The Path to the Chromosome Theory of Heredity

The Path to the Chromosome Theory of Heredity

The notion of the chromosome theory of heredity is very important to the understanding of evolution and genetics. There were many ideas, some correct and others not, which influenced the revolutionary discovery of chromosomal heredity. The main stones in the path to the chromosomal theory are the pangenesis hypothesis, the germ-plasm theory, and Mendel’s Laws.

Pangenesis

Lamarck expressed the idea that by simply using or not using certain organs they may be developed or atrophied and their offspring can then inherit these acquired characteristics. (Milner p. 375, 1993) It should be noted that this theory was not widely accepted largely due to the fact that the French word that Lamarck used in the sense of “must” was translated as “wants to,” which makes it sound as though the organism decides to change its body. Furthermore, Lamarck provided no mechanism by which this could take place. (www.ridgenet.net/do_while/sage/v1i8f.htm) Darwin was the one to give a mechanism for Lamarck’s idea. This mechanism is known as pangenesis.

Darwin’s ideas of evolution were well founded in the beginning; however, as his theory progressed he reverted to Lamarckian thought to explain his observations. To begin his theory he started with the observations that there is variation in offspring. He wrote, “no one supposes that all the individuals of the same species inhabiting the same confined locality, are cast of the same mold…I am convinced that the most experienced naturalist would be surprised at the number of the cases of variability…as I have collected” (Chapter 2 of Origin of Species). He sees that there is a struggle for existence saying, “as more individuals are produced than can possibly survive, there must in every case be a struggle for existence” (Chapter 3 of Origin of Species). Finally, Darwin recognized that there is a survival of the fittest in that the organisms with the best variations for the continuance of the species live and those with variations that are not as useful to the continuation of the species die (Chapter 4 of Origin of Species). Darwin begins to diverge into Lamarckian thought at this point. Darwin wrote that, “ slightly different changes in the conditions of life add to the vigor and fertility of all organic beings…the crossing of forms which have been exposed to slightly different conditions of life or which have varied, favours the size, vigor, and fertility of their offspring” (Chapter 9 of Origin of Species).