Hockenbury Discovering Psychology 5th txtbk

568 CHAPTER 13 Psychological Disordersschizophrenia—is the same evidence that underscores the importance of environmentalfactors (Joseph & Leo, 2006; Torrey, 2006). If schizophrenia were purely amatter of inherited maladaptive genes, then you would expect a risk rate muchcloser to 100 percent for monozygotic twins. Obviously, nongenetic factors mustplay a role in explaining why half of identical twins with a schizophrenic twin do notdevelop schizophrenia.Nevertheless, scientists are getting closer to identifying some of the specific geneticpatterns that are associated with an increased risk of developing schizophrenia.New research using sophisticated gene analysis techniques confirms the incrediblycomplex role that genes play in the development of schizophrenia. Three differentresearch teams compared DNA samples from thousands of people diagnosed withschizophrenia with DNA samples from control groups of people who did not haveschizophrenia (Shi & others, 2009; Stefansson & others, 2009; Purcell & others,2009). Some of the people in the control groups had other mental or physical disorders,but most were healthy. The researchers were looking for specific geneticvariations that were more common in the genomes of people with schizophreniathan in people without schizophrenia.Collectively, the studies found that schizophrenia was associated with literallythousands of common gene variations. Some of the specific variants were quite rare,while others were quite common. Taken individually, none of the gene variants iscapable of “causing” schizophrenia. Even in combination, the genetic variants areonly associated with an increased risk of developing schizophrenia. As yet, no specificpattern of genetic variation can be identified as the genetic “cause” of schizophrenia.Three particularly interesting results stand out. First, some of the same uniquegenetic patterns associated with schizophrenia were also found in DNA samplesfrom people with bipolar disorder but were not associated with any other psychologicalor physical disorder (Purcell & others, 2009). This finding suggests thatbipolar disorder and schizophrenia might share some common genetic origins. Second,also implicated were several chromosome locations that are associated withgenes that influence brain development, memory, and cognition. Finally, a largenumber of the gene variants were found to occur on a specific chromosome that isalso known to harbor genes involved in the immune response (Shi & others, 2009).Later in this section, we’ll discuss some intriguing links between the immune systemand schizophrenia.Paternal AgeOlder Fathers and the Risk of SchizophreniaDespite the fact that family and twin studies point to the role of genetic factors inthe risk of developing schizophrenia, no genetic model thus far explains all of thepatterns of schizophrenia occurrence within families (Insel & Lehner, 2007;Kendler & Diehl, 1993). Adding to the perplexity is the fact that schizophrenia oftenoccurs in individuals with no family history of mental disorders.One explanation for these anomalies is that for each generation, new cases ofschizophrenia arise from genetic mutations carried in the sperm of the biological fathers,especially older fathers. As men age, their sperm cells continue to reproduceby dividing. By the time a male is 20, his sperm cells have undergone about 200 divisions;by the time he is 40, about 660 divisions. As the number of divisions increasesover time, the sperm cells accumulate genetic mutations that can then bepassed on to that man’s offspring. Hence, the theory goes, as paternal age increases,the risk of offspring developing schizophrenia also increases.Researcher Dolores Malaspina and her colleagues (2001) explored this notion byreviewing data on more than 87,000 people born in Jerusalem from 1964 to 1976.Of this group, 658 people had been diagnosed with schizophrenia by 1998. Aftercontrolling for various risk factors, the researchers found that paternal age was astrong and significant predictor of the schizophrenia diagnoses. Specifically,Malaspina and her colleagues (2001) found that: