****Essay Prompt-After reading Heritability; It’s All Relative, think about the following questions and write a 500-word essay. Answer the following questions in the essay, and use support and proof from the readings. There are two other readings attached to use for more information attached but the bulk of the info should be researched or found in the Heritability; It’s All Relative, article.
Why are twin studies valuable in behavioural genetics research? What does the research say about the effect of the environment on IQ scores in poor homes versus affluent homes? What does this suggest? What is the conclusion of the article how might these findings be useful to other researchers?
Articles and info Heritability: It’s all relative (apa.org) Heritability: it’s all relative Heritability is a statistical measure defined in relation to a particular environment and a particular population. April 2004, Vol 35, No. 4 Print version: page 44 2 min read Heritability is one of the foundational concepts of behavioural genetics, but its meaning is not always clear.
Does a study showing that IQ is highly heritable among affluent children in Denmark have any implications for poor children in the United States? Or is it largely irrelevant? New research is making it increasingly obvious that the answer is: “It depends.”
Heritability, as the term is used by behavioural geneticists, is a statistical measure defined in relation to a particular environment and a particular population. The only way to find out whether the heritability of a trait is the same for other environments and populations is to go out and study them.
In a recent study, University of Virginia psychologist Eric Turkheimer, PhD, and his colleagues did just that. Their study explored the heritability of children’s IQ in different populations within the United States–those with high socioeconomic status (SES) and those with low SES.
Previous studies of children’s IQ have produced conflicting results. On the one hand, some studies of twins and adoptees have found large genetic effects. On the other hand, studies of impoverished children adopted by well-to-do families suggest that the environment plays an important role.
For their study, published last year in Psychological Science (Vol. 14, No. 6), Turkheimer and his colleagues analyzed data from several hundred monozygotic and dizygotic twins included in the National Collaborative Perinatal Project, which followed more than 48,000 mothers and their children from birth to age 7.
Turkheimer and his colleagues found that, among poor families, children who grew up in the same household tended to have similar IQ scores, regardless of how genetically similar they were. Around 60 per cent of the variance was accounted for by the environment, while genes contributed almost nothing.
Among affluent families, the reverse was true. Monozygotic twins with identical genes tended to have much more similar IQ scores than dizygotic twins, regardless of the family environment. The findings suggest that it makes little sense to speak in general about the heritability of a trait such as IQ. For large populations of people who live in diverse environments, such as children in the United States, such broad statements may be meaningless.
The environment can make genes extremely important in some subpopulations, but insignificant in others, notes Turkheimer. Such findings do not challenge the traditional definition of heritability–the proportion of variance on a particular trait that is accounted for by genetic factors within the population as a whole, says Terrie Moffitt, PhD, of the University of Wisconsin and King’s College London.
But they are important reminders that heritability can vary dramatically depending on the population and the environment that is being studied. –E.S. BENSON Behavioral genetics: meet molecular biology The wedding of techniques from molecular biology with traditional twin and family studies has ushered in a ‘postgenomic era’ in behavioural genetics.
By ETIENNE S. BENSON April 2004, Vol 35, No. 4 Print version: page 42 8 min read 0 During its first 30 years, from roughly 1960 to 1990, the modern discipline of behavioural genetics was based almost entirely on twin and family studies. Those studies made a strong case for the importance of genes in behaviour, but the connection always remained loose and statistical.
Only in rare cases could a direct connection between a particular gene or set of genes and a particular behaviour be made. In the past decade and a half, all that has changed with the introduction of bioinformatics, genetic engineering and other techniques that allow researchers to measure, analyze and manipulate genetic material rapidly and easily.
These techniques have changed the composition of the field of behavioural genetics, engaging the interest of new groups of researchers beyond psychology–molecular biologists, medical doctors and others–who had previously seen behaviour as too slippery for biological research.
This shift took place during a time when interest in genetics was exploding. The announcement in 2000 of a completed draft of the human genome–the total complement of genes found in the nucleus of each human cell–and the 50th anniversary of Francis Crick and James Watson’s discovery of the structure of DNA in 2003 marked the high points.
Today, expectations of quick rewards from the use of these new techniques are lower than they were during the first flush of excitement. It is now clear that a single gene for complex disorders such as depression is unlikely to exist, let alone be found, even with the most sophisticated methods. Complex behavioural traits, researchers are finding, are influenced by tens if not hundreds of genes, each interacting with the environment and each other in unpredictable ways.
Nonetheless, behavioural genetics continues to hold out the promise of a better understanding of the biological basis of behaviour–hence the field receives strong support from the National Institutes of Health and other grant-making institutions concerned with the intersection of behaviour and health.
