Heredity and Development: Second Edition (1972)

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1 HEREDITY AND DEVELOPMENT 

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HEREDITY AND DEVELOPMENT: SECOND EDITION 3 CONCEPTS IN GENETICS INTRODUCTION An individual is a product of his heredity and of his development. His hered- ity is the substance he receives from his parents—his biological inheritance. An ovum and a sperm, the hereditary substance of man, unite to form a fertil- ized ovum, the zygote. The essence of this substance is the set of instructions that it contains. The zygote contains all the instructions required to produce another human being. Shortly after its formation, the zygote undergoes a series of changes that leads, if its luck holds, to an adult. These changes are its development. Devel- opment is species-specific, that is, the sequence from zygote to adult is con- trolled by the nature of the hereditary instructions. For one zygote the instruc- tions may be “make a man,” for another “make a mouse.” If all goes well, one will in due time be a man, the other a mouse. Development, then, may be thought of as a carrying out of the hereditary instructions contained in the zygote. The problems of heredity and development have been central to biology since this field began, in the mid-nineteenth century, to become a rigorous science. The problems of heredity and development will always be central to biology. This is inevitable since they are closely associated with the unique feature of life itself—the ability to replicate. This book is concerned with heredity and development. Its basic purpose is to show how ideas in these two fields were first formulated and then stud- ied. The intellectual history of the two has been quite different. Genetics, the science of heredity, effectively began in 1900. By 1930 the general laws of inheritance seemed to be well established, 

HEREDITY AND DEVELOPMENT: SECOND EDITION 4 in the sense that the rules for the transmission of genes from parent to off- spring were understood. Furthermore the rules, with only slight modifica- tions, were found to apply to all animals and plants that geneticists studied. Classical genetics was essentially complete by 1930. It was in a state simi- lar to that of physics in 1899, when A.A.Michelson said, The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote…. “Our future discoveries must be looked for in the sixth place of decimals.” For both sciences the period of intellectual calm was brief. Physics was soon revolutionized by studies of the nucleus of physical matter; genetics was revo- lutionized by studies of the nucleus of living matter. In the decade beginning with 1944, geneticists discovered the chemical substances of which genes are composed, and from this base they have gone on to show how the chemical structure of the gene is responsible for its specific activity. It is hard to exaggerate the importance of these discoveries. So far as biol- ogy is concerned, they are matched in importance only by the theory of evolu- tion. For science in general they represent one of the great intellectual achievements of the twentieth century. Embryology, the science of development, has not had such an intellectu- ally satisfying history. There was a vigorous beginning and the main events and problems were soon demarcated. But explanation has come hard and the fundamental causes of events in development are still poorly understood. It was inevitable that this be so. There could be no satisfactory explanations in embryology until there was a reasonable comprehension of the action of genes. Now that we seem close to such an understanding, greater progress becomes possible. Embryology is a science of tomorrow. The science of biology has few stories more interesting than the history of man’s attempts to explain heredity and development. We will begin with heredity. Some of the most mysterious aspects of heredity are taken for granted. We ‘naturally expect’ the offspring of human beings to be other human beings; of pine trees to be pine trees; of Amoeba to be Amoeba. But a scientist cannot ‘naturally expect’ anything. Natural events must have as their basis laws that are either known or knowable. At least, a belief of this sort is the working philosophy of scientists. Establishing the laws that explain why it is that offspring resemble 

HEREDITY AND DEVELOPMENT: SECOND EDITION 5 their parents has been one of the most exciting chapters in modern biology. A bewildering mass of observations has been united in a conceptual scheme that is rational and concise. This is the science of genetics. In all probability men have speculated about the nature of heredity since the dawn of history. It can be said safely, however, that their speculations led to nothing that could be called science until quite recently. Our method of approach to the field of genetics will be a historical one. We shall begin with the year 1868, when Charles Darwin published The Varia- tion of Animals and Plants under Domestication, and trace the increase in man’s understanding of genetic phenomena down to the present day. This method of approach will illustrate how a science develops. It is possible that this approach will reveal some things about science that you never realized before. We are so accustomed to a succession of triumphs in science that we fail to realize that this is not an inevitable consequence of the application of the ‘scientific method.’ Perhaps you have some knowledge of the way scientific understanding is thought to come about. The explana- tion is usually given as follows: 1. A scientist is confronted with a natural phenomenon that he wishes to explain. 2. He invents a hypothesis to explain the unknown phenomenon in terms of known phenomena. 3. Deductions are made from the hypothesis. 4. The deductions are tested. If they are found to be true it becomes more probable that the hypothesis is true. The more deductions that are tested and found to be true the more probable it is that the hypothesis is true. 5. Eventually the scientist convinces himself and his contemporaries that his hypothesis adequately explains the phenomenon. 6. In the course of explaining the first unknown phenomenon other unknown phenomena will be encountered. The same procedure is fol- lowed to explain these and so successive phenomena become under- standable and organized into the ever-growing body of scientific knowledge. At least that is the way that science is thought to work. To the non-scientist it may appear that if one has the equipment, time, intelligence, and abides by the rules, scientific progress is inevitable. To those working in science, this view is clearly a distortion. Progress is not inevitable. In our survey of genet- ics, for example, we shall find that many blind alleys have been entered and, in addition, we shall learn that an ‘estab- 

HEREDITY AND DEVELOPMENT: SECOND EDITION 6 lished fact’ of one period may turn out to be an ‘erroneous conclusion’ in the light of later discoveries. The idea of straight-line progress is a distortion due to looking back on the history of science. When we do this we select the experiments, observations, and ideas that subsequent events have shown to be correct and, not surprisingly, we ignore the inadequate experiments, incor- rect observations, and faulty ideas. We shall attempt, therefore, to describe the progress of genetics as it took place and in so doing try to understand some of the problems facing scientists who are working on unknown phenomena. It is most important that we make the attempt; we are living in an age when scientific knowledge is of the utmost concern to all mankind. The proper use of scientific knowledge can result in unparalleled benefits to mankind and a misuse can lead to unimagin- able disasters. It is essential that those who will make the decisions, and they will be the non-scientists primarily, have as much knowledge of the nature of scientific methods as possible. Our purpose, therefore, in studying genetics is twofold. We should gain some appreciation of the data and concepts in the field of genetics and, in addition, an understanding of the manner in which science develops.