Evolution of Biological Inheritance Theories
Theory of Pangenesis
Charles Darwin proposed that all cells emit gemmules that travel through the blood to the gonads, congregating there, each contributing to the creation of offspring.
Theory of the Continuity of Germ Plasm
Friedrich Weismann postulated the existence of discrete particles, the determinants, located in the chromosomes of the zygote, so that each factor would contribute to the formation of a body part. During embryonic development, the determinants of each cell entering into mitosis would be shared by their daughter cells. Finally, each cell is left alone with the determinants necessary for differentiation; for example, intestinal cells receive only the determinants of the intestine.
Weismann also coined the terms somatic, to refer to cells that differentiate and give rise to body tissues, and germ for others that remain undifferentiated, with their nucleus, which he called germ plasm, an unaffected carrier of all the determinants, and whose function would be the formation of gametes. According to this theory, the multicellular body or soma is perishable, and only the germline is transmitted from generation to generation, hence the term 'continuity of germ plasm.'
Intracellular Pangenesis Theory
It was developed by the Dutch botanist Hugo Marie de Vries and is a modification of Darwin's hypothesis of pangenesis. Gemmules, which he called pangenes in homage to Darwin, would be located on chromosomes forming a set of instructions that would determine each particular trait (the color of the flower, for example), not a collection of determinants for each body part.
He ventured further, stating that the nucleus of every cell would contain all pangenes, although only some will be expressed. For the 'active' pangenes to achieve their end, the information was carried by migrating from the nucleus to the cytoplasm, which induces cell differentiation processes, but without the pangenes leaving the cells. For this reason, he called his theory, formulated in 1889, intracellular pangenesis.
Mendelism
It is the theory of biological inheritance by the Austrian monk Gregor Johann Mendel, published in 1866 in his book 'Experiments in Plant Hybridization.' For hybridization experiments, Mendel used the legume Pisum sativum (pea). In his theory, he states that the characters that parents transmit to their offspring are carried by discrete units of information, known as hereditary factors, produced and transmitted independently of each other and coming together in pairs in the offspring.
The main supporters and promoters of Mendelism were the English biologist William Bateson and the Danish botanist Wilhelm Ludvig Johannsen. The latter coined the terms gene (derived from the pangene of De Vries), which replaced the 'hereditary factor' used by Mendel, genotype (the gene pool of the organism), and phenotype (the outward expression of the genotype).