Genetics and Human Development
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Genetics and Human Development Genetics and Human Development Document Transcript

  • Genetics and Human Development Ty Partridge and Jennifer DeGroot Hanawalt Wayne State University (An entry for Applied developmental psychology encyclopedia) It has been over a half-century since Watson and Crick’s (1953) landmark paper on the structure of DNA and its implications for the transmission of heritable information. In that time, genetics has become one of the most exalted fields in all of science. Indeed, more of the pages of Science, the journal of the American Association for the Advancement of Science, are devoted to research, which in one way or another involves genes than any other facet of science. Genetics has also captured the popular press like few other areas of science. Completion of the sequencing of the human genome has been celebrated as one of the greatest achievements in the history of human intellectual endeavor. Perhaps nowhere has genetics seeped into popular culture more pervasively, nor more erroneously, than in the area of human behavior. This is, in part, based on the accolades and hubris with which genes have been promoted as the key to unraveling the mysteries of humanity. The National Institute of Mental Health has put identifying the genetic bases of complex behaviors and genetic roots of mental illness as cornerstones of their funding initiatives. The American Psychological Association (APA) and the American Psychological Society (APS) have joined this movement with broad-scale support of behavioral genetics research. The APA genetics in psychology website proclaims, for instance, that “Soon the genetic bases of common psychiatric disorders, such as schizophrenia and depression, are likely to be unraveled.” (Flint, 2004). Picking up on the euphoric optimism that mainstream behavioral science has for genes in explaining behavior, the popular press is replete with headline grabbing stories about scientist identifying the 1
  • genetic basis for schizophrenia, depression, homosexuality, aggression, perceptions of injustice, infidelity, and the list continues. However, when one reads the science of genetics closely a key, if not the cornerstone, aspect of genetic science is missing from the scientific and popular press headline accounts: Development. As geneticists are increasingly learning everyday, the role of genes cannot be separated from the process of development. In this brief essay, we hope to highlight the inseparable entwinement of genes and development. First, we will discuss how improper metaphors for the functioning of genes have lead to the subjugation of development in human behavior genetics and popular accounts of genes and behavior. Further we offer an alternative metaphor that is more congruent with what genetic science is learning about genetic functioning. Secondly, we will discuss the two sciences of genetics; population and molecular, and how their relation to each other has led too much of the confusion about the role of genes in development. Finally, we will discuss the role of genes and development as they pertain to our growing understanding of the origins of human behavior. Metaphors of the gene: Blueprints and algorithms Contemporary debates regarding the role that genes play in the development of human behavior are largely extensions of the historical debate between preformationism v. probabilistic epigenesis (Gottlieb, 1992). Preformationism holds that the adult phenotype (i.e., biological and behavioral traits) is inherent in the embryo at conception. In other words, our adult traits are largely pre-specified. Indeed, the classic preformationist claim was that the sperm cell contained miniature adults or homunculi, which simply grew in size until they reached adulthood. Modern preformationism certainly does not take this view, but rather a modified perspective that the information for guiding the process of development from embryo to adult must be contained somewhere in the embryo. Behavioral genetics has largely adopted this preformationist line of thought, leading to the ubiquitous metaphor of genes as the blue-prints for life: an instruction 2
  • manual for humanity. Genes therefore, are set apart from development and are viewed as the governors or prime causes of developmental processes. Thus, the reasoning is that if we can just read the blue-print encoded in the genome we would know with relative certainty how development going to proceed for a given individual. More recently, behavioral geneticists have reworked the blue-print metaphor into the seemingly more sophisticated metaphor of the genome as a computer algorithm – a set of programming code which specifies the developmental program. While this latter metaphor is couched in the vogue terminology de jour, the assumptions and implications are the same as the preformationist blue-print metaphor. The historical alternative to preformationism is probabilistic epigenesis, which has been incorporated into the field of developmental psychobiology. Epigenesis quite literally means to create from what came before, and the probabilistic epigenetic perspective argues that development proceeds through probabilistic transformations of one biological / behavioral state to another. These probabilistic transformations are the result of complex dynamic transactions between the integrated organism, its developmental history, and ecological contingencies. In short, the developmental process is “governed” by the act of developing via organism-environment transactions. The rules guiding development are not encoded somewhere outside the developmental process, but rather are created with each step of development. Rather than a blue-print or computer program, a more apt metaphor for this perspective is that of a game of make-believe among children where rules are constantly being invented by the children playing the game and implemented via pre-school arguments toward consensus. Where the game ends no one knows and it literally takes on a unique form of its own. From this perspective genes are critical, indeed essential, to the developmental process and yet they do not hold any special exalted place. Genes are simply one of many important participatory factors. It is important to note that the majority opinion, whether from a behavioral genetics 3 View slide
