GENETICS & HUMAN BEHAVIOUR

Genetics is a complex science but what are the fundamentals of Genetics?

The cell carries genetic material which consists of Chromosomes and Genes. Each chromosome contains many genes.

The Chromosomes in the Egg & Sperm carry the genes responsible for heredity. Each animal species has a characteristic number of chromosomes per cell. Human cells have 23 pairs of chromosomes. Of these one is the sex Chromosome which determines the individual’s sex. As per convention the sex chromosomes are labelled X and Y. The remaining 22 pairs of Chromosomes are known as Autosomes. Autosomes carry genes that are not sex linked.

The egg and sperm cells go through a stage called Meiosis in which the chromosomes are split apart leaving only one chromosome out of each pair. The single set of chromosomes from the egg and the sperm combine to form a new cell known as the Zygote.

As a result of the fusion of the chromosomes into a pair with one each coming from the father and from the mother, the characteristics of both the parents on those chromosomes get passed on to the offspring.  

The genes themselves are parts of DNA & RNA in the chromosomes. The genes contain codes for production of various substances called enzymes. Enzymes are proteins that are necessary for the production of various substances that are required for the cells to grow. By controlling the production of various substances required for the cell to grow and sustain the body, genes shape human behaviour along with the environmental conditions.

Whether it is the Nature (Genes) or the Nurture (Environment) that predominantly shapes an individual’s behaviour is not clearly known but most Psychologist say that Nurture or Environment is more powerful than Nature.  

One should remember that it is not a single gene that gives a particular trait, a multiple number of genes on the chromosome cause them. As already mentioned, there is a pair of genes from each parent in an individual. One gene of the pair decides the trait of the individual and is hence known as the dominant gene and the other gene is called a recessive gene. While the gene pair is known as the Genotype, the trait induced by the dominant gene is called the Phenotype.

Only in the case of identical or monozygotic (born by division of the same zygote) twins the hereditary inheritance is identical.  Monozygotic twins are very useful for studying human behaviour with respect to genetic inheritance. Twins that originate from two different Zygotes are called fraternal twins also known as Dizygotic twins.

Some forms of mental retardation are known to occur due to the abnormality in sex chromosomes. Similarly anti-social behaviour may also be related to abnormalities in the sex chromosomes. The Downs syndrome is due to a chromosomal abnormality.

The behavioural traits in humans we see are Polygenic and are determined by not one but many genes.

Human intelligence is not easy to define. Generally, we take intelligence to be level of 1. Verbal comprehension, 2. Word fluency, 3. Perceiving Mathematical relationships, 4. Perceiving spatial relationships, 5. Memory, 6. Perception & 7. Reasoning. These form the basis of assessing the IQ of an individual. However, this is not the only way to define intelligence.

In IQ testing a correlation of 1 means completely identical and 0 means no commonality at all. It is found that the IQs of Monozygotic twins reared together have a very high correlation of 0.87 while for those who reared apart is 0.75.

For fraternal twins, this drops to 0.53 irrespective of the same sex or opposite sex. For siblings, this is 0.49. That means there is hardly any difference between siblings and Dizygotic or Fraternal twins.

There is a difference of 0.12 in correlation between the Monozygotic twins reared together and reared apart. Clearly, that difference of 0.12 is on account of the environment. This is a substantial difference.

These findings indicate that there is a strong influence of inherited characteristics on the intelligence of the individual. There are also environmental factors that shape behaviour. So both Nature and Nurture contribute to behaviours.

The human brain is very complex and has about 10-12 billion neurons or nerve cells. Each neuron is connected to the others making the number of connections truly immense. These connections are called Synapses. Information is passed from one cell to another by chemicals known as Neurotransmitters.

There is a cell body that keeps the cell alive and there are fibres of two types. They are Dendrites & Axons. Dendrites are relatively short and axons can be very long. The Axon connecting the toes to the spinal cord is more than a meter in length. Both of them transmit electrical pulses to the cells.

The drugs that affect behaviour and experience –Psychoactive drugs like LSD, Mescaline, Marijuana, Heroin, Amphetamines, Cocaine, Caffeine, and Alcohol work on the nervous system by influencing the flow of information across the Synapses.