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Scientists have developed a number of methods to study the mechanism of heredity: 1. Pedigree Method 2. Cytological Studies 3. Breeding Experiments 4. Twin Studies.
Method # 1. Pedigree Method:
This method involves studying certain traits or characteristics covering several generations of the same family. Based on the evidence, inferences are made about hereditary transmission of each characteristic. The geneticists employing this method involved what is called a family tree which shows the distribution of the particular trait or characteristic among the members of the family over generations.
Method # 2. Cytological Studies:
In this type of study, the body cells are examined carefully with the help of very sensitive microscopes. On the basis of these observations inferences are made about the transmission. The discovery of the conditions causing Down’s syndrome was based on this method.
Method # 3. Breeding Experiments:
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This is a very common method of studying genetic transmission. In fact. Mendel employed this method by selectively breeding different kinds of garden peas. A very famous experiment from the point of view of psychology was conducted by R.C. Tryon who selected two groups of rats, one capable of running fast, the other being slow runners, in a maze-learning situation.
Subsequently, he went on selectively breeding; the slow rats being made to mate only with other slow rats and the fast rats with other fast rats. After a few generations of such breeding, he found that two distinct types of rats called ‘maze bright’ and “maze dull” rats resulted.
This experiment is illustrative of the use of the breeding method to study the hereditary transmission of behavioural characteristics. Obviously this method cannot be employed with human subjects. But at the level of animals this method is very useful.
This method is also very useful for studying the role of various environments. For example, one can breed a pure strain of an animal and then vary the environmental conditions and study their influence on the animal.
Method # 4. Twin Studies:
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This is the most important method from the point of view of psychology. Nature has provided very convenient sources of studying the influences of heredity through twins. Twins are of two types. When a male and a female unite sexually sometimes two cells are fertilised. This results in the birth of two children. Twins born as a result of two cells being fertilised at the same time are known as fraternal twins or dizygotic twins.
On other occasions it has been found that even one fertilised cell splits into two. Thus, two children are born out of one fertilised cell splitting into two. Such twins are called identical twins or monozygotic twins. A number of experiments have been carried out with both fraternal and identical twins.
Identical twins have a greater similarity of hereditary characteristics than fraternal twins. Twins in general, have a stronger affinity for each other than ordinary brothers and sisters. In these experiments, comparisons have been made among identical twins, fraternal twins and siblings to study the role of heredity.
Similarly, comparisons have been made between twins living and growing together on the one hand, and twins where the members have been separated and reared apart. These investigations help us to understand the impact of the environment on the behaviour of an individual.
Twin studies have been of great significance in making possible the comparative study of the roles of heredity and environment and also their interactions in determining human behaviour. A large number of these studies have related themselves to intelligence, particularly measured by intelligence tests.
Briefly, these studies have shown that identical twins resemble each other much more than fraternal twins and ordinary siblings. It has also been shown that identical twins, even when reared apart, show a greater degree of resemblance in intelligence then fraternal twins reared together.
Some evidence from the field of mental abnormality has been advanced to show that when two members of a pair of twins show symptoms of schizophrenia, there is a likelihood of family history than when only one member is affected.
It is perhaps needless to go into the details of a large number of twin studies. It is sufficient to say that twin studies brought out the fact that it is difficult to conclusively answer the question about the roles of heredity and environment in determining behaviour. They also pointed out that the real question is not how much heredity contributed and how much the environment contributed but the manner in which the two interact.
The controversy between heredity and environment, to a large extent, was abandoned towards the early part of this century. But again, during the sixties and seventies, the work of ethnologists like Lorenz and Tinbergen revived this controversy. Basing their observations on the behaviour of organisms in natural settings, Lorenz and Tinbergen pointed out the existence of inherited specific patterns of behaviour common to members of a particular species.
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These behavioural patterns showed a lot of similarity to instincts. But psychologists criticised the observations and argued that many of these are results of learning during the prenatal and subsequent periods. While this controversy still remains, fortunately it does not occupy a focal position in psychological researches.