“There’s more and more a proper recognition that you have to understand behaviour and genetics and how they work together if you want to understand how people stay healthy or become unhealthy,” says John Hewitt, PhD, director of the Institute for Behavioral Genetics at the University of Colorado at Boulder, and chair of the APA Science Directorate’s task force on genetics.
New tools and collaborations The new techniques have not replaced the classic methods in behavioural genetics: twin and family studies that used genetic relatedness to search for genes associated with behaviour (see page 46). In fact, twin studies remain one of the best ways of identifying genetic markers linked to complex behavioural traits, according to researchers such as John DeFries, PhD, founder of the journal Behavioral Genetics and former director of the Institute for Behavioral Genetics.
Increasingly, however, such studies are being used not as end-points in themselves, but as stepping stones toward molecular genetics studies that can identify particular genes and their functions, says DeFries. Ten years ago, before the Human Genome Project and the proliferation of inexpensive genetic tests, a researcher studying a particular behavioural disorder might have had access to tests for three or four genes, says Jonathan Flint, MD, a behavioural geneticist at Oxford University–and the information available about those genes would have been minimal.
“Now you click on the Internet and you can find information for the whole genome,” he says. Such information is now available not just for the human genome, but also for common laboratory animals such as mice. This flood of data means that the ability to gather, organize and analyze biological information is becoming increasingly critical. Flint’s lab, like many others, has recently hired a bioinformatics specialist to stay up-to-date on methods for mining the gigabytes of data now available.
New techniques are also providing scientists with ways of directly manipulating genes in animals and observing the altered genes’ effects on behaviour. Mice have proved to be especially amenable to such manipulation. There are now thousands of different strains of single-gene mutants and “knockout” mice–animals in which a single gene has been altered or disabled. APA genetics task force member Jeanne Wehner, PhD, of the University of Colorado at Boulder, is among those who have studied such knockouts.
Although her training is in biochemistry, she and her laboratory do work that is primarily psychological. Using tests of learning and cognition, they look for behavioural differences in strains of genetically manipulated mice. One knockout-mouse strain they have studied is missing the gene for protein kinase C gamma, a cellular “second-messenger” that communicates between surface receptors and the internal machinery of neurons in the brain and spinal cord.
Like standard mice, these knockouts can be trained to respond to a stimulus in exchange for a reward. However, in experiments where rewards are given for withholding a response–rather than for responding immediately after each stimulus–the mice tend to perform poorly. This, together with their tendency to drink more alcohol than standard mice, is taken as an indication of their impulsivity.
Wehner and her colleagues at the University of Colorado are now trying to flesh out the links between protein kinase C gamma and its possible effects on human behaviours such as drug abuse and alcoholism. One set of studies, led by a member of Wehner’s lab, neuroscientist Barbara Bowers, PhD, is examining the effects of protein kinase C within the cell. She is testing the hypothesis that the missing gene affects serotonin receptors, which are known to be involved in emotion and motivation.
Another set of studies, led by Marissa Ehringer, PhD, a human genetics researcher also at the Institute for Behavioral Genetics, is trying to bridge the gap between animals and humans. As part of a larger project on adolescent anti-social behaviour, Ehringer is looking for evidence that humans show variation in the gene for protein kinase C gamma and whether that variation has consequences for behaviour.
As with much of today’s behavioural genetics research, the protein kinase C studies would be impossible without the collaboration of people from a variety of disciplines: the biologists who created the knockout animals, the neuroscientists and psychologists who designed and implemented the animal behaviour studies, and the psychologists and medical geneticists who are looking for genetic variation in humans.
Promises and challenges The proliferation of new techniques has raised expectations of what behavioural genetics can do. But, as many researchers are quick to note, those expectations can sometimes be seriously out of touch with the real promises and challenges of the field. “The most common misunderstanding–and it’s almost willful misunderstanding right now–is that there’s going to be a simple answer to a complex question,” says Hewitt.
Typically, this takes the form of claims that “the gene” for some complex trait–sexual orientation, for instance, or alcoholism–has been discovered. The media deserve some blame for exaggerating the significance of new research findings, but as Hewitt notes, researchers are not guilt-free:
The temptation to play along with the hype in order to increase support for the field is strong. Recent research is making such a stance increasingly untenable, however. The deeper scientists delve into the genetics of complex behaviours, the more they find that such behaviours are influenced by tens or hundreds of interacting genes, each accounting for only a small portion of the overall variance.
That it is not genes alone, but rather genes in interaction with the environment that produce complex behaviours, is also receiving increasing support, says psychologist Terrie Moffitt, PhD, of the Institute for Psychiatry at King’s College London. Moffitt and her colleagues, for instance, have studied two genes that affect the breakdown and uptake of neurotransmitters in the brain.