  • perspective or a developmental psychobiological perspective, is that genes alone do not determine phenotypic outcomes – environmental influences also do play a role. However, the degree of environmental influences is distinct for each approach. Behavioral genetics considers environmental influences with the concept of the reaction-range. The reaction-range is the narrow range of deviation from the pre-specified genetic program within which the environment can exert a modest influence. Conversely, developmental psychobiology employs the norm-of-reaction concept to explain gene-environment transactions. This concept refers to the fact that a wide range of phenotypic outcomes are possible given a single genome, however, only a limited range of those potential phenotypes are generally observed because the environmental context of development tends to be fairly homogeneous across individuals. This gives the illusion of a more limited phenotype-genotype relationship than is actually the case. “Because we cannot reasonable manipulate and test development of every genotype in every environment possible, we cannot know the developmental limitation for any genotype. We estimate norm of reaction by the observed variation of a genotype within the range of environments observed in a study. Reaction range cannot be applied to biological genetics or to development.” (Michel & Moore 1995 p. 184-185). Gottlieb (1992; 1997) illustrates this with a wide array of empirical examples of neophenotypes – new phenotypes that arise from the same genome as a product if differential developmental histories. Indeed, differences or similarities in animal phenotypes occur due to sharing or lacking similar developmental systems rather than sharing or lacking similar genotypes (Lickliter & Ness, 1990). The two sciences: population and molecular genetics In part, the separation human behavioral genetics and developmental psychobiology stems from the fact that there are actually two sciences of genetics: population genetics and molecular genetics. Behavioral genetics is more closely aligned to the former and developmental 4 View slide
  • psychobiology to the latter. To understand this divergence it is important to put it in an historical context. The concept of a gene has a long history, much longer than Watson and Crick’(1953) discovery. Indeed, the practice of animal and plant husbandry clearly demonstrated that the traits of off-spring resembled the traits of parents. Thus, through selective breeding, one could relatively reliably produce animals and plants with desirable traits. It seemed apparent that there must be some mechanism through which parents could pass on phenotypic traits to off-spring via reproduction – and the concept of a gene was born (though as noted earlier, shared developmental systems rather than shared genes account for the common appearance of traits in parent and off- spring populations.) The important point here is that the notion of a gene extends back hundreds of years, long before basic knowledge of cell biology, let alone chromosomes, DNA, and nucleotide sequences. Building on this long history of animal and plant husbandry, population genetics has its origins in the work of Gregor Mendel, the 19th Century priest and biologist, who first quantified heritability by working out the statistical relationship between the phenotypic characteristics of off-spring given the phenotypic characteristics of parents. Population genetics has little to do with the functioning of genes or their actual relationship to phenotypic development. Rather population genetics is chiefly concerned with the development of statistical models relating parent population traits to the likelihood of those same traits appearing in off-spring populations, and the statistical manner in which those traits spread through a species population. Molecular genetics, on the other hand, is a much newer science stemming from traditions in experimental embryology and cell biology. First suggested by Schrödinger (Schrödinger, 1948) the molecular search for genes began with Watson and Crick (1953). Contemporary molecular genetics focuses on the simultaneous tasks of identifying genes and articulating the processes through which they synthesize proteins. Gene identification alone is not a simple task. Genes, while we typically think of them as objects, can really be thought of more accurately as events. Genes are sequences of 5
  • nucleotides that have the capacity to produce proteins, which are in turn the foundational building blocks of cell biology. Most of the DNA in the human genome is referred to as “junk DNA” because it is not involved in protein synthesis. We can not simply look at strands of DNA and see the genes – we must catch them in action and only through their behavior can we distinguish genes from junk DNA. It is in this way that genes are in many ways events rather than objects and it is the reason that it is so very difficult to identify them. This is also the reason that the process of identifying genes is dependent upon simultaneously understanding how they work to synthesize proteins. In the last five years, revolutionary advances in research technology have been made in both the identification and functional facets of molecular genetics. This has led to an equally revolutionary revision of our understanding of genes and how they work. Some of the more notable findings are that: 1. The human genome is contains far fewer genes than was previously estimated – only approximately 30,000 as compared to the 120,000 historically assumed. This is of importance because, given that this is far fewer than the approximately 100,000 genes of the nematode (flat- worm), it suggests a complex relationship between genes and the complex biological structures and behaviors of human beings. 