The Human Genome Project
Today scientists are mapping the entire genome — the DNA contained in the 23 human chromosomes. This enterprise is enormous. The chromosomes of each person contain 3 billion permutations of four chemical bases arrayed in two interlocking strands. This DNA may be divided into between 50,000 and 100,000 genes. But the same DNA can function in more than one gene, making the concept of individual genes something of a convenient fiction. The mystery of how these genes, and the chemistry underlying them, cause specific traits and diseases is a convoluted one.
The Human Genome Project has, and will continue to, advance our understanding of genes and suggest preventive and therapeutic strategies for many diseases. Some diseases, like Huntington’s, have been linked to a single gene. But the search for single genes for complex human traits, like sexual orientation or antisocial behavior, or mental disorders like schizophrenia or depression, is seriously misguided.
Most claims linking emotional disorders and behaviors to genes are statistical in nature. For example, differences in the correlations in traits between identical twins (who inherit identical genes) and fraternal twins (who have half their genes in common) are examined with the goal of separating the role of environment from that of genes. But this goal is elusive. Research finds that identical twins are treated more alike than fraternal twins. These calculations are therefore insufficient for deciding that alcoholism or manic-depression is inherited, let alone television viewing, conservatism, and other basic, everyday traits for which such claims have been made.
The Myth of Mental Illness
In the late 1980s, genes for schizophrenia and manic-depression were identified with great fanfare by teams of geneticists. Both claims have now been definitively disproved. Yet, while the original announcements were heralded on TV news and front pages of newspapers around the country, most people are unaware of the refutations.
In 1987, the prestigious British journal Nature published an article linking manic-depression to a specific gene. This conclusion came from family linkage studies, which search for gene variants in suspect sections on the chromosomes of families with a high incidence of a disease. Usually, an active area of DNA (called a genetic marker) is observed to coincide with the disease. if the same marker appears only in diseased family members, evidence of a genetic link has been established. Even so, this does not guarantee that a gene can be identified with the marker.
One genetic marker of manic-depression was identified in a single extended Amish family. But this marker was not apparent in other families that displayed the disorder. Then, further evaluations placed several members of the family without the marker in the manic-depressive category. Another marker detected in several Israeli families was subjected to more detailed genetic analysis, and a number of subjects were switched between the marked and unmarked categories. Ultimately, those with and without the putative markers had similar rates of the disorder.
Other candidates for a manic-depression gene will be put forward. But most researchers no longer believe a single gene is implicated, even within specific families. In fact, genetic research on manic-depression and schizophrenia has rekindled the recognition of the role of environment in emotional disorders. If distinct genetic patterns can’t be tied to the disorders, then personal experiences are most likely crucial in their emergence.
Epidemiologic data on the major mental illnesses make it clear that they can’t be reduced to purely genetic causes. For example, according to psychiatric epidemiologist Myrna Weissman, Ph.D., Americans born before 1905 had a 1 percent rate of depression by age 75. Among Americans born a half century later, 6 percent become depressed by age 24! Similarly, while the average age at which manic-depression first appears was 32 in the mid 1960s, its average onset today is 19. Only social factors can produce such large shifts in incidence and age of onset of mental disorders in a few decades.
Last reviewed: By John M. Grohol, Psy.D. on 27 Aug 2007
