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CHAPTER 2 HEREDITARY INFLUENCES ON DEVELOPMENT OUTLINE I. Principles of Hereditary Transmission Determining how one’s genotype, the genetic endowment that an individual inherits, is expressed as a phenotype, or the ways in which a person’s genotype is expressed in observable or measurable characteristics A. B. C. Genetic material 1. Conception is the moment of fertilization, when a sperm penetrates an ovum, creating a zygote, the single cell formed at conception from the union of a sperm and an ovum 2. Chromosome is a threadlike structure made up of genes; humans have 46 chromosomes in the nucleus of each body cell 3. Genes are the hereditary blueprints for development that are transmitted unchanged from generation to generation 4. Deoxyribonucleic acid or DNA is the long, double-stranded molecules that make up chromosomes Growth of the zygote and production of body cells 1. Mitosis is the process in which a cell duplicates its chromosomes and then divides into two genetically identical daughter cells The germ (or sex) cells 1. Production of gametes through meiosis a. Meiosis is the process in which a germ cell divides, producing gametes (sperm or ova) that each contain half of the parent cell’s original complement of chromosomes; in humans, the products of meiosis contain 23 chromosomes. 2. Crossing over is a process in which genetic material is exchanged between pairs of chromosomes during meiosis. Box 2.1 – Focus on research: Crossing over and chromosome segregation during meiosis 3. 4. Hereditary uniqueness a. Independent assortment is the principle that each pair of chromosomes segregates independently from all other chromosome pairs during meiosis Multiple births a. Monozygotic (or identical) twins develop from a single zygote that later divides to form two genetically identical individuals Chapter 2 b. D. E. II. Dizygotic (or fraternal) twins result when a mother releases two ova at roughly the same time and each is fertilized by a different sperm, producing two zygotes that are genetically different Male or female? 1. Autosomes are the 22 pairs of human chromosomes that are identical in males and females 1. X chromosome is the longer of the two sex chromosomes; normal females have two X chromosomes, whereas normal males have only one 2. Y chromosome is the shorter of the two sex chromosomes; normal males have one Y chromosome, whereas females have none What do genes do? 1. At the most basic level, genes call for the production of amino acids 2. Genes also guide cell differentiation 3. Some genes regulate the pace and timing of development 4. Environmental factors influence how genes function in several ways a. At the intracellular level, a molecular gene-environment interaction occurs b. At the extracellular level, a cellular gene-environment interaction occurs c. At the level of the external environment, both experience-expectant (experienced by all humans) and experience-dependent (experienced by some humans) interactions may occur How Are Genes Expressed? A. Single-gene inheritance patterns are a pattern of inheritance in which one allele dominates another so that only its phenotype is expressed 1. Simple dominant-recessive inheritance is a pattern of inheritance in which one allele dominates another so that only its phenotype is expressed a. Allele is an alternative form of a gene that can appear at a particular site on a chromosome b. Dominant allele is a relatively powerful gene that is expressed phenotypically and masks the effect of a less powerful gene c. Recessive allele is a less powerful gene that is not expressed phenotypically when paired with a dominant allele d. Homozygous indicates having inherited two alleles for an attribute that are identical in their effects e. Heterzygous indicates having inherited two alleles for an attribute that have different effects f. Carrier is a heterozygous individual who displays no sign of a recessive allele in his or her own phenotype but can pass this gene to offspring Box 2.2 – Applying research to your life: Examples of dominant and recessive traits in human heredity 2. Codominance 36 Hereditary Influences on Development a. B. III. Codominance is a condition in which two heterozygous but equally powerful alleles produce a phenotype in which both genes are fully and equally expressed b. Sickle-cell anemia is a genetic blood disease that causes red blood cells to assume an unusual sickle shape and to become inefficient at distributing oxygen 3. Sex-linked characteristic is an attribute determined by a recessive gene that appears on the X chromosome; more likely to characterize males Polygenic inheritance 1. Polygenic trait is a characteristic that is influenced by the action of many genes rather than a single pair Hereditary Disorders A. B. C. D. Congenital defects are is present (though not necessarily apparent) at birth; such defects may stem from genetic and prenatal influences, or from complications of the birth process Chromosomal abnormalities 1. Abnormalities of the sex chromosomes 2. Abnormalities of the autosomes b. Down syndrome is a chromosomal abnormality (also known as trisomy21) caused by the presence of an extra 21st chromosome; people with this syndrome have a distinctive physical appearance and are moderately to severely retarded Genetic abnormalities 1. Mutation is a change in the chemical structure or arrangement of one or more genes that has the effect of producing a new phenotype Predicting, detecting, and treating hereditary disorders 1. Predicting hereditary disorders a. Genetic counseling is a service designed to inform prospective parents about genetic diseases and to help them determine the likelihood that they would transmit such disorders to their children i. Fragile X syndrome is an abnormality of the X chromosome caused by a defective gene and associated with mild to severe mental retardation, particularly when the defective gene is passed from mother to child 2. Detecting hereditary disorders a. Amniocentesis is a method of extracting amniotic fluid from a pregnant woman so that fetal body cells within the fluid can be tested for chromosomal abnormalities and other genetic defects b. Chorionic villus sampling (CVS) is an alternative to amniocentesis in which fetal cells are extracted from the chorion for prenatal tests; CVS can be performed earlier in pregnancy than is possible with amniocentesis c. Ultrasound is a method of detecting gross physical abnormalities by scanning the womb with sound waves, thereby producing a visual outline of fetus 37 Chapter 2 3. Treating hereditary disorders a. Phenylketonuria (PKU) is a genetic disease in which the child is unable to metabolize phenylalanine; if left untreated, it soon causes hyperactivity and mental retardation b. Germline gene therapy is a procedure, not yet perfected or approved for use with humans, in which harmful genes would be repaired or replaced with healthy ones, thereby permanently correcting a genetic defect