[Encyclios]

From origins to Mendel

Until recent times, the problem of biological inheritance has been predominantly related to that of generation, that is, the ways in which an organism produces another organism similar to itself. In ancient times, according to Aristotle’s conception, it was assumed that it was the action of the soul inherent in the seed that guaranteed the unity of the species and thus generative similarity.

According to other theories, due to Hippocrates and Democritus, particles detached from each organ of the body are collected in the seed, which reproduce its characteristics when the seeds of the two parents are fused, forming an organ similar to the parent organ. In the seventeenth century, the preformist conception was advanced that in the egg or sperm is contained in miniature the organism already formed and assumed to be a pre-existing germ created by God at the beginning of the world; parents and children therefore resemble each other because they are derived from germs created according to the same pattern.

P.-L. Maupertuis and G. Buffon observed, however, that if the germ is either the egg or the spermatozoon, the child should resemble only the mother or only the father, which is contrary to experience, and taking up the conception of Hippocrates and Democritus they proposed the theory of organic molecules. C. Darwin in the nineteenth century also formulated such a hypothesis, but it was precisely with the development of the theory of evolution that the problem imposed itself in a new way. It emphasized the importance not so much of similarity as of dissimilarity from the parents due to variations affected by selection, a fact long known to breeders and horticulturists who, through appropriate crosses, had obtained new breeds.

Such researches were conducted on the assumption that inheritance was continuous, that is, that the characters of the parents mixed in the descendants resulting in an intermediate value. Starting from this conception, F. Galton advocated the theory of ancestral inheritance whereby an individual would derive 1/4 of his characters from his parents, 1/16 from his grandparents, and so on. With statistical research, he also formulated the law of regression that a certain character, e.g., stature, tends to move away from that of the average parent and toward that of the average population.

[Encyclios]

The theories of Mendel

This theory, in the light of subsequent research, turned out to be unfounded, while the opposite theory, due to G. Mendel and formulated in an 1866 memoir, was confirmed, according to which hereditary traits are transmitted as units that remain constant and distinct in offspring. Mendel made crosses of plants of the same species distinguished by paired and contrasting characters (high-low, smooth-wrinkly) and by following the statistical distribution of these characters in successive generations he was able to identify his famous laws. He further admitted that each observed character corresponded to an element contained in reproductive cells capable of linking with another in a lasting or temporary way in descendants, without alteration.

The hypothesis that hereditary characters were produced by material particles contained in reproductive cells, also advanced by A. Weissmann after 1880, was rejected by many as a speculative conception inspired by materialism and mechanism. Mendel’s laws therefore remained unknown and in any case not understood by the few who knew them until the early 20th century, when H. De Vries, C. Correns and E. von Tschermak confirmed their soundness with their work.

[Encyclios]

The twentieth century

From the early twentieth century onward it was a succession of discoveries and formulations of theories. K. E. Correns and T. Boveri in 1902, W. S. Sutton in 1903 pointed out the close parallelism between the behavior of chromosomes in gametogenesis and fertilization and the trend of Mendelian characters from one generation to the next. They thus came to formulate the “chromosomal theory of inheritance,” according to which the genes carrying inherited characters are located in chromosomes and transmitted with them, through gametes, from one generation to the next. Subsequently, other important discoveries were those made by T. H. Morgan and his students.

Morgani discovered sex chromosomes, correctly interpreted sex-linked inheritance and formulated the concept of linkage (association) and crossing-over (exchange) of genes. Other discoveries followed, but perhaps the most resounding came in 1953, when J. D. Watson, F. H. Crick and M. Wilkins elucidated the structure of the nucleic acid molecule. The code by which information is recorded in DNA molecules was discovered by M. W. Nirenberg and S. Ochoa. Thus, the analysis of genetics has truly reached the basis of life phenomena and it is now possible to study how genes function and how they are expressed.

The fields of application of genetics are the most varied, from medical to chemical, pharmaceutical, industrial, agricultural and animal husbandry. Of long-standing tradition and experience is the obtaining in animal husbandry and agriculture of animal (transgenic animals) and plant (GMO, genetically modified organisms, obtained by genetic manipulation) varieties endowed with particular requirements and well adapted to breeding and production in particular environments (for example, the obtaining of short-legged cattle that less easily can escape from pens and that of numerous varieties of wheat each adapted to cultivation in particular places). Even more numerous are the research fields of genetics, three in particular have had very extensive developments: bacterial genetics, population genetics, and gene genetics.