Morgan knew that each chromosome contains a definite number of hereditary elements which are called genes. Experiments have made certain that these genes lie along the chromosome in a straight line. However, like electrons, they have never been seen, since they are far too infinitesimal for even the most powerful microscope to reveal. But as with electrons, the influence of the genes was so apparent that their existence could not be doubted.
It might be stated here that sex, in the higher animals at least, is predetermined at fertilization. The sex chromosomes are elements of the sex-determining mechanism, and if hereditary characteristics are borne upon them, it will be seen that the mechanism which is determining whether the individual shall be male or female is also determining whether or not the individuals shall exhibit some particular characteristic.
Let us suppose that a chromosome from the male and one from the female lie side by side in the original cell of a new organism. The genes are exactly opposite each other. In other words, the gene controlling eye color in the male chromosome lies opposite the corresponding gene which regulates eye color in the female chromosme. And so on down the line. The corresponding genes in the male and female chromosome are always opposite each other.
Suppose that a part of a chromosome which has come from the male becomes interchanged with that identical part of the female chromosome; then the normal linkage group is broken. The paternal chromosome will now contain elements of the maternal, and conversely, the maternal chromosome will contain elements of the paternal. As only the dominant elements are inherited by the first generation, this will not alter the characteristics of the children. However, when the next generation, or grandchildren, are produced by crossing offspring of the first generation, the change in linkage will be seen. Instead of the grandchildren being altogether like either the grandfather or the grandmother, something like one per cent will combine certain features of the grandfather with certain features of the grandmother.
When this takes place it is called "crossing over," and there is considerable visual evidence that it actually takes place. Viewed through a microscope as they lie alongside each other in the original cell before it divides, the male and female chromosomes are sometimes seen to wind themselves around each other. They may cross at one point or more, and there is every reason to believe that they exchange parts of themselves when they do this. This is only an hypothesis, but it has enabled investigators in genetics who use "statistical" methods, such as research workers in physics make use of in their quest for electrons, to actually locate on the microscopic field the position of five hundred genes of the fruit fly.