Box 2.3 – Applying research to your life: Ethical issues surrounding treatments for hereditary disorders IV. Hereditary Influences on Behavior A. B. Behavioral genetics is the scientific study of how a genotype interacts with the environment to determine behavioral attributes such as intelligence, personality, and mental health Methods of studying hereditary influences 1. Heritability is the amount of variability in a trait that is attributable to hereditary factors 2. Selective breeding experiment is a method of studying genetic influences by determining whether traits can be bred in animals through selective mating 3. Family studies a. Kinship is the extent to which two individuals have genes in common b. Twin design is a study in which sets of twins that differ in zygosity (kinship) are compared to determine the heritability of an attribute c. Adoption design is a study in which adoptees are compared with their biological relatives and their adoptive relatives to estimate the heritability of an attribute, or attributes 4. Estimating the contribution of genes and environment a. Concordance rates are the percentage of cases in which a particular attribute is present for one member of a twin pair if it is present for the other b. Gene influences i. Heritability coefficient is a numerical estimate, ranging from .00 to +1.00, of the amount of variation in an attribute that is due to hereditary factors c. Nonshared environmental influence (NSE) is an environmental influence that people living together do not share that should make these individuals different from one another d. Shared environmental influence (SE) is an environmental influence that people living together share that should make these individuals similar to one another e. Myths about heritability i. They cannot tell us whether we inherited a trait ii. They apply only to populations, never individuals 38 Hereditary Influences on Development iii. C. D. E. V. They refer only to a particular trait displayed by a particular population under particular environmental circumstances iv. Heritable traits can be modified by environmental influences Hereditary influences on intellectual performance 1. As children mature, genes seems to contribute more to individual differences in IQ 2. Both twin studies and adoption studies support this conclusion Hereditary contributions to personality 1. Core dimensions genetically influenced a. Introversion/extroversion: the opposite poles of a personality dimension, where introverts are shy, anxious around others, and tend to withdraw from social situations, and extroverts are highly sociable and enjoy being with others. b. Empathic concern is a measure of the extent to which an individual recognizes the needs of others and is concerned about their welfare 2. How much genetic influence? Personality traits appear to be moderately heritable 3. Which aspects of environment influence personality? 4. Measuring the effects of nonshared environments 5. Do siblings have different experiences because they have different genes? a. Children raised in the same home generally do not have the same personality b. Even within the same home, environmental differences can occur due to gender (sons treated differently than daughters) or birth order (firstborns treated differently than later born children) Hereditary contributions to behavior disorders and mental illness 1. Schizophrenia is a serious form of mental illness characterized by disturbances in logical thinking, emotional expression, and interpersonal behavior 2. Bipolar disorder is a psychological disorder characterized by extreme fluctuations in mood 3. Neurotic disorder is an irrational pattern of thinking or behavior that a person may use to contend with stress or to avoid anxiety 4. Although there are genetic predispositions for mental illness, it usually takes a number of stressful experiences to trigger it Theories of Heredity and Environment Interactions in Development A. B. C. Canalization is genetic restriction of phenotype to a small number of developmental outcomes; a highly canalized attribute is one for which genes channel development along predetermined pathways, so that the environment has little effect on the phenotype that emerges Range-of-reaction principle is the idea that genotype sets limits on the range of possible phenotypes that a person might display in response to different environments Genotype/environment correlations 1. Passive genotype/environment correlations reflect the notion that the rearing environments biological parents provide are influenced by the parents’ own genes, and hence are correlated with the child’s own genotype 39 Chapter 2 2. D. IV. Ethological and Evolutionary Viewpoints A. B. C. D. VII. Evocative genotype/environment correlations reflect the notion that our heritable attributes affect others’ behavior toward us and thus influence the social environment in which development takes place 3. Active genotype/environment correlations reflect the notion that our genotypes affect the types of environments that we prefer and seek out 4. How do genotype/environment correlations influence development? a. The relative influence of these different types of correlations changes over the lifespan, with active correlations taking on greater influence as the child matures 5. Separated identical twins Contributions and criticisms of the behavioral genetics approach 1. Genes and environment interact to influence development 2. Critics point out this is more a descriptive overview than a well-articulated explanation Assumptions of classical ethology 1. Ethology is the study of the bioevolutionary basis of behavior and development with a focus on the survival of the individual 2. All animal species are born with biologically programmed characteristics that evolved as a result of natural selection (an evolutionary process, proposed by Charles Darwin, stating that individuals with characteristics that promote adaptation to the environment will survive, reproduce, and pass these adaptive characteristics to offspring; those lacking these adaptive characteristics will eventually die out) Ethology and human development 1. Critical period is a limited time span during which developing organisms are biologically prepared to display adaptive patterns of development, provided they receive the appropriate input 2. Sensitive period is period of time that is optimal for the development of particular capacities, or behaviors, and in which the individual is particularly sensitive to environmental influences that would foster these attributes Modern evolutionary theory is the study of the bioevolutionary basis of behavior and development with a focus on survival of the genes Contributions and criticisms of the ethological viewpoint 1. Present view of child as a biological creature 2. Difficult to test, and cultural learning may overshadow innate evolutionary mechanisms Applying Developmental Themes to Hereditary Influences on Development A. B. C. D. The Active Child