Academy of Science, USSR. (Leningrad) 1931, p
THE GENETIC CONCEPTION OF LIFE
CALVIN B. BRIDGES
When the Academy of Sciences invited me to come to Leningrad to lecture on the theory of heredity, I came gladly. First, because I personally have looked across the ocean with great admiration at the splendid achievements of your economic and social program. Second, because this invitation to a so-called theoretical scientist, is in itself a symptom of the general increase in scope and power which marks your activities.
Here you must maintain a balance between theory and practice, keeping theories in their rôle of servants of practice. But the more ambitious the practice, the more elaborate is the theoretical assistance which must be called upon. It is easy to build a small dam of wood or of stone across a little river. But to fling a great and enduring dam of steel and concrete across a mighty river uses all the resources of advanced mathematics and of highly developed theories of stresses.
A similar advanced stage has now been reached in the great programs for the improvement of animals and plants undertaken by your practical breeders. Your workers have now advanced so far that in order to make their advance as rapid as possible they must make use of new higher levels of theory. New methods must be developed and used.
The improver of plants and animals meet tremendous obstacles. Living things follow their own laws and are not easily turned to strange new ways. One must be patient. Especially one must allow time. Even when it is known exactly how to proceed, the carrying out of a breeding program may take many years. Often some particular process cannot be shortened, no matter how many workers or how much money is devoted to it. The demands which you make are very high; and they shall be met. But you must not forget that these great tasks could not have been done at all until within the last few years, and now only slowly.
There are great differences in the rate at which the same goal can be reached with different plants and animals. This is in spite of the fact that the correct method is much the same for all forms. The laws of heredity show a high, a startlingly high, similarity throughout the whole range of plant and animal life. There is a very real reason for this. The reason is that the special machinery inside plants and animals which produces characters is nearly the same in all plants and animals.
Our first task is to discover as fully as possible the structure of this machinery and how it does its work to produce the characters, which we desire. The fuller our knowledge in this field the greater is our power to control the process. We find what each lever and wheel does in order to know what lever to pull to produce a given character.
The speed of discovery of the structure of this machinery and of the general laws of its operation similarly varies greatly with different kinds of animals and plants. If one breeds plants, a whole year is generally used to follow through a single generation. With domestic animals several years may have to be spent in merely waiting for the animals to grow to maturity. You especially cannot afford to wait. To discover the general laws, your breeders must search for and use that particular animal where the laws can be found out most rapidly.
The most favourable animal so far found is the small fly Drosophila melanogaster with which I and others have been working for more than twenty years. This fly gives a complete new generation from parent to full-grown children in tens days. It can be raised throughout the year, giving 30 or even 36 generations each year. The families are large, each mother giving hundreds or thousands of children. From such large families, of full brothers and sisters, it is easy to discover the exact proportion of children affected by a new character. Also one can see the full number of different kinds of children possible for a given pair and can select the desired kind even though it is as rare as one in many thousands.
The fly is very small. It takes approximately a million to weigh one kilo. Because of this small size, one little laboratory room can be used instead of vast fields or large stables. The food for several hundred for the entire span of their life costs only a few kopecks.
This little fly shows immense variety in its characters. New characters arise under our very eyes. For example, a race all of whose members have eyes of a dark red color suddenly gives one individual whose eyes are snow white. We have found out how to breed from such a new type in order to get a pure race where all the thousands of children show this character. Such new primary racial characters arise frequently, so we have much opportunity to learn the general rules whereby one establishes a pure race of a new character. We use the name «mutation» for a new character which arises suddenly from the original, uniform, pure-breed ancestral stock.
Another difficult task, which occurs frequently in the breed of plants and animals, consists in taking desirable characters which exist only in separate races and combining them into a new race, better than either old race. Practice in this art can be acquired in the shortest time with the races of this fly. The variety of separate types is so vast that the number of separate distinct races which could be made by putting together these primary characters in various ways is greater than the number of grains of sand in all the sand banks of the entire world.
