themselves), their offspring (the second filial generation) exhibited the characters of the grandparents in a rather consistent ratio of three tall to one short. Furthermore, if allowed to self-pollinate, the short plants always bred true—they never produced anything but short plants. From those results Mendel developed the concept of dominance, based on the supposition that each plant carried two trait units, one of which dominated the other. Nothing was known at that time about chromosomes or meiosis, yet Mendel deduced from his results that the trait units, later called genes, could be a kind of physical particle that was transmitted from one generation to another through the reproductive mechanism.

Mendel’s most-important concept was the idea that the paired genes present in the parent separate or segregate during the formation of the gametes. Moreover, in later experiments in which he studied the inheritance of two pairs of traits, Mendel showed that one pair of genes is independent of another. Thus, the principles of segregation and of independent assortment were established.

Mendel’s findings were ignored for 35 years, probably for two reasons. Because the distinguished Swiss botanist Karl Wilhelm von Nägeli failed to recognize the significance of the work after Mendel sent him the results, he did nothing to encourage Mendel. Nägeli’s great prestige and the lack of his endorsement indirectly weighed against widespread recognition of Mendel’s work. Moreover, when the work was published, little was known about the cell, and the processes of mitosis and meiosis were completely unknown. Mendel’s work was finally rediscovered in 1900, when three botanists independently recognized the worth of his studies from their own research and cited his publication in their work.

Elucidation of the hereditary mechanism

By 1901 it was understood how the hereditary units postulated by Mendel are distributed; it was also known that the somatic (body) cells have a double, or diploid, complement of chromosomes, while the reproductive cells have a single, or haploid, chromosome number. The experimental demonstration of the chromosomal basis for heredity had been firmly established by the German cytologist Theodor Boveri soon after the turn of the century and subsequently confirmed by others. To account for the large number of observed hereditary characters, Boveri suggested that each chromosome in a pair can exchange the hereditary factors it carries with those of the other chromosome. At first, the American geneticist Thomas Hunt Morgan dismissed that concept. Later, however, when he found that it agreed with his own laboratory findings, Morgan and his collaborators assigned the hereditary units (genes) specific positions, or loci, within the chromosomes. With the genes established as the carriers of hereditary traits, the English biologist William Bateson coined the term genetics for the experimental study of heredity and evolution.