Nature and Nurture Interactions in Development Qualitative and Quantitative Changes in Development The Holistic Nature of Child Development 40 Hereditary Influences on Development LECTURE 2-1: HEREDITY AND ENVIRONMENT AS CO-DETERMINERS OF DEVELOPMENTAL OUTCOMES This text identifies four basic developmental themes. One was the nature-nurture controversy over whether people are primarily the product of environment or of their heredity and biological predispositions. Today, behavior geneticists emphasize how heredity and environments interact to determine developmental outcomes. Chapter 2 explores the mechanisms of heredity and reviews our current understanding of how heredity and environment work together to determine both physical and behavioral characteristics. This interactionist view is more in line with reality, and students will benefit from seeing examples of that. These examples all illustrate a basic tenet of genetics: a variety of phenotypes are possible for each genotype. How broad a variety will depend on the degree of canalization of the trait (Waddington’s notion) or, put differently, the range of reaction (Gottesman’s notion). According to Gottesman and others, genetics sets the upper and lower limits on the possible variations. All characteristics show variation. Environment does not change genetic makeup, but differentially selects one of several potentials. Examples help students see how wide a range of phenotypes is possible for a given genotype. Example 1: Physical Changes at Adolescence Pubertal changes provide a very striking example of the interaction of heredity and environment. They illustrate the notion that there are a variety of possible outcomes given a particular genetic makeup, while at the same time, some limits are set by heredity. In this example, environment refers to an internal environment, the cell environment. The cell environment changes with pubertal increases in levels of androgens and estrogens. The genes do not change, but there are dramatic changes in physical characteristics. All that has changed is the chemical makeup of the cell environment, yet the difference in outcome is striking if you compare the physical appearance of a 16-year-old girl to pictures of her at age nine. Example 2: Graying at 40 Similarly, when adults begin to get gray hair in their 40s, they have not undergone genetic mutation, but some changes have occurred in the chemical makeup of the cell environment that influences hair color. In both Examples 1 and 2, note that there are some limits set by heredity for what outcomes can occur. The hormones may set in action the physical changes that occur during adolescence, but heredity will have something to say about what the ultimate height of the girl will be, the size of the girl’s breasts, the timing of menarche, etc. The full range of outcomes is not possible for all genotypes. The unique genetic makeup of an individual determines what particular range of outcomes will be possible. Similarly, in the hair color example, chemical changes associated with the aging process set graying into action, but there are limits in the possible outcomes. We do not see too many people with green or purple hair as they age (maybe blue, though—out of a bottle!). We see shades of gray, silver, and white. Heredity plays a role in the commonality we see across individuals in physical and behavioral outcomes and also plays a role in the individuality we see, likewise for environment. Example 3: Himalayan Rabbit 41 Chapter 2 A particularly striking example of the interaction of heredity and environment comes from an experiment with Himalayan rabbits. Temperature has been found to exert considerable influence on the fur color of these rabbits. Himalayans are typically white except for the extremities, nose, and ears. We can turn any part black, however, by shaving the area and applying ice. If the area is kept cold, the fur will grow back black where there once was white fur. We could create a very interesting rabbit (with patterns). The potential for either black pigmentation or no pigmentation (white) is present in every cell all the time. Which potential is released apparently depends on environmental conditions (specifically, temperature). White to black is quite a range of reaction. Note here again that the options are constrained—no red, purple, or green fur has resulted from the application of cold. Example 4: IQ and Gottesman’s Range of Reaction Notion Note: This example can be used with Activity 2-7 and Handout 2-1. Handout 2-1 provides a graphic representation of the hypothetical outcomes for IQ test score when (a) heredity is the sole determining factor (top graph), (b) environment is the sole determining factor (middle graph), and (c) heredity and environment interact as Gottesman suggests (bottom graph). The examples so far have illustrated the interactive role of heredity and environment in determining physical characteristics. Students will enjoy extending this thinking to behaviors as well, particularly looking at Gottesman’s notion as applied to IQ. It may help to begin by reviewing the evidence for heredity and environment separately, and then work toward a discussion of the interaction of these features and the complexity of it. Evidence for Role of Heredity in Determining IQ Kinship studies reveal that the closer the kinship, the higher the correlation (e.g., r = .47 for siblings reared together, but r ranges from .72 to .91 for identical twins) IQs of identical twins reared apart are more strongly correlated than the IQs of fraternal twins reared together. The correlation between the IQs of adopted children and their biological parents has been found to be greater (r = .49) than the IQ correlation between adopted children and their adoptive parents (r = .29). Evidence for Role of Environment IQs of biologically unrelated children who grow up in the same home are more strongly correlated (r = .34) than are the IQ scores of unrelated children living in different homes (r = .01). IQ scores of African-American adoptees reared by middle-class Caucasian parents were 20 points higher than the IQs of comparable children raised by African-American parents. Disadvantaged children show IQ gains after participation in compensatory education programs, but these gains are lost if the stimulation necessary to maintain intellectual development is not continued. 42 Hereditary Influences on Development Genetically normal children reared by feebleminded mothers show dramatic drops in IQ because these mothers are unable to provide the intellectual stimulation needed to promote development past the early elementary level. Children who have extended illnesses that prevent normal experiences promoting intellectual development will often show marked gains in IQ after their health permits normal interaction with a normally stimulating environment. The IQs of identical twins reared in different homes having similar environments are strongly correlated (r = .75 to .91). Identical twins reared in very different environments have been found to differ by as much as 24 points, and the correlations are much lower (as low as r = .26). Gottesman’s Model Both heredity and environment clearly influence IQ test performance. How can their joint contribution best be conceptualized? Gottesman has formulated a useful conceptual model. 