The mutant characters studied in the flies are of the same sorts as those with which one must work in practical breeding. For example, some mutations turn the flies into giants, weighing twice as much as normal. In one of these giant races the amount of feeding determines whether a fly shall be of normal size or twice as large. This is like the queen bee which is made to grow to large size by the food she receives.
Some characters are physiological. One mutation determines whether the flies shall have or lack the instinct to fly toward the light. The duration of life is subject to change; some mutant forms having their life cycle shortened to thirty days instead of the normal sixty days. The ability to live at all is subject to mutation. In some races only half or only a tenth of the flies ever see the light of day. Poor viability is not always a disadvantage, for it can be used to kill off automatically at an early stage the sex which is not wanted.
In the fly we can change the ratio of the sexes at will, from families consisting mainly or only of daughters, to families of two or three hundred sons but no daughters, with all intermediate ratios. The fertility of races can be changed. And in special mutant types the females are all sterile while the males are all fertile, or vice versa. The sex can be changed so that one and the same individual is female on the left side but is male on the right. Or we can raise females which are speckled with small spots of maleness. Other flies are intersexes, that is intermediate between males and females, or supersexes, that is, superfemales more female-like than normal or supermales with exaggerated male sexuality. In these changes of sex the whole build of the fly is changed, including many secondary sex characters and characters which are usually regarded as non-sexual characters.
As the results of studies on this exceptionally favourable animal, the heredity in it is now more thoroughly known than that of any other plant or animal. The studies have resulted in what may be called a genetic conception of life, based on the structural relations found.
The physicists have shown that such an object as a particle of sugar is best understood in terms of the presence, in definite relations, of certain substructures, namely molecules, atoms and electrons. In a similar manner the far more complex and different activities of living things are being understood in terms of a series of genetic units. Thus, a fly is built out of cells, and cells are controlled by nuclei. Nuclei are themselves composed of chromosomes. The chromosome is in turn composed of hundreds of smaller genetic units called genes.
All genes work together in producing each effect. The exact character produced corresponds to the point of equilibrium between the genes and conditions tending to make the character more extreme and the genes and conditions tending to restrict its development.
The genes are contained inside the chromosome, somewhat like marbles in a bag. The exact arrangement has been studied and found to be a linear series, a chain or string of genes. The genes are organically connected to each other in single line, gene to gene, probably by an intergenic substance. This chain or string of genes does not lie straight along the axis of the chromosome, which is too short to hold the long gene-string in this manner The gene-string lies spirally coiled just inside the pellicle of the chromosome, like a spring in a tube. The fact that the genes are inside the chromosome now admits no doubt, and the details of the internal arrangement are deduced from enormous experimental data.
We have found that changes in anyone of these genetic units namely, nuclei, chromosomes and genes changes the development and the characters of the fly. Thus, doubling of the nuclear contents changes the fly into a particular kind of giant. A similar doubling of the nuclear contents gives the giant cabbages which Karpetchenko has produced. Many of our flowers, vegetables and grains are the result of such doubling.
Changes by the addition or loss of one particular chromosome, through processes called non-disjunction or elimination, lead to other mutant types. Certain datura plants also show this type of mutant character. Changes by loss of a piece of a chromosome, called deficiency, or by gain of an extra-piece of chromosome, called a duplication, give still other kinds of changes in the characters of the fly. Finally, changes in the smallest unit of all, the gene itself, give the great majority of mutant characters.
Through the discovery of the effects of X-rays we are now in position to increase a hundred fold or even more the rate at which new mutant forms can be secured. The details of these new discoveries and their importance in the development of plants and animals with desired characteristics are being given in special lectures at the Academy of Sciences.
All these discoveries, made principally with the fly, have made it possible to elaborate a new theory of heredity, a new genetical conception of life on the basis of which it has been necessary to rebuild the entire set of methods in use in practical breeding. The investigator now really begins to govern and control the process of production of new varieties.
At this meeting I wish mainly to thank you for the privilege of participation in such stirring enterprises and to assure you that whatever knowledge I may possess is at the disposal of your workers.
I wish all success to the Soviet Republic in its program.
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