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After reading this article you will learn about the factors influencing the development process in individuals.
A very basic question regarding the developmental process has been the prolonged controversy between the relative roles of heredity and environment. How much of human behaviour is due to the role of environment and how much of it is inherited? This was a central question in psychological researches for a long time.
In the very early stages psychologists were sharply divided. Those who were influenced by the biological sciences tended to emphasise the role of heredity; and those with sociological bias tended to give emphasis to the environment. Theories based on the instinct concept naturally emphasised the hereditary view.
A new phase in this controversy commenced with the work of Mendel in genetics. Mendel’s work laid the foundation for the science of genetics. Through his experiments, Mendel brought out his theory of genes, the dominant and recessive genes. Mendel showed how certain characteristics were transmitted across generations because of genetic characteristics.
The real carriers of heredity are the chromosomes which are long, cylindrical structures found in the nuclei of those cells which are capable of dividing. In the human body each living cell carries 23 pairs of chromosomes. It has been shown that these chromosomes are very important in the carriage and transmission of hereditary influences.
Chromosomes are complex chemical structures. The chemicals include DNA and RNA and there are specific points along the DNA to carry the coded instructions which are necessary for the development and functioning of the individuals. Thus, genes play a crucial role in passing on hereditary characteristics.
When a human child is conceived, 23 chromosomes of a male cell and 23 chromosomes of a female cell combine to form a single cell called a zygote. The entire ground plan for the development of the individual, whether it is going to be a male or female and several other things are determined by these forty six chromosomes.
Out of these one pair of chromosomes determines the sex. There are two types of sex chromosomes, the X and the Y. If the linked pair contains two X- chromosomes, then the resulting offspring will be a female. If on the other hand, the pair contains one X-chromosome and one Y-chromosome, the resulting offspring will be a male.
In addition to determining the sex characteristics, chromosomes and gene structures play a very important role in determining a number of other characteristics. Certain disorders like mental retardation PKU, Down’s syndrome, colour blindness, etc. are transmitted through the genes.
In recent years, a number of experiments have been carried out in the area of behavioural genetics which studied the genetic transmission of behavioural characteristics. It has been shown that two children of the same parents have very little chance of being exactly identical. This shows that the genetic structure ensures the uniqueness of every individual.
Geneticists explain this on the basis of the principle of independent assortment. This principle states that when two sets of chromosomes combine, they combine and recombine in a completely independent manner with the result that one cannot definitely say that children cannot be very different from their parents. This has been shown with reference to characteristics like the colour of the eyes, the hair etc.
In recent years, genetic scientists have undertaken extensive research to study the process of gene structuring, changes in gene structure, etc. They have been very successful in creating changes in gene structuring and combination.
These efforts have succeeded in producing combinations of genes .in a very short time which might have otherwise taken a very long time. The breeding of hybrid varieties is an example. A hybrid is an offspring produced by a combination of genetically different parents.
It has been shown that most human traits are determined not by single genes but by the interaction of a number of genes. Such traits which are determined by the interaction of a number of genes are called polygenic traits. This is why many human characteristics are found to occur in a continuum rather than in discrete categories. If a trait is determined by a single gene then people are found to belong to one of two categories. For example, the colour of the eyes is determined by a single gene, so people are either brown-eyed or not.
On the other hand a trait like intelligence is continuous and ranges from a low degree to a high degree and people are found to vary all along the range. This is because intelligence is a polygenic trait. Since there are a number of genes determining intelligence, we find very few people becoming ‘genius’ and very few people becoming mentally retarded. It is again because of the fact that behavioural characteristics are determined by a large number of genes that we find it possible to influence them through the provision of a proper environment.