They have found that the genes have significant effects on depression and antisocial behaviour–but only in people who are exposed to particular environmental stressors (see Further Reading below). Other research is showing that the idea that the heritability of a given trait can be determined once and for all is mistaken. In reality, heritability for complex behavioural traits–the amount of variance in a population accounted for by genetic factors–can vary dramatically within populations (see sidebar).
Even those conducting animal research, which in many ways is easier to interpret than research on humans, have faced challenges. With knockout mice, for instance, developmental psychologists have been quick to point out that removing a gene from an embryonic stem cell and allowing that cell to grow into a genetically modified mouse is not the same as turning the gene off in an otherwise normal adult.
The missing gene could have widespread effects on how the organism develops. In response, says Wehner, geneticists are now producing mice with conditional or inducible knockouts–genes that are inactive only during certain developmental stages, or that can be turned on or off using drugs or changes in environmental conditions. Even so, progress has been slow. Such knockouts are extremely difficult to make, she notes, and they have limitations of their own.
New techniques may help researchers overcome at least some of those challenges. One particularly promising area, says Flint, is the combination of behavioural genetics with visualization tools in biology. In living animals, including humans, functional MRI and other brain-imaging techniques are providing increasingly high-resolution maps of large-scale neural activity.
Meanwhile, in cells, molecular techniques such as tagging enzymes with green fluorescent protein are allowing researchers to watch changes in gene expression as they occur. Researchers are also hoping to make increasingly direct connections between animal models and clinical research, says Hewitt. Right now, a number of interesting candidates genes have been identified in animals, but links to human behaviour are sparse.
Visualization tools such as those described by Flint may help bridge the gap. These techniques may bring behavioural geneticists one step closer to their ultimate goal: discovering how neurons–shaped by interactions between genes and the environment–give rise to behaviour.
“The marrying of those different technologies will enable some of the most exciting science in the next 10 years,” says Flint. Etienne S. Benson is a science writer in Cambridge, Mass. New research opportunities where genes and behaviour intersect The new Social and Behavioral Research Branch of the National Human Genome Research Institute gives psychologists and other social science and health researchers a new opportunity. April 2004, Vol 35, No. 4 Print version: page 43 1 min read.
The creation of the new Social and Behavioral Research Branch of the National Human Genome Research Institute (NHGRI) gives psychologists and other social science and health researchers a new opportunity to make use of the discoveries of the Human Genome Project.
The new branch will fund research that seeks cutting-edge approaches to translating Human Genome Project information into interventions for health promotion and disease prevention, and for counselling patients and families dealing with the impact of devastating genetic disorders, according to NHGRI spokesman Geoff Spencer.
The branch is headed by behavioural epidemiologist Colleen McBride, PhD, of Duke University, whose past work has emphasized population-based interventions for improving public health. She plans to foster research in four conceptual domains:
Testing communication strategies aimed at best elucidating an individual’s risk for developing a genetic condition. Developing and evaluating interventions aimed at reducing genetically susceptible individuals’ risk of acquiring a disease. Translating genomic discoveries to clinical practice. Understanding the social, ethical and policy implications of genomic research.
–K. KERSTING Behavioral Genetics–A second look at twin studies (apa.org) COVER STORY A second look at twin studies As behavioural genetics enters a second century, the field’s oldest research method remains both relevant and controversial. By LEA WINERMAN Monitor Staff April 2004, Vol 35, No. 4 Print version: page 46 6 min read 3 “Twins have a special claim upon our attention; it is, that their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their afterlives.” — Sir Francis Galton, 19th-century behavioural genetics pioneer, Inquiries into Human Faculty and its Development,1875
More than a century after Galton’s observation, twin studies remain a favourite tool of behavioural geneticists. Researchers have used twin studies to try to disentangle the environmental and genetic backgrounds of a cornucopia of traits, from aggression to intelligence to schizophrenia to alcohol dependence. But despite the popularity of twin studies, some psychologists have long questioned assumptions that underlie them–like the supposition that fraternal and identical twins share equal environments or that people choose mates with traits unlike their own.
The equal environments assumption, for example, has been debated for at least 40 years. Many researchers have found evidence that the assumption is valid, but others remain skeptical (see Further Reading below). Overall, twin studies assumptions remain controversial, says psychologist James Jaccard, PhD, a psychologist who studies statistical methods at the University at Albany of the State University of New York.
In response, though, researchers are working to expand and develop twin study designs and statistical methods. And while the question of the assumption remains a stumbling block for some researchers, many agree twin studies will continue to be an important tool–along with emerging genome and molecular research methods (see article page 42)–in shedding light on human behavioural genetics.
Methods and theory: The classical twin study design relies on studying twins raised in the same family environments. Monozygotic (identical) twins share all of their genes, while dizygotic (fraternal) twins share only about 50 per cent of them.
