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The inheritance of schizophrenia is now a well-established fact. This section focuses on the theories on the inheritance of schizophrenia and their evidences. The following figure shows the outline of this section.

10 to 15 percent of the immediate families of schizophrenic patients suffer from the disorder.

Among children who have either a schizophrenic mother or a schizophrenic father, about 10 percent will develop the disease.

Approximately 50 percent of the children whose parents are both schizophrenics go on to develop schizophrenia themselves. On average, 1 to 1.5 percent of the people will develop schizophrenia sometime during their lives.

Genes are tools that transfer physical makeup from our parents into our bodies. Half of the genes come from the mother, the other half from the father. In the nucleus of each of the somatic cells in our body, there are some 6 million genes, located on 46 chromosomes. Genes determine such inherited traits as our skin color, the upper limits of our height, how intelligence we can be, and whether we produce the correct amount of enzymes, hormones, and other substances the body necessary for metabolism. They are not, however, the sole factor that determine our qualities and abilities. Inheriting a certain gene or genes can sometimes cause fatal diseases or render us vulnerable to others if we encounter certain things in the environment.

Schizophrenia is now considered as a disorder in which a predisposition-instead of the disease itself-is inherited from the parents.

There are two types of twins: identical or fraternal. Identical twins are produced when an egg splits after fertilization. As a result, they are genetically identical. On the other hand, fraternal twins result from the fertilization of two separate eggs. About half the genes of fraternal twins are identical, with the other half being different. Fraternal twins share no more genetic similarities than they do with any other child born to the same parents.

Facts

The likelihood that both identical twins will suffer from schizophrenia is 2 to 4 times greater than for both fraternal twins.

The chances for both twins to become schizophrenic increase as their ages grow.

Conclusion

The fact that both the identical twins are more likely to suffer from this mental disorder than both the fraternal twins do imply the importance of genetic factors in the etiology of the disorder.

Although the fact that schizophrenia runs in families supports the inheritance of schizophrenia, it can also suggest that schizophrenics raise their children in a unusual way that increases their offspring's chances to develop schizophrenia themselves. This debate is known as the "nature-nurture" controversy, an issue over the two possible causes of schizophrenia: their inheritance or their environment.

The nature-nurture controversy is about whether schizophrenia is a congenital or acquired disease. It is widely accepted that the biological causes of schizophrenia include a variety of factors, which fall in two major categories: the biological and environmental. However, to determine the precise role of either factor (i.e. biological and environmental) in the etiology of schizophrenia is a big challenge for researchers.

Studies involving adoption offered evidences against the nurture argument of the "nature-nurture" controversy.

FACTS
In the situation where identical twins are raised in two separate households with no awareness of one another, one will become schizophrenic 50% of the time if the other develops schizophrenia.

Therefore, it can be argued that identical twins that are raised together or apart have equal chances to develop schizophrenia, thereby eliminating adoption as a factor affecting in the incidence of the disease.

Adoption studies have cast further light on whether schizophrenia is inherited. According to these studies, adopted children who had a schizophrenic natural parent are at far higher risk of developing the mental disorder than adopted children who did not.

Based on the studies and facts mentioned above, it is very convincing that genetics plays a role in the incidence of schizophrenia. The fact that one of the identical twins is not 100% concordant with the other in developing the disease means that more than genetics is involved in the disease. The biological and environmental influences remain a debated issue.