1. There is a range of possible IQ outcomes for each genotype. Have students consider four genotypes: Genotype A ranges from 78 to 165 Genotype B ranges from 68 to 125 Genotype C ranges from 40 to 75 Genotype D ranges from 20 to 38 Note: the wide range of reaction (outcomes) that Gottesman includes in his schematic for some genotypes is compatible with the finding that as many as 15 percent of a sample followed from age 2 to 18 years changed over 40 IQ points (McCall et al., 1973). 2. We can say that, for each genotype, there is a “ceiling” on how much IQ test performance can improve under the most favorable conditions and a performance “floor” below which an individual of that genotype is unlikely to fall. 3. Note that the Gottesman model provides a clear way of explaining why we can say very little about a person’s potential (their genotype) knowing only their IQ test scores. If we open up a file and read that an individual’s IQ score is 78, we can conclude nothing about this child’s potential. This child could be a Genotype C in an enriched environment and operating near the top limit of potential, or could be a Genotype A in a restricted environment. Ask students to discuss the implications of educational placement based on IQ scores. 4. Gottesman’s model shows us that if two individuals or groups are found to differ by 20 IQ points, we really cannot conclude anything about genotype or potential unless we can assume these individuals or groups have experienced equally favorable or equally unfavorable environmental stimulation. (Note: Gottesman’s model is useful for explaining why concluding that the difference between the IQ scores of African Americans versus Caucasians reflects genetic differences is unwarranted—we cannot assume equal favorableness of environment for both racial groups.) 43 Chapter 2 LECTURE 2-2: PKU and TAY-SACHS AS ILLUSTRATIONS OF HEREDITY-ENVIRONMENT INTERACTION Note: Use separately or as an extension of Lecture 2-1. Use Handouts 2-2a and 2-2b with this lecture PKU—Phenylketonuria, a Treatable Recessive Genetic Disease The condition PKU provides a powerful example of the practical importance of understanding how heredity and environment interact to determine outcomes. Discovery of this interaction has made the difference between normal and subnormal intelligence for hundreds of children afflicted with the condition. Environmental intervention early in life can “block” the expression of this disorder. How does the recessive gene for PKU exert its influence? PKU is short for phenylketonuria, a condition resulting from a genetically-transmitted inability to produce an enzyme. This enzyme, called phenylalanine hydroxylase, is necessary to metabolize phenylalanine (a substance found in many foods) into a usable substance, tyrosine. Tyrosine is an amino acid, one of the constituents of protein. Tyrosine is important for normal formation of thyroxine (a hormone) and epinephrine (a neurotransmitter). It is also important for the formation of pigment for the skin and hair. In the absence of the necessary enzyme to convert phenylalanine into tyrosine, a toxic build-up of pyruvic acid results, which is damaging to the central nervous system, particularly the developing one. If nothing is done to counter the build-up, the result is severe mental disability (mean IQ is about 40 for children untreated). How is this genetic disorder treated? Environmental intervention can counter the effects of the inherited inability to convert phenylalanine into tyrosine—that intervention is diet therapy. If infants are started on a diet low in phenylalanine during the first weeks of life, these children will not suffer severe damage to the nervous system and the resulting disability, and will have IQs in the normal range. Research suggests that there may be some deficits remaining that are not detected by IQ testing. Welsch, Pennington, Ozonoff, Rouse, and McCabe (1990) report evidence that executive functions carried out in the prefrontal areas were impaired in a diet-controlled group of PKU children. They suggest that even mild elevations in phenylalanine blood levels can result in a lowering of dopamine and other amines essential for normal intellectual functioning. Most foods contain some protein and, hence, phenylalanine. Nutritional therapy requires strict adherence to a very restrictive diet that excludes or restricts intake of milk, meat, and most other normal protein sources. Even normal pastas and breads are too high in protein for these children. To fulfill their nutritional requirements while restricting phenylalanine, a special formula called Lofenalac has been developed for infants. After age three, a preparation called Phenyl-free is given along with natural foods that are low in phenylalanine, such as fruit ices instead of ice cream, low-protein spaghetti with tomato sauce instead of spaghetti with meatballs, and jelly sandwiches with low-protein bread instead of peanut butter and jelly sandwiches. The child must learn at an early age which are the YES foods and which are the NO ones and how to assume responsibility for food management. Diet therapy used to be discontinued when children started school or, more recently, at around age 10, however, some nutritionists now think that individuals with PKU benefit from a phenylalanine-restricted diet throughout their lives, not just during the periods of greatest brain development (Trahms, 1984). What have we learned from PKU about the way heredity and environment work to affect outcomes? 44 Hereditary Influences on Development What is of particular interest in the case of PKU is that, by learning how environment (diet) and heredity (genetic abnormality in enzyme production) interact, it has been possible to help these children. To ensure that children who need help will be identified early, PKU testing is mandatory in most states a few days after birth. PKU provides a good illustration of the broad range of IQ scores (outcomes) that are possible given a particular genetic makeup—from IQs of 20 to 120!