Experimenting with dog breeding
Mendel spent ten years on his experiments, but when he had given his results to the world they were promptly forgotten, while students of heredity wrangled over the theories of Weismann and Lamarck.
However, in 1900 three men simultaneously brought it all to light again. At the start of their investigations the exceptions to Mendel's laws received more attention than they should have, and many conservative investigators decided that Mendel was wrong. About this time the work was taken up by a young professor of biology, Thomas Hunt Morgan by name.
Previous to 1900, investigators had thought of inheritance as belonging to the adult individual. In other words, they believed that the parents were responsible for the characteristics of the son, and that in his turn, the son would hand them on combined with those of his mate, to his own sons or daughters. Even earlier than that, Weismann had successfully proposed the theory to his associates that it was not the physical characteristics which were handed on from father to son, but rather the father's germ plasm, which is the essential part of the reproductive cells. Weismann also stated that this plasm is held in minute rods called chromosomes, lying in the center of each cell. This is now an established fact.
When fertilization takes place, the chromosomes from the male sperm lie beside the chromosomes of the female in the ovum, or egg. The first cell of any new organism contains one complete set of chromosomes from each of the parents, male and female. Growth is then carried on by the splitting of this first cell into two, and of these two into four, and so on until the body is formed by billions of individual cells. However, each time a cell is split, the original male and female chromosomes are handed down exactly as in the first case. So it may be seen that the chromosome is the unit of heredity.
This was not known to Mendel. He called his units of heredity "elements", and they were altogether things of his own imagination, since he was not working with a microscope. When Dr. Morgan took up the work he made use of this valuable aid, but nobody understood just what the functions of the chromosomes were even when they saw them, and it was not known whether they were Mendel's elements or not.