Twins: And What They Tell Us About Who We Are by Lawrence Wright Page B
Authors: Lawrence Wright
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gene, such as cystic fibrosis, phenyl-ketonuria, and Huntington's disease. In several cases, scientists have trumpeted genetic markers for such things as manic depression or schizophrenia and have then quietly withdrawn their findings.
The brain depends upon enzymes for the neurochemical process we call thinking, and those enzymes are created by genes. From a biochemical perspective, there is an obvious genetic contribution to intelligence, but no single gene that makes one person smarter than another. Instead, more than half of the body's 100,000 genes donate in some way to the general fund of intelligence. Robert Plomin, a behavioral geneticist of the Institute of Psychiatry in London, who at that time was at Pennsylvania State University, examined two groups of children, one with high IQs and another with low IQs, to see if there were particular markers that were in or near genes thought to be associated with cognition. He found five markers that had significant associations with differences in intelligence, but four of them failed to be replicated in an independent sample.
Perhaps the most successful example of twin studies leading to molecular discovery is the example of Tourette's syndrome, a bizarre neurological disorder characterized by chronic tics, sometimes violent twitching, intrusive thoughts, and the uttering of repetitious, meaningless phrases. Tourette's runs in families and has
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all the characteristics of a genetic disease, and yet the search of the human genome has so far yielded no obvious genetic markers for the disorder. Daniel Weinberger, chief of the Clinical Disorders Branch at the National Institute of Mental Health in Baltimore, studied five sets of identical twins. In each set, one twin meets the so-called "300-yard diagnosis," which means that his symptoms are so apparent that an observer can diagnose them at that distance, whereas the other twin has less severe symptoms. When the researchers examined the brains of the twins, they found that the more affected twin had extremely sensitive receptors for the brain chemical dopamine. Dopamine affects the section of the brain called the caudate nucleus, the region where motor actions are planned. Because each set of twins has identical genes, the researchers reason that the cause of the difference in their dopamine receptors may have to do with birth traumait was usually true that the twin with the lower birth weight grew up to be the more severely affectedor else the stress of different life events. In either case, it appears that there is a genetic basis for the disease, but it requires an environmental trigger to awaken its catastrophic potential.
Alcoholism also runs in families, and identical twins are far more alike in respect to their drinking behavior than fraternal twins. Moreover, identical twins who have been raised apart are about as alike in their drinking as identical twins raised together. This suggests, as strongly as any other twin studies, that alcoholism is an inherited disorder. In 1990, researchers at the University of Texas Health Science Center at San Antonio claimed to have linked the D-2 dopamine receptor gene to alcoholismthe same location where scientists found the discordance for Tourette's syndromebut other scientists failed to replicate the finding. Even if
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there is an alcoholic gene, however, it is clear that environment influences much of drinking behavior. Children of alcoholics who have been adopted into families where drinking is not a problem rarely become alcoholics themselves; such cases seem to depend upon the stability of the adopting family. Twin studies correlating drinking behavior with religious affiliation found that alcoholism was five times higher in people with no religious affiliation than in the most fundamentalist group. Presumably Muslims and Amish people would carry at least some genetic vulnerability for alcoholism, if there is such a thing, but alcoholism is rarely a problem
The brain depends upon enzymes for the neurochemical process we call thinking, and those enzymes are created by genes. From a biochemical perspective, there is an obvious genetic contribution to intelligence, but no single gene that makes one person smarter than another. Instead, more than half of the body's 100,000 genes donate in some way to the general fund of intelligence. Robert Plomin, a behavioral geneticist of the Institute of Psychiatry in London, who at that time was at Pennsylvania State University, examined two groups of children, one with high IQs and another with low IQs, to see if there were particular markers that were in or near genes thought to be associated with cognition. He found five markers that had significant associations with differences in intelligence, but four of them failed to be replicated in an independent sample.
Perhaps the most successful example of twin studies leading to molecular discovery is the example of Tourette's syndrome, a bizarre neurological disorder characterized by chronic tics, sometimes violent twitching, intrusive thoughts, and the uttering of repetitious, meaningless phrases. Tourette's runs in families and has
Page 80
all the characteristics of a genetic disease, and yet the search of the human genome has so far yielded no obvious genetic markers for the disorder. Daniel Weinberger, chief of the Clinical Disorders Branch at the National Institute of Mental Health in Baltimore, studied five sets of identical twins. In each set, one twin meets the so-called "300-yard diagnosis," which means that his symptoms are so apparent that an observer can diagnose them at that distance, whereas the other twin has less severe symptoms. When the researchers examined the brains of the twins, they found that the more affected twin had extremely sensitive receptors for the brain chemical dopamine. Dopamine affects the section of the brain called the caudate nucleus, the region where motor actions are planned. Because each set of twins has identical genes, the researchers reason that the cause of the difference in their dopamine receptors may have to do with birth traumait was usually true that the twin with the lower birth weight grew up to be the more severely affectedor else the stress of different life events. In either case, it appears that there is a genetic basis for the disease, but it requires an environmental trigger to awaken its catastrophic potential.
Alcoholism also runs in families, and identical twins are far more alike in respect to their drinking behavior than fraternal twins. Moreover, identical twins who have been raised apart are about as alike in their drinking as identical twins raised together. This suggests, as strongly as any other twin studies, that alcoholism is an inherited disorder. In 1990, researchers at the University of Texas Health Science Center at San Antonio claimed to have linked the D-2 dopamine receptor gene to alcoholismthe same location where scientists found the discordance for Tourette's syndromebut other scientists failed to replicate the finding. Even if
Page 81
there is an alcoholic gene, however, it is clear that environment influences much of drinking behavior. Children of alcoholics who have been adopted into families where drinking is not a problem rarely become alcoholics themselves; such cases seem to depend upon the stability of the adopting family. Twin studies correlating drinking behavior with religious affiliation found that alcoholism was five times higher in people with no religious affiliation than in the most fundamentalist group. Presumably Muslims and Amish people would carry at least some genetic vulnerability for alcoholism, if there is such a thing, but alcoholism is rarely a problem
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