Dog Breeding Genetics
In 1902 William Sutton discovered evidence to prove it, but the way in which chromosomes actually worked was still very much in doubt. The solving of this problem took an immense amount of work. It meant keeping the pedigrees of thousands of breedings, and tabulating the variations of all the individuals. It can be seen that this was a tremendous task, since the average animal breeds at most only several times a year. To collect sufficient data to solve the problem of genetics in this
way was impossible for any one man. It was here that Dr. Morgan decided to make use of the common fruit fly which breeds all year round. Since only nine days are needed for a generation, it is possible, starting with one pair of flies, to raise twenty-five generations in the period of one year. This would correspond to something like five hundred years in human beings, or thirty to forty in dogs.
The first conclusion Morgan reached is that every species of animal or plant has a characteristic number of chromosomes, and that this number is invariable in practically every instance. He also discovered that inheritance characteristics seemed to come in groups, as though certain of them were linked together.
The fruit fly has eight chromosomes, four male in the sperm and four female in the ovum. The question then arose whether the four characteristics exhibited by the flies might not correspond to the four male and four female chromosomes. Before this, Morgan had discovered that each generation of flies showed four outstanding characteristics. For instance, a black body tended to be accompanied by purple eyes and small wings, with a dot of color at the base. He kept careful records of the appearance of all the changes of the flies, and he found four different groups. If it could be assumed that the four groups corresponded to the male and female chromosomes, the chromosome might then be said to be the actual unit of heredity. This is now known to be the case. Observing various kinds of plants and animals has shown the linkage of groups, and the number of these groups is always half the number of chromosomes possessed by an individual of that species.
Mendel had concluded that various combinations of inherited characteristics were the result of chance only. But it is obvious, if certain characteristics always fall into groups, that their inheritance cannot be attributed to chance. However, there still remained some important exceptions which linkage of the chromosomes could not explain. For instance, on some occasions when it was thought that all of the second generation descended from two flies should inherit either yellow wings and white eyes, or gray wings and red eyes, it turned out that something like one percent of the second generation would actually have gray wings and white eyes, or yellow wings and red eyes. Morgan concluded that something was wrong with the linkage groups, something which must be solved before the work could proceed. His answer to this question is his greatest contribution to the science of genetics.