Chromosomes as Gene Carriers
Almost 400 years ago the versatile philosopher Montaigne admitted being completely baffled by the mystery of heredity. He thought that he had inherited from his father a disease--a stone in the bladder; but his father suffered from this disease some years after Montaigne was born. How, then, could the father transmit to his son something which he himself did not have at the time the son was conceived? And, besides, the semen was believed to be mere liquid. How could a liquid transmit a stone in the bladder?
To dispel even a part of the mystery which worried Montaigne much biology had to be learned. At present we would say that Montaigne did not inherit a stone in the bladder; what he inherited were genes which engendered a constitution, a development pattern, which included a predisposition towards formation of bladder stones. Furthermore, we know that the genes have a physical basis in the chromosomes in the nuclei of the sex cells, which are highly organized structures with complex and orderly behavior that makes heredity possible. The physical basis of heredity is necessarily also the physical basis of evolution.
Sex Cells and Fertilization. Leonardo da Vinci ( 1452-1519), who was so much ahead of his times in so many things, realized the basic fact that the father and mother contribute equally to the heredity of the child, as shown by the following quotation: "The black races of Ethiopia are not the products of the sun: for if black gets black with child in Scythia, the offspring is black. But if a black gets a white woman with child, the offspring is gray. And this shows that the seed of the mother has power in the fetus equally with that of the father." From Aristotle on, until as late as the eighteenth century, most people