50 Years of DNA: What It Has Meant to Psychological Science
Celebrate the discovery more than the discoverers …
Discovering the structure of DNA was a race that others would soon have won if Watson and Crick had not beaten them to it. In part, Watson and Crick won the race because they had seen an unpublished X-ray photograph of DNA by Rosalind Franklin that showed unmistakable evidence of a helical structure. Two weeks later, they built their famous model of DNA as a double helix. Franklin died 5 years later at the age of 37 not knowing that Watson and Crick had seen her DNA photograph before they published their model of DNA. Although Franklin has been used as an example of the misogyny of the science establishment because she did not share in the Nobel prize for discovering the structure of DNA, Nobel prizes are never awarded posthumously – Franklin died in 1958 and the Nobel prize was awarded in 1962 (Maddox, 2002).
The 20th century has been called the century of the gene. The century began with the re-discovery of Mendel’s laws of heredity. The word genetics was first coined in 1903. In 1944, deoxyribonucleic acid (DNA) was shown to be the stuff of genetics – DNA had previously been dismissed as a possibility because it seemed too simple a molecule. Then, on April 25, 1953, the most famous paper in science was published in which James Watson and Francis Crick described the double helix of DNA, the premier icon of science. We will hear a lot this month about the far-reaching impact of this discovery on the life sciences (Dennis & Campbell, 2003).
What would be different about psychology today if we still did not know the structure of DNA? Obviously, the obligatory half page in introductory psychology textbooks that shows the double helix would disappear but this would be no great loss because this information is rarely integrated in the rest of the textbooks. However, a real conceptual benefit for psychology has been that understanding the structure of DNA – and the advances that continue to follow from this discovery – have demystified and legitimized the process of heredity. But beyond such vague conceptual benefits, has the discovery of the structure of DNA affected psychology?
DNA has certainly changed the course of the rest of the life sciences. Understanding the structure of DNA led directly to understanding the two major functions of DNA. The first is the essence of heredity: how DNA reliably duplicates itself by unzipping and dividing the helix up the middle, with each half of the helix re-creating its complement. The second is how DNA codes for proteins, the building blocks of life, although the actual DNA code was not broken for more than a decade after Watson and Crick’s paper. This knowledge of the two functions of DNA spawned the biotech revolution whose foundation is the sequencing and cloning of DNA. The ability to sequence DNA culminated in the Human Genome Project’s working draft of the 3 billion bases of DNA in 2001. However, there is no human genome: We each have a unique genome because one in a thousand of DNA bases varies for at least one percent of the population. Life scientists are more interested in the generalities of the genome while behavioral scientists are more interested in the variations in the genome. Millions of DNA variants have been identified and making it possible to find genes that affect psychological disorders, as explained below. By recognizing unique DNA sequences in genes, microarrays are now able to assess the expression of more than ten thousand genes simultaneously. Genes can then be cloned and manipulated and re-inserted in the same species (gene targeting) or another species (Ridley, 1999).
But what about psychology? Mouse model research in psychology has begun to benefit from these advances in gene expression and gene targeting. In the last two years, many behavioral studies have begun to study coordinated patterns of gene expression across thousands of genes in response to learning and drugs. Dozens of studies have used gene targeting to modify specific genes in order to study the genes’ effects on behavior, especially learning and behavioral responses to drugs (Plomin & Crabbe, 2003).
For human psychology, gene targeting is not possible as a research tool, nor are gene expression studies of brain tissue possible except for post-mortem brains. The most general benefit for psychology so far has been the ability to identify specific genes that affect human behavior, which is the direct result of the identification of millions of DNA variations in the human genome. Gene identification has been most successful for the thousands of rare single-gene disorders in which a mutation in a single gene is necessary and sufficient to cause a disorder. Many of these single-gene disorders have psychological effects – for example, more than 200 of these disorders include cognitive effects among their symptoms. Such single-gene disorders are very rare, with prevalences of .0001 or less.
The DNA revolution also provides tools to identify genes responsible for the heritability of common psychological disorders and dimensions. These are usually called complex traits because they are likely to be influenced by multiple genes as well as by multiple environmental factors. Psychological disorders such as schizophrenia, affective disorders, dementia, autism, reading disability, alcoholism, and hyperactivity are the target of much of this DNA research. Although some genes have been identified for these disorders, the process of identifying genes for complex traits in psychology as well as in medicine has been much slower and more difficult than anticipated, probably because complex traits are influenced by many more genes of much smaller effect size than had been assumed (Plomin & McGuffin, 2003).
The most far-reaching ramifications for psychology will come after these genes are identified. Psychology will be central to the new era of genetic research called the post-genomic era in which the focus will shift from finding genes to understanding how these genes work. Such post-genomic research is usually considered in relation to the bottom-up strategy of molecular biology in which a gene’s product is identified by its DNA sequence and the function of the gene product is traced through cells and then cell systems and eventually the brain. Psychology lies at the other end of the continuum of levels of analysis in the sense that psychology represents an integrationist top-down level of analysis that begins with the behavior of the whole organism rather than a reductionist bottom-up level of analysis that begins with a single molecule in a single cell. For example, psychologists can ask how the effects of specific genes unfold in behavioral development, how they interact and correlate with experience, and how they affect response to treatment (Plomin, DeFries, Craig & McGuffin, 2003).
This top-down psychological level of analysis is likely to pay off more quickly in prediction, diagnosis, and intervention, and eventually for behavioral preventions that use genes as early warning systems. Bottom-up and top-down levels of analysis of gene-behavior pathways will eventually meet in the brain. The grandest implication is that DNA will serve as an integrating force across all of the life sciences, including psychology.
Dennis, C, & Campbell, P. (2003). The eternal molecule. Nature, 421, 396. [Introduction to a special issue: The double helix – 50 years.]
Plomin, R, & Crabbe, J. C. (2001). DNA. Psychological Bulletin, 126, 806-828.
Plomin, R, DeFries, J. C., Craig, I. W., & McGuffin, P. (Eds). (2003). Behavioral Genetics in the Postgenomic Era. APA Books: Washington, DC.
Plomin, R, & McGuffin, P. (2003). Psychopathology in the postgenomic era. Annual Review of Psychology, 54, 205-228.
Ridley, M. (1999). Genome: The autobiography of a species in 23 chapters. London: Fourth Estate.
Watson, J.D, & Crick, F H C. (1953). A structure for deoxyribose nucleic acid. Nature, 171, 737-738.
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