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PSYB30-Chapter 6 Notes .docx

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Connie Boudens

Chapter 6:Genetics  Genotypes: The genetic makeup that codes for a specific trait.  Behavioural genetics is the study of the genetic and environmental contributions to individual differences in personality and behaviour. Nature and Nurture as Allies  We call the combination of nature and nurture a genotype-environment interaction.  What if people change environments and environments change people, so that we really can’t separate the impact of genes on environment? This is called a genotype-environment correlation.  In fact, some researchers advocate an even more complicated equation for understanding the manifestation of complex human traits – a concept known as phenotype. Phenotype = Genotype + Environment + Gene-environment correlation + Gene- environment interaction Genes and Environment as Co-actors Heritability  Heritability (h ) is the amount of observed individual differences in some characteristic that can be accounted for by genetic differences. Heritability refers to differences across a group or population of people- not to a specific person.  Heritability therefore, refers to the inheritance of a particular trait in a particular population at a particular time, so sometimes heritability estimates differ depending on the exact sample and methods used by researchers. Environmentality  Environmentality (e ) estimates the extent to which observed individual differences can be traced in any way to individual differences in environments. Shared and Nonshared Environments  One way of thinking about environmental influences on characteristics such as height or personality is to identify aspects of the environment that are shared and unshared among relatives living together in a household.  Shared environments includes aspects of the family environment that are generally the same for all the children in the household including physical, psychological, and social aspects.  In contrast, the nonshared environment includes experiences that relative have which make them different from one another. This may include unique experiences within the family, or outside the family. When it comes to personality most of the environmental influence ends up being of the nonshared variety.  One measure of heritability is to calculate the correlation (r) between twins on a given trait and compare the correlation between MZ twins and DZ twins. The exact formula is to double the difference between these correlations or h = 2(r mz– rdz  A second way of estimating heritability is to compare identical twins who have been raised in separate environments. We call these kinds of twins MZA twins (monozygotic twins raised apart). If such twins score similarly in a trait such as Extraversion, then we know that Extraversion has a strong genetic component. Studies of MZA twins are particularly powerful in disentangling the effects of gens and environment because they have identical genes but different environments. Here is a second formula for estimation heritability: h = r MZA  The double-the-difference method assumes that twins were reared under equal environments. That is, it assumes that people have not treated MZ twins more alike than DZ twins.  This equal environments assumption applies only to similar treatment that is related to the specific characteristics under study. For example, people often dress their twins alike in identical sailor suits or matching dresses. This probably happens more often to MZ twins than to DZ twins. Of we were studying something like fashion sense, then this would violate our equal environment assumption. But unless wearing matching outfits, affects a specific personality characteristic like shyness, then this assumptions still holds.  The double-the-difference formula also assumes that twins are typical of the population. We call this the assumption of representativeness.  Twins may be more alike on a certain characteristic due to selective placement during the adoption process and not their genetics. Selective placement makes it impossible to see the effect of genetics apart from the effect of environment because it confounds the two. Research Methods Illustrated: Correlational Designs I: The Logic of Adoption and Twin Studies  When two variables are related, there are always at least three possible explanations of the findings. First, it’s possible that one variable causes the other, like being around the same people all the time cause people to develop a similar sense of humor. Second, it’s also possible that the second variable causes the first one, like people who have a similar sense of humor spend more time hanging around each other. Finally, it’s also possible that some third variable – like genetics – causes people to hang around each other and to find the same jokes funny. Heritability of Common Personality Characteristics  A solid finding in the research – one that has been well replicated across many samples and for both self-report and other report – is that virtually all individual differences in human behaviour including cognitive abilities, personality, social attitudes, psychological interests, and psychopathology are moderately heritable.  The variance in personality traits typically breaks down like this: Observed differences in personality traits = 40% Genetics + 0% Shared environment + 40% Nonshared environment + 20% Error. Then and Now: The Science of Genetics  People believed that inherited characteristics- of people and plants-were blended. That is, the offspring of a pea plant with wrinkled peas and a plant with smooth peas would have moderately wrinkled peas. But that is not what Mendel found. Instead, he found that the next generation was all smooth. What happened to the genetic information for wrinkly peas? Mendel reasoned that the trait must still be present in the genotype even though it was not expressed in the phenotype. If this was true, then the genotype ought to be passed on to the next generation. Indeed, about 75% of the plants in the next generation were smooth and 25% were wrinkled.  From such observations, Mendel developed two hypotheses, part of what we now consider his first law of inheritance.  First, each parent plant passes on one form of the gene (which he called element) for a given characteristic to its offspring, who get two forms of the gene, one from each parent (note that different forms of the same gene is called alleles). These two alleles can either be the same or different. When the alleles are different, one characteristic will be dominant over the other. However, both alleles will be passed on to the next generation.  This concept of dominance explains the pattern of seeds Mendel observed in successive generations. In fact, such inheritance pattern where one trait dominates over the other is called Mendelian inheritance and Model is now known as the founder of modern genetics.  Epigenetics: The study of how the environment changes the function of genes without changing the genes themselves.  We know that a gene is a sequence of DNA that codes for a specific trait. Genes are composed of coding regions called exons and noncoding regions called introns. Of the 3.3 billion base pairs of DNA in the human genome, only about 2 to 3% are functioning genes. The remainder of the DNA – nearly 2 meters of it – was once thought to do nothing because it occurs outside genes. This so-called junk DNA is actuall
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