genetic factor of schizophrenia

Genetic Factor of schizophrenia

Schizophrenia is a combination of disordered thinking, perceptual disturbances, behavioral abnormalities, affective disruptions, and impaired social competency. This means the person has difficulty thinking clearly, knowing what is real, managing feelings, making decisions, and relating to others.

Schizophrenia is not a single disorder but rather a syndrome with multiple variations and multiple etiologies, both of which are complex and inadequately understand. In some, a genetic defect may contribute to abnormal development of the brain or a neurochemical malfunction, while in other cases factors such as nutrition, toxins, or trauma might interact in genetically vulnerable persons, resulting in schizophrenia.

There is a genetic component in schizophrenia is well recognized. However, the exact genetic vulnerability is not known, as no single gene has been identified as a risk factor for schizophrenia. Research is focusing on chromosomes 6, 18, and 22. It is likely that more than one gene is involved and that different families may have different genes involved. A persons has an 8% risk of schizophrenia if a sibling has the disorder, a 12% risk if one parent is affected, a 14% risk of sharing the disorder with a dizygotic twin, a 39% risk if both parents are affected, and 50% risk if a monozygotic twin has schizophrenia (Kennedy, 1996; Straub, 1997).

In a 1967 review paper Edith Zerbin - Rudib listed 17 major family studies of schizophrenia involving first degree relatives. By 1980 at least nine other major family studies had been reported. Parental relationships are exceptional and are not considered here because it is well established that person affected by schizophrenia exhibit reduced fertility. Since, by definition, a parent must have successfully reproduced at least once, parents of schizophrenia offspring are less likely to have expressed the symptoms of the disorder, despite carrying predisposing risk that may have been transmitted to the offspring. Aside from parental relationships the studies consistently showed a much greater risk of schizophrenia in the close relatives of schizophrenia persons than would be expected in the general population.

In monozygotic twins, prenatal factors do not always affect each twin to the same extent. Because the hands are formed at the same time cells are migrating to the cerebral cortex during the second trimester of pregnancy, they have been a site for indirectly studying brain development. In studying sets of twins in which one has schizophrenia and the other does not, it was found that affected twins had a number of small deformities in their hands and greater differences in their fingerprints compare to their siblings. There was also a significant prenatal size difference between the twins during the second trimester. Conditions that could result in brain injury at this stage of development include anemia, anoxia, ischemia, maternal alcohol or drug abuse, toxin exposure, or viral infections (Davis and Bracha, 1996).

Genetic theory predicts that if all familial aggregation of schizophrenia were attributable to genetic factors, then the heritability of liability should be approximately double the correlations in liability found in first degree relatives (because, on average, first degree relatives share half of their genes in common). The range the heritability of liability to schizophrenia calculated from the twin studies is approximately twice the range of correlation in liability to schizophrenia found in first degree relatives in most family studies.

Degree relative		% schizhophrenia
3rd (12,5%)	General populations 1st cousins Uncle/aunts Nephews/nieces	1 2 2 4
2nd (25%)	Grandchildren Half siblings Parents	5 6 6
1st (50%)	Siblings Children Fraternal twins	9 13 17
100%	Identical twins	48

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