So, if a researcher compares the similarity between sets of identical twins to the similarity between sets of fraternal twins for a particular trait, then any excess likeness between the identical twins should be due to genes rather than environment. Researchers use this method, and variations on it, to estimate the heritability of traits:
The percentage of variance in a population is due to genes. Modern twin studies also try to quantify the effect of a person’s shared environment (family) and unique environment (the individual events that shape a life) on a trait. The assumptions those studies rest on–questioned by some psychologists, including, in recent work, Jaccard–include:
Random mating. Twin researchers assume that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar in a particular trait. If instead, people tend to choose mates like themselves, then fraternal twins could share more than 50 per cent of their genes–and hence more similarities in genetically influenced traits–because they would receive similar genes from their mothers and fathers.
Equal environments. Twin researchers also assume that fraternal and identical twins raised in the same homes experience equally similar environments. But some research suggests that parents, teachers, peers and others may treat identical twins more similarly than fraternal twins. Gene-environment interaction. Some researchers think that interactions between genes and environment, rather than genes and environment separately, may influence many traits.
A recent study from Science (Vol. 297, No. 5582) by Avshalom Caspi, PhD, of King’s College London, for example, suggests that a gene might moderate the propensity for violence, particularly in people who are severely maltreated as children. Many twin study designs don’t take this type of complication into account. Genetic mechanisms. Traits can be inherited through different genetic mechanisms.
For traits governed by dominant genetic mechanisms, a dominant gene inherited from one parent trumps a recessive gene inherited from the other parent: If a person inherits a recessive gene for blue eyes from one parent and a dominant gene for brown eyes from the other parent, then the dominant brown gene wins, and the person’s eyes are brown. Additive genetic mechanisms, in contrast, mix together–a plant that receives one red gene and one white gene might, if the genes are additive, turn out pink.
Epistatic mechanisms are complex cases where interactions among multiple genes may determine the outcome of one trait. Twin studies, in general, assume that only one type of genetic mechanism–usually additive–is operating for a particular trait. The value of twin studies Twin researchers acknowledge that these and other limitations exist. But, they say, the limitations don’t negate the usefulness of twin studies.
For traits that are substantially influenced by heredity, the approximately two-fold difference in genetic similarity between the two types of twins should outweigh any complications, says John Hewitt, PhD, director of the Institute for Behavioral Genetics at the University of Colorado at Boulder. And the extent to which different assumptions matter may depend on which trait is being studied.
Studies have suggested, for example, that people are more likely to select mates with similar levels of intelligence than they are mates with similar levels of neuroticism, extraversion and other personality traits (see page 50). So, researchers who use twins to study intelligence might have to worry more about nonrandom mating than researchers who study personality.
Twin study designs and statistical analysis methods are also constantly evolving and improving. The original twin study design has expanded to include studies of twins’ extended families, longitudinal studies and other variations. Some of these variations allow researchers to address previous limitations–they can investigate the effects of nonrandom mating, for example, by including the spouses of twins in studies.
In fact, says psychologist Dorret Boomsma, PhD, of Vrije Universiteit in the Netherlands, all of these assumptions can be tested, given the proper data. She argues that they should not be seen as assumptions at all, but instead as mechanisms whose relevance can be tested using study designs that go beyond the classical twin study design. Analysis methods, likewise, don’t remain static.
“People are always thinking about ways to improve the analyses,” Hewitt says. Jaccard acknowledges that this is true. “For some designs, we don’t have to make as strong assumptions as we used to make,” he says. “Instead of having to assume away four constructs, we only have to assume away two or three.” In the age of molecular genetics, meanwhile, the classical twin study design is only one aspect of genetics research.
Twin studies estimate the heritability of a trait, but molecular genetics attempts to pinpoint the effects of a particular gene. The future of twin research will involve combining traditional twin studies with molecular genetics research, according to Hewitt, who believes that day is already here. “When we conduct a study of twins these days, we always get DNA on everyone,” Hewitt says. “And we’ll use that DNA to try and identify specific individual genes that contribute to the overall pattern of heritability.”
Further Reading Boomsma, D., Busjahn, A., & Peltonen, L. (2002). Classical twin studies and beyond. Nature Reviews Genetics, 3(11), 872-882. Kendler, K.S., Neale, M.C., Kessler, R.C., Heath, A.C., & Eaves, L.J. (1993). A test of the equal-environment assumption in twin studies of psychiatric illness. Behaviour Genetics, 23, 21-28. Neale, M.C., & Cardon, L.R. (1992). Methodology for genetic studies of twins and families.
Dordrecht, The Netherlands: Kluwer Academic Press. Pam, A., Kemker, S.S., Ross, C.A., & Golden, R. (1996). The “equal environments assumption” in MZ-DZ twin comparisons: An untenable premise of psychiatric genetics? Acta Geneticae Medicae et Gemellologiae, 45(3), 349-360.
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