—depending on environment (diet, in this case). To use Gottesman’s terminology, that is a wide range of reaction. Addendum: further evidence of the interactive influence of heredity and environment An even more striking example of the wide range of possible reaction is evident when we look at the IQs of the children of PKU mothers. A pregnant PKU woman may be carrying a genetically normal child (PKU is a recessive trait) who we would expect to have normal IQ. However, if the PKU mother’s diet is not carefully controlled during pregnancy, the levels of unconverted phenylalanine can get high enough to damage the developing fetus, resulting in cardiac defects, retarded growth, mental disability, and microcephaly. The possibility exists that a genetically normal baby can be normally intelligent or mentally disabled, depending on the degree of control of the mother’s phenylalanine blood levels. Clearly, neither heredity nor environment is key. They work in an interactive fashion to determine outcomes, and the range of outcomes is quite broad. Resources Trahms, C. (1984). Nutritional care for children with metabolic disorder. In M. Krause & L. K. Mahan (Eds.), Food, Nutrition, and Diet Therapy. Philadelphia: Saunders. Verp, M. S. (1993). Genetic counseling and screening. In C. Lin, M. S. Verp, & R. E. Sabbagha (Eds.), The high-risk fetus: Pathophysiology, diagnosis, management. New York: Macmillan. Welsh, M., Pennington, B., Ozonoff, S., Rouse, B., & McCabe, E. (1990). Neuropsychology of early-treated phenylketonuria: Specific executive function deficits. Child Development, 61, 1697–1713. Tay-Sachs, a Not-Yet-Treatable Recessive Genetic Disease Tay-Sachs is another example of a recessive genetic disorder. But unlike PKU, the disease is fatal: TaySachs children die by three to five years of age. There is no known therapy. Tay-Sachs newborns appear normal, but symptoms emerge by about four to eight months of age: Severe brain damage Enlargement of head Seizures Blindness, deafness; reddish spot on retina Increasing delays in development Increasing unresponsiveness to parents and environment How is Tay-Sachs transmitted? Tay-Sachs is a recessive disorder, which means that both parents must be carriers for a Tay-Sachs child to be born. If both parents are carriers, then their chances are 1 in 4 with each pregnancy that a Tay-Sachs child will be born. See Handout 2-2b for illustration. (Note: same type of diagram can illustrate PKU transmission as well, a similarity that may need to be pointed out to students.) How many people carry this recessive gene? 1 in 150 in the general population 1 in 30 in the Jewish population 45 Chapter 2 Actual occurrence rate? 1 in 300,000 in the general population 1 in 3,600 of Ashkenazic Jews (of Eastern European origins) How does the presence of the recessive gene for Tay-Sachs exert its influence? Tay-Sachs children lack a vital enzyme called HEX A. This enzyme is essential in breaking down a fatty material called ganglioside (GM2) in brain cells. The build-up begins prenatally and reaches a potentially harmful level around six to eight months of age. In normal infants, the HEX A enzyme has begun to break it down by then, preventing any damage to the brain. In Tay-Sachs infants, the GM2 continues to build, eventually causing the symptoms mentioned above. The result is fatal. See Handout 2-2b for an illustration of the normal brain cell and the Tay-Sachs brain cell. Point to Make The mechanism of genetic transmission is through physiological functioning. In this case, the result of abnormal physiology is brain deterioration and the death of the child. This example, like PKU, provides concrete illustration of a way that heredity actually produces its effects on behavior. It also opens the way for a discussion of how knowing the mechanism of influence stimulates and directs research efforts for an intervention that will prevent the build-up of GM2 and the fatal outcome for Tay-Sachs children. Understanding that genetic makeup affects physiology, which in turn affects physical and behavioral outcomes, takes some of the mystery out of the notion of genetically transmitted diseases and/or characteristics. 46 Hereditary Influences on Development LECTURE 2-3: SCARR ON UNDERSTANDING INDIVIDUAL DIFFERENCES Note: lecture coordinates with Handouts 2-3a and 2-3b and Activity 2-4. Sandra Scarr’s 1991 presidential address to the biennial meeting of the Society for Research in Child Development (also in Child Development, 63, 1–19) can provide the basis for an extension of the material in the text on individual differences. Her address evoked strong reactions (see Baumrind, 1993; Jackson, 1993) and illustrated the ongoing controversy about the relative contribution of heredity and genetics and the debate over the emphasis many behavior geneticists place on the genetic contribution to individual differences. The heated reaction occurs because of the apparent threat of a strongly hereditary position on the effectiveness of interventions or attempts to optimize development. A discussion of the question, “What does evidence for a role of heredity imply regarding the potential for interventions to be effective?” is relevant to students’ roles as the policy makers and voters of tomorrow. As was evident in the responses to the publication of The Bell Curve in 1994, this issue is not just academic; how individuals interpret evidence of a genetic contribution can have far-reaching social policy ramifications. Suggestion Start the class with excerpts from Murray and Herrnstein’s controversial book The Bell Curve and examples of reactions in the media (e.g., Newsweek, Oct. 24, 1994, pp. 52–62) as a way of illustrating the implications of the heredity-environment debate for social policy. Most psychologists would argue that there is no reason for a debate—both factors influence outcomes in an interdependent fashion. Nevertheless, any mention of heredity playing a role still triggers the erroneous assumption in the public mind that hereditary influence implies no room for change. Some scientists have fueled that misconception. Scarr has provided some fuel to the debate by making comments to the effect that environmental characteristics (including parenting) cannot cause predictable outcomes in individuals. Scarr’s 1992 Presidential Address and Child Development Article Scarr and McCartney’s theory of genotype/environment interaction is presented in the text and helps to capture some of the ways that genotype works indirectly to influence an individual’s experience. In her address, Scarr further elaborated on the theme of her research of the prior 25 years—the notion that organisms and environments combine to produce human development. Some points that Scarr made in her address: 1. Cross-cultural research clearly demonstrates that the environmental variation within our species affects normal development. The children within each society, however, grow up to be individually different—a reflection of the influence of the child’s genotype on development. Scarr presented a path model illustrating the causal role of both genes and environments on enduring phenotypes. She elaborated on environment, showing how enduring phenotypes vary in actual expression depending on developmental context (home, school, etc.), and on the transitory situation (school party, football game, family dinner, etc.). This is an important notion, not unlike Bronfenbrenner’s conceptualization of various levels of environmental influence—mesosystem, exosystem, and macrosystem. She has incorporated into the model the fact that a particular genotype and rearing condition do not add up to a single phenotype but rather to an array. Expressed phenotype also depends on the characteristics of the environment at the moment. 