The Secret of Life

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Here was one of those extraordinary simplicities that can revolutionize a whole field of science. Mendel's observations proved that inside the cells of plants—and presumably animals too—is a mysterious mechanism, incredibly small, that rules heredity in accordance with precise mathematical laws. In 1866 Mendel published a paper to this effect in the proceedings of the Brünn Natural Science Society, but nothing happened. The world was not ripe for his ideas. In 1868, when he was appointed abbot of his monastery, his scientific career came to an end.

At the turn of the century, three scientists (Hugo De Vries in The Netherlands, Karl Correns in Germany, and Erich Tschermak in Austria) independently rediscovered Mendel's principles. They also rediscovered his long-forgotten paper, and gave him full credit; the basic principles of genetics are still known as Mendel's laws. Genetics, born at last to science's estate, went to work on the interwoven mysteries of life and heredity.

Key Chromosomes. For a while, as often happens after a scientific breakthrough, additional discoveries came easily. Several biologists, notably Walter S. Sutton in the U.S., connected Mendelian inheritance with the known behavior of chromosomes, which are threadlike bodies in the nuclei of cells. When a cell divides nonsexually, as in a growing plant or animal tissue, the chromosomes replicate (make copies of) themselves. Each daughter cell gets a full set, and unless something has gone wrong, it is exactly like the chromosome set of the parent cell (see diagram).

In sexual reproduction, the chromosomes behave differently. The sex cells (sperm and egg) are the end results of a complicated process (meiosis or reduction division) that gives each of them half as many chromosomes as in the nonsexual cells. This reduction is necessary because the sex cells join during fertilization of the egg, and if each contributed a full set of chromosomes, the fertilized egg would have twice the normal number. But if both sperm and egg contribute half as many chromosomes, the fertilized egg gets just the right number.

Many years before the birth of the science of genetics, the chromosomes had been observed behaving in this way, but no one knew why they did. Genetics supplied the answer. Reduction division is a kind of lottery that deals the fertilized egg half a set of chromosomes from each parent, like cards dealt out to players in a two-handed card game. When maternal and paternal chromosomes are slightly different, which is generally the case, their dominant genes (units of heredity) suppress recessive genes, as Mendel's red-flowered peas suppressed white-floweredness. Each recessive gene is still riding its chromosome, and biding its time in obscurity. It can assert itself only when the corresponding gene from the other parent is also recessive. It may have to wait for many generations (in the case of humans, for hundreds of years) before it gets its innings. Then, free of suppression by a dominant gene, it produces a white-flowered plant or a blue-eyed baby. Or, if it is a bad gene, it may produce a deformed baby or a plant that bears no flowers.