Math Science Chemistry Economics Biology News Search
The aim of this article is to provide a brief history of the development of the modern genetics. The new science of heredity and inheritance, thus the law of heredity are explored. Classical genetics is now regarded as a triumph of modern biology, because it allowed the scientists to unlock the secrets of nature and life and eventually give the mankind control of the material world.
Genetics before Mendel
As you might imagine every discussion with regard to the history of a scientific subject starts with Aristotle and Hippocrates. They both developed the theory of pangenesis, saying that each part of the body produces something, germ cells as we now call them. These are, broadly speaking, the material bases of heredity, since these cells transform into characters of the offspring. Characters not yet presented in an individual may also be transmitted by certain effects of mutilation or loss.
More or less casual observations in plants and animals were made over a long period, beginning with the observations of Cotton Mather on maize in 1716.
He made many crosses, studied the pollination process , and also recognized the importance of insects in the process. After all the research done, he recorded a few cases where the offspring resembled only one parent, consequently emphasizing the identity if the hybrids from reciprocal crosses.
Following Mather, Darwin collected a vast amount of information from the works of the plants hybridizers, practical breeding of domestic animals and cultivated plants. He himself carried out numerous experiments with pigeons and with various plants. He recognized two more or less distinct types of variations, continuous and discontinuous that show dominance and often being transmitted unchanged through many generations.
His conclusions were clear: crossing has a unifying effect, since hybrids are generally intermediate between parents and the next generations.
Darwin had, however, more to say: he thought that changed conditions such as domestication stimulated variability and also affected the inheritance both in selection within strain and in crosses between strains.
The next important biologist, Herbert dealt with crosses among ornamental plants. Perhaps his most important contribution was his discussion of the idea that crosses between species are unsuccessful while crosses between varieties yield fertile offspring.
Experiments on Plant Hybridization carried by Naudin, a contemporary of Mendel studied a series of crosses involving different types of plants. In several respects he made real advancements that helped the next researchers to develop notions that completed the concept of genetics.
The New Era of Mendelism in Genetics
Everyone agrees that the new science of genetics was established due to the discoveries made by Gregor Johann Mendel.
His life was difficult due to the fact that the political regime did not allow a priest to have scientific views (it was considered unorthodox).Trying to combine his love for nature and love for God, in 1856 he began the series of experiments that led to his paper hence his laws.
The interest in honeybees made him attempt to cross strains of bees, apparently without success. He is also known to have kept mice but he hesitated as a priest to carry out experiments on mammals.
The only details that we know about his experiments and life appear in Mendel’s letters to another correspondent of the Austrian Regional Report Magazine.
The picture that emerges is of a man very actively and effectively experimenting, aware of the importance of his discovery, fitting into the image of the frustrated genius.
Over the next few years Mendel performed his classic experiments on hybridization, using peas grown in the monastery garden. These were undertaken to show that in this species characters can be treated as distinct units or elements which are transmitted unchanged from parents to offspring. Using what suitable techniques of artificial fertilization, Mendel performed the appropriate crosses and showed that in the first hybrid (F1) generation, one character out of each pair appears in all hybrid plants.
The other character has apparently disappeared, which he described by saying that one character in the pair was dominant. In the case of height when tall and short plants were crossed all the hybrids were tall, so tall is the dominant character. Mendel then self fertilized the hybrids to produce the second hybrid (F2) generation, where he observed the famous 3:1 ratio. What Mendel called the recessive character reappeared in one quarter of the F2 plats, giving a ratio of three dominant (e.g. tall) to one recessive (e.g. short).
Mendel explained this phenomenon of the segregation of characters by suggesting that the first hybrid generation combined the hereditary potential of both characters, the recessive being hidden but nevertheless available for transmission to the next generation.
A rather different problem was pointed out by scientists, who noted that the results obtained by Mendel are too good to be true. He would have had to be very lucky indeed to get ratios as close as he did with the theoretical prediction.
Drosophila is as important as humans, at least for geneticists. It was not chosen at the beginning as a experimental material because the ratios obtained were very poor.
The deviations obtained from the theoretical Mendelian ratios were rarely close. The phenomenon was due to considerable differences in the mortalities of insects in pupal stages, before experiments were made and results were counted.
The problem was not the Drosophila, but the scientists who did not prepare the culture method carefully.
Morgan was the first scientist to report the first sex-linked lethal allele in Drosophila by introducing marker genes and detecting them.
It soon became evident that such recessive lethals represent the largest single class of mutant in Drosophila.
This is why this small insect became very useful for studying mutations.
Theories of the determination of sex were already numerous in Aristotle’s time, and he discussed many of them.
His own view was that there is, in each embryo, a sort of contest between the male and female potentialities, and the question of which prevails, that is, the frequencies of the two sexes, may be influenced by many factors, such as the age of parents, the direction of wind, etc.
The constancy of chromosome number for a species was known, and it was known that this number was usually even, equal numbers coming from the egg and from the sperm.
It was known that each chromosome divided longitudinally at each somatic division, and that this division is initiated by an equal division of each visible granule along the length of the chromosome, It was also known that the reduction in chromosome number is accomplished by the last two divisions before the production of the mature gametes.
Further, it was generally supposed that the chromosomes are the bearers of the essential hereditary materials. There were, however, a number of things, now part of common biological knowledge that were not known. It was generally supposed that, when the chromosomes reappear at the end of the resting stage, they first do so as a single continuous thread, which then breaks into the number of chromosomes characteristic for the particular species.
Classical genetics starts with the parallelism between the segregation of Mendelian characters and the behavior of the chromosome during meiosis and fertilization.
The Chromosome Theory appeared later as a crucial step on the pathway of establishing genetics as an autonomous science of heredity.
Johanssen used the concept of the genotype to dismiss traditional studies of the phenotype (individual characters) as irrelevant for the understanding of heredity.
Since the phenotype could be influenced by all sorts of non-hereditable environmental factors, only the geneticists could see through to the underlying content of the germ plasm which alone was responsible for determining the character of future generations.
There is a general view that scientific discoveries are more of less inevitable, and that it makes little difference for the general public whether or not a particular individual makes a discovery at a given time.
In most of the cases it will not be understood and it will not have any effect on future events until someone else makes the same discovery again, and eventually confirm the theory.
The development of genetics is one of the best examples of this kind.
All of the discoveries led to the utilization of new ideas and new techniques and to rapid, sometimes spectacular, advancements in genetics and in other fields of medicine.
The history of this science is primarily a history of ideas, as I reckon that chromosomes or other biochemical products helped us develop a better life standard.