[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.