47 Chapter 2 Suggestion Prepare transparencies from Figures 2, 3, and 4 in the Scarr article (Child Development, 1992, pp. 6–8) to illustrate her theory of how different levels of genetics and environment influence behavioral development and expression. 2. An important contention of Scarr’s theory is that the genetic uniqueness of individuals results in the creation of unique environmental experiences by these individuals. The manner in which genetic makeup influences environment is stated in three propositions: Proposition 1: There are three kinds of genotype/environment effects: passive, evocative, and active (summarized in Chapter 2 and used for Activity 2-4 below). Proposition 2: With development, genotype/environment effects become decreasingly passive and increasingly active. As children become increasingly able to choose their own friends, activities, and interests, self-selected niches (active genotype influence) become a stronger influence than the parent-provided (passive genotype influence) environment. People tend to choose work, home, and recreational settings that match their personal characteristics. For example, a withdrawn or unsociable person is not likely to seek employment as a car salesperson or as a sales representative for a book company. Proposition 3: As a necessary corollary of Proposition 2, Scarr maintained that genetic differences become more potent determiners across development as individuals become increasingly capable of and responsible for making their own environments. For example, children who do not possess characteristics that allow them to thrive in the public school environment have little choice but to stay there until high school. At that point, more options are available, and students begin to sort themselves among them. Some choose advanced placement classes, some take college preparatory programs, some gravitate toward vocational-technical programs, some choose work-study, and others emphasize sports, debate, drama, or band and do as little academic work as possible to maintain eligibility and keep their parents placated. After high school, even more options open up—postal worker, mechanic, truck driver, secretary, gardener, ski instructor, professional athlete, librarian, bartender, and more. The diversity of options allows young adults much more opportunity to actively niche-pick than they had in childhood. This model differs from the more traditional conceptualization that environmental variations are the causes of the individual variations we observe in phenotypes. In the triarchic theory of experience, Scarr argued that variations in environment are best understood as products of parental and child characteristics, not as causes of their unique characteristics. Scarr strongly emphasized that individuals differ in how they encode and experience their environments and that variations in encoding are evident at various points in their development. For example, all the families in a neighborhood may subscribe to the same daily newspaper, all have access to the local public library, and all have a television set. In those ways, their environments are identical. Yet, how those identical environmental features affect each family and each individual will depend on each person’s unique genetic makeup. People who read well and easily are more likely to read the newspaper, spend more time in leisure reading, and make use of the library. What each chooses to read in the newspaper or for pleasure reading will depend on the person’s 48 Hereditary Influences on Development individual characteristics. Not only will the individual who is a capable reader select more and different reading material than someone with different capabilities, but the voracious reader also will evoke different reactions from others. Others who enjoy reading will respond positively, while those who do not may find the voracious reader boring. During childhood, adults will typically respond positively unless a child is reading a novel at an inappropriate time (e.g., during math or music class, during meals or chore time) or is reading Playboy instead of Wind in the Willows or the Tolkien trilogy. 3. Throughout her address and the 1992 article, Scarr emphasized that environmental variations do exist, as suggested by traditional developmental theory. She also agreed with Plomin and Thompson (1987), that considerable environmental variance is contributed by nonshared environmental influences (i.e., those environmental characteristics that are not common for all family members). For example, only the oldest child of two has the experience of being the firstborn, or being age 10 when the parents are ages 33 and 37, or of having a younger sibling. Scarr argued, however, that the unique characteristics of an individual’s environment are due less to circumstantial factors such as those just noted and more to (a) the differences in opportunities actively selected or ignored by the individual, and (b) the differences in responses the individual evokes from others. Scarr maintained that the developmental data simply do not support the view that particular environmental characteristics can cause predictable outcomes in individuals. For example, the correlations between parenting practices and behavior outcomes are not very high. Children adopted into the same family do not turn out to be clones of each other. Not all children reared with a depressed mother show depressed behavior. Not all abused children become aggressive adolescents. The data do not support the view that “common family and larger context have common influences.” Even when children are genetically related (and especially when they are not), Scarr maintained that there is ample evidence that they respond in unique ways to common experiences and may even create unique experiences. 4. Scarr further argued that this lack of one-to-one correspondence between common environment and outcomes should alleviate some of the concern and guilt parents often feel about not being perfect. Parents go into the parenting role assuming that if they do A, B, and C and do not do E, F, and G, that their children will turn out with the characteristics they desire. Scarr maintained that children will turn out in a particular way with a relatively wide range of parenting behaviors simply because of who those children are genetically. Basically, Scarr argued that a wide range of circumstances will foster normal development and that parenting in some specific way will not necessarily mold a child to be something that he or she is not. Children who are not receiving parenting and environmental supports within the range normal for the species (i.e., those who are malnourished, deprived of stimulation, or abused) can benefit from a change in their environments. However, she suggested that parenting that falls within the normal range will be “good enough” parenting to support normal development and the emergence of the child’s genetic potential physically, cognitively, socially, and otherwise. The result may not always be to the parents’ liking; sometimes children take an undesirable trajectory despite parenting that falls within the normal range. Interventions in such cases may have only temporary and limited effects according to Scarr and others (e.g., Clarke & Clarke, 1989). 