Twins and Genetics

Identical twins share the same genetic code and thus are natural clones. Because identical twins share all of their genes, it has been said that it is the environment—rather than genetics—that accounts for any differences between them. Twins can either be monozygotic ("identical"), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic ("fraternal") meaning that they develop from two single eggs that are fertilized by two separate sperms. In a study design that includes both identical and fraternal twins, a researcher can estimate the relative influence of genes and environments on behavioural differences among people. The use of twin and adoption studies has helped us to understand how much abnormality might be due to genetic factors. Researchers who are interested in genetic influences may use twin, family history and adoption studies. All of these involve comparing people with different degrees of genetic relatedness to see how similar they are in relation to a particular trait or behaviour. Studies of schizophrenia patients and their families, for example, have shown that the more closely a person is related to a schizophrenia patient, the greater their own risk of developing the disorder, which supports a role for genes in the disorder. Schizophrenia caused by biological factors is strongly supported as it can be heritable (Carlson, 1994). The linkage between gene factors and schizophrenia can be researched by twin studies and adoption studies. They investigated a group of schizophrenic people, who had been adopted when they were children. The incidence of schizophrenia in the family which had schizophrenic adoptees was similar with the general population which is about 2%. However, the incidence in the schizophrenic adoptees' biological family (parents and siblings) was comparatively high (13%). The founding indicates that the patients became schizophrenia is not because they are growing up in a schizophrenia family but