2. A single gene is capable of synthesizing more than a single protein, which has severe consequences for relating genes to phenotypic outcomes as compared to the old dogma that there was a one to one relationship between genes and proteins. Thus, knowledge of the gene sequence itself, without the developmental conditions differentiating when a given protein might be synthesized, provides no information about how gene sequences relate to proteins, let alone complex behaviors such as depression and schizophrenia. 3. That both gene expression and the process of protein synthesis are much more probabilistic and complex than previously thought. Indeed, gene expression is dependent upon the activity many other genes in the genome and highly sensitive to cellular and extra-cellular contexts (see Garcia-Coll, Bearer, & Lerner, In press) 6
  • The two science redoux: behavioral genetics and developmental psychobiology Behavioral genetics is primarily subset of population genetics and has as its aim the statistical estimation of behavioral traits given the presence of those traits in parents, non-twin siblings, fraternal-twin siblings, and identical-twin siblings. The key statistic of behavioral genetics is the heritability quotient, signified as h2. This statistic is derived from the differential correlations in a given behavioral trait (e.g., schizophrenia, depression, alcoholism, extroverted personality, etc.) among various relative relationships (i.e., parent-child, non-twin siblings, fraternal-twin siblings, and identical-twin siblings). The h2 statistic can range in value from 0 to 1 and is thought to reflect the percent of variation in a given behavioral trait due to shared genes (e.g., if h2 = .75 for a given trait, then it is argued that 75% of the variance in that trait is due to genes). However, this interpretation is a substantial misinterpretation of the true meaning of the statistic. The h2 coefficient simply reflects the likelihood of a phenotypic trait given that trait is present in a relative – it in absolutely no way indicates a mechanism of heritability (Michel & Moore, 1995). The implication of the gene as the mechanism is an assumption based on an adherence to the non-developmental preformationist approach to genetics. Indeed, the genetic basis of human behavior was estimated using this statistical approach before any scientist had ever identified an actual gene! Moreover, the more we learn about genes and their functioning, the statistical assumptions underlying the h2 coefficient are largely invalid (Wolfe, Brodie, & Wade, 2000). From molecular genetic perspective, behavioral genetics has largely relied on gene knock-out techniques using animal models of human behaviors. This technique involves identifying animal model analogues for behaviors such as depression and substance abuse, and systematically blocking the expression of a specific gene or chromosomal region and observing the effect on the analogue behavior. More recently, behavioral geneticists have begun to employ newer techniques such as quantitative trait loci (QTL) and single nucleotide polymorphisms (SNP) to correlate variation in 7
  • DNA sequencing with behavioral outcomes. As human behavioral genetics has employed more molecular genetic techniques to understanding the relationship between genes and human behavior two things are becoming clear – that the relationship is much more complex than supposed by traditional behavioral genetics (see Plomin & McGuffin, 2003) and that behavioral genetics without a fully integrated developmental perspective is untenable. The view of gene-behavior relationships as understood by developmental psychobiology seems to be providing a more adequate fit to contemporary genetic findings. This view is a complex one, in which genes are important participatory factors, but are just one factor in a hierarchically organized developmental system (Gottlieb, 1996). This developmental system involves DNA, RNA, Proteins, Cells, Organs, Organ systems, biological entities, and ecological environments engaging in bi-directional transactions across development. This is a much more complicated situation than envisioned by classic behavioral genetics. As it turns out, with respect to the relationship between genes and behavior, the herald of the human genome marks the prelude to our scientific voyage rather than a call to port. 8
  • References Fling, J. (2004). Genetics in Psychology [online]. Available: http://www.apa.org/science/genetics/intro.html Garcia-Coll, C., Bearer, E., & Lerner, R. M. (In press). Nature and nurture: The complex interplay of genetic and environmental influences on human behavior and development. Mahwah, NJ: Lawrence Erlbaum Associates. Gottlieb, G. (1992). Individual development and evolution: The genesis of novel behavior. New York: Oxford University Press. Gottlieb, G. (1997). Synthesizing nature-nurture: Prenatal roots of instinctive behavior. Mawah, NJ: Earlbaum. Lickliter, R., & Ness, J. W., (1990). Domestication and comparative psychology: Status and strategy. Journal of Comparative Psychology, 104(3), 211-218. Michel, G. F. & Moore, C. L. (1995). Developmental psychobiology: An interdisciplinary science. Cambridge, MA: MIT Press. Plomin, R., & McGuffin, P. (2003). Psychopathology in the post-genomic era. Annual Review of Psychology, 54, 205-228. Schrödinger, E. (1948). What is life? The physical aspect of the living cell. Cambridge, UK: Cambridge University Press. Watson, J. D., & Crick, F. H. C., (1953). A Structure for Deoxyribose Nucleic Acid. Nature, 171, 737. Wolf, J. B., Brodie, E. D., & Wade, M. J. (2000). Epistasis and the evolutionary process. Oxford, UK: Oxford University Press. 9