49 Chapter 2 Responses to Scarr Two articles were published in reaction to Scarr’s address, one by Baumrind (1993) and one by Jackson (1993). Baumrind (1993) takes strong exception to Scarr’s notion that the “average expectable environment” is “good enough.” She argues that the “details of socialization patterns are crucial to an understanding of normal and deviant development,” and “what parents do or fail to do crucially affects their children’s development” (p. 1299). Baumrind supports her view by citing evidence showing that parenting over and above “good enough” parenting results in more positive outcomes for children. Baumrind objects to Scarr’s pessimistic view of interventions or “better than good enough” parenting efforts. In Baumrind’s view, there are simply too many studies showing that quality of rearing environment matters. She argues that Scarr has relied too heavily on heritability analyses to support her views. Heritability analyses have, in Baumrind’s view, overestimated the role of genetic factors (e.g., identical twins reared in different environments are typically not reared in radically different environments, inflating the correlation). Baumrind expresses concern that Scarr’s emphasis on “good enough” parenting as all that is requisite for normal development will lead to a parental and societal abdication of responsibility to provide optimal environments for development—not out of malice but out of a sense of disempowerment. After all, if it doesn’t really matter what I do as a parent, why should I spend the extra money to send my child to a private school, provide a tutor for a struggling young student, or give up my bowling night to go to a school open house? Jackson is an African-American researcher who expresses concern for the negative implications of Scarr’s views on interventions, particularly for African-American children. Jackson questions the adequacy of the evidence Scarr uses to support her theory of genotype/environment effects, arguing, for example, that reliance on biracial adoptive studies may not be appropriate. Scarr’s work has not adequately acknowledged the potentially confounding of adoptive status, of being an African-American or biracial child reared in a Caucasian home, or of being a twin. Neither adoptees nor twins are typical individuals reared in “typical” environments unless one assumes that adoptees are treated no differently than biological children or that having a twin does not affect a child’s experience (a point Baumrind makes as well). Of considerable concern to Jackson (as well as to Baumrind) is the pessimism of Scarr’s theory with respect to the effectiveness of interventions or of high-quality care. Jackson and Baumrind both present several studies that challenge Scarr’s pessimism regarding the effectiveness of intervention. Scarr’s Response to Baumrind and Jackson 50 Hereditary Influences on Development Scarr’s response to her critics comes in the form of an elaboration of the theory presented in her presidential address. In addition, she acknowledges the importance of instruction for specific, nonuniversal skills, citing the differing levels of mathematical knowledge across cultures as a function of different cultural practices. Scarr also acknowledges that sometimes interventions have effects on development that are quite large. She emphasizes that the behavior genetics view does not mean that behavior is intractable. To say that much of human development is programmed to unfold under a wide variety of “good enough” environments does not preclude the possibility of raising the mean. Scarr concludes her response by cautioning psychologists not to be blind to the role of heredity in the name of humanitarianism. Resources Baumrind, D. (1993). The average expectable environment is not good enough: A response to Scarr. Child Development, 64, 1299–1317. Clarke, A. M., & Clarke, A. D. B. (1989). The later cognitive effects of early intervention. Intelligence, 13, 289–297. Jackson, J. F. (1993). Human behavioral genetics, Scarr's theory, and her views on interventions: A critical review and commentary on their implications for African American children. Child Development, 64, 1318–1333. Plomin, R., & Daniels, D. (1987). Why are children in the same family so different from one another? Behavioral and Brain Sciences, 10, 1–60. Plomin, R., & Thompson, R. (1987). Life-span developmental behavior genetics. In P. B. Baltes, D. L. Featherman, & R. M. Lerner (Eds.), Life-span development and behavior (Vol. 8, pp. 111–123). Hillsdale, NJ: Erlbaum. Scarr, S. (1992). Developmental theories for the 1990s: Development and individual differences (Presidential Address). Child Development, 63, 1–19. Scarr, S. (1993). Biological and cultural diversity: The legacy of Darwin for development. Child Development, 63, 1–19. 51 Chapter 2 LECTURE 2-4: THE IMPACT OF THE SAME ENVIRONMENT IS NOT THE SAME FOR ALL INDIVIDUALS Use with Handout 2-4. A particularly powerful example of the point that genetic factors set limits on the way environment affects behavior is found in a study by King and Eleftheriou (1959) with deer mice. (It is necessary to use animals rather than people for this demonstration to achieve experimental control over environmental factors.) In this study, King and Eleftheriou compared the effect of an experience, systematic handling, on two subspecies of deer mice (bairdii and gracilis). Half of the gracilis and half of the bairdii deer mice experienced 10 minutes of daily handling from ages three to 25 days old (in an automatic handling machine to control for experimenter bias or the possibility of the handler responding differentially to subspecies differences in reactions to the handling); the other half of each subspecies remained in their home cages and were not exposed to the handling. Experimental Environment (handled) Genotype 1 gracilis (Gs) Control Environment (nonhandled) Genotype 1 gracilis (Gs) Genotype 2 bairdii (Bs) Genotype 2 bairdii (Bs) What might we expect handling to do? For most young animals, handling is frightening initially. Frightened animals tend either to freeze in place or become frantic and urinate and defecate. Deer mice (like pet kittens, puppies, hamsters, rabbits, etc.) become less afraid and more docile as they become habituated to the handling. This habituation effect typically transfers to other contexts. Knowing this, the experimenters looked at how the Gs and Bs reacted in an open-field situation, a situation that is moderately stressful for most animals. When assessed at 79 days of age, these questions were of interest: Would the two subspecies differ in their reactions to that environment? That is, would the control Bs and Gs differ in how many squares of the gridded floor that they crossed? Results: Control Bs were much more active than the control Gs in the open field, indicating a “baseline” subspecies difference in boldness. Control Bs also learned more quickly to avoid shocks than did Control Gs. Clearly, genetic makeup affected phenotype. Would one subspecies be more active in the open field than the other after experiencing the handling? Would the subspecies differ in how quickly it learned to avoid the shock? Results: On the open-field test, the handled Bs increased their activity level considerably compared to control Bs, while handled Gs crossed even fewer squares than control Gs. On the shock avoidance learning tasks, handled Gs performed better than control Gs in learning to avoid shocks. In contrast, handled Bs performed less well than control Bs. Again, the effects were in opposite directions. 52 Hereditary Influences on Development The early handling experience did not just have an effect that differed in magnitude for the two subspecies—it actually had an opposite effect! A treatment that increased activity in one subspecies decreased it in the other; a treatment that facilitated learning for one subspecies interfered with learning for the other. (Sound familiar, parents? Does what worked well with one child sometimes fail miserably with the other, or make things worse?) The results of the King and Eleftheriou study are a clear demonstration that the response to a given experience may differ with genotype; i.e., not everyone responds the same way to the same treatment because genetic factors set limits on the way environment can affect behavior. Resource King, J. A. & Eleftheriou, B. E. (1959). Effects of early handling upon behavior in two subspecies of deer mice, peromyscus maniculatus. Journal of Comparative and Physiological Psychology, 52, 82–88. 53 Chapter 2 LECTURE 2-5: EXPERIENTIAL CANALIZATION To balance off the emphasis on genetic factors of the other lecture suggestions, consider discussing Gottlieb’s (1991) Developmental Psychology paper. Like Scarr, Gottesman, Plomin, and others, Gottlieb argues that outcomes are not the result of purely genetic or maturational forces. He emphasizes the role of environment to a greater extent. In fact, he argues that canalization may reflect the influence of environment as much as heredity. Some characteristics (e.g., intelligence) may show stronger canalization during infancy and the preschool years because experience is more common across children during that period than later. It is possible that early intelligence is much more open to variation than we typically observe. Resource Gottlieb, G. (1991). Experiential canalization of behavioral development: Theory and commentary. Developmental Psychology, 27, 4–13. 54 Hereditary Influences on Development LECTURE 2-6: THE DNA REVOLUTION The 50th anniversary of the discovery of DNA was in 2003, and there were a number of very good celebrations. The following resources can assist you in preparing a lecture on various aspects of the discovery. Nature provides a website, containing news, features and web specials celebrating the historical, scientific, and cultural impacts of the discovery of the double helix. http://www.nature.com/nature/dna50/ Here is the original paper, from James Watson and Francis Crick, published in Nature on April 25, 1953 http://www.nature.com/nature/dna50/watsoncrick.pdf It was Rosalind Franklin’s photograph of the double helix that was responsible for the discovery of the structure of DNA. Franklin has often been overlooked; she didn’t share the Nobel Prize for the discovery because she died of ovarian cancer at age 37. You can hear an interview, from NPR, with author Brenda Maddox about the role of Rosalind Franklin.http://www.npr.org/programs/atc/features/2002/oct/darklady/index.html National Public Radio News produced a special series of short programs on The DNA Revolution. http://www.npr.org/news/specials/dnaanniversary/index.html 55 Chapter 2 STUDENT ACTIVITIES ACTIVITY 2-1 DETERMINANTS OF SIMILARITIES AND DIFFERENCES This is a good activity to assign before a lecture on the interaction of heredity and environment. Ask students to speculate on what factors might have contributed to making them the way they are in terms of such characteristics as sociability, aggression, personality characteristics, temperament, and intelligence. List them on the board or on an overhead projector and discuss to what extent they reflect heredity, environment, or an interaction of the two. Activity 2-1 can be used as a lead-in to make the following points: Point A: The assumptions we make about what factors lay behind our characteristics are not always supported by scientific evidence. For example, some students may have listed parental and experiential factors as lying behind your sociability, your lack of aggression, or your personality characteristics. Others may have attributed your level of intelligence to genetic factors. The evidence presented in the text, and that will be presented in class, indicates that the factors you identified do play a role, but the evidence strongly suggests that all human characteristics are determined by the combined influence of heredity and environment. It is not consistent with current evidence to attribute most human characteristics to either one or the other. (Evidence could be described at this point or students could be referred to the text.) (Option: insert Lecture 2-1 here. The question could be raised at this point: How can the interaction of heredity and environment be best conceptualized? The notions of canalization and range of reaction could be presented or students could be referred to the text.) Point B: Heredity has been shown to play an important role in all of the characteristics mentioned as part of Activity 2-1 (sociability, aggression, personality, temperament, and intelligence). You might well ask: Just how has a role been determined? How is it possible to determine to what extent a characteristic is heritable? (Methods for estimating hereditary influences can be reviewed at this point, or students can be referred to the text.) Point C: Twin studies have shown that identical twins reared apart are more similar than fraternal twins reared together on many characteristics. How can the remarkable similarities between identical twins reared in different environments be explained solely based on shared genes? (Discuss Scarr and McCartney’s notions of evocative and active genotype/environment correlations.) ACTIVITY 2-2 CREATE A ZYGOTE Assign to be due on the day you will go over this material in class or in place of lecture material on this topic. This activity asks students to answer questions about cell division and parental genetic contributions and then draw a magnified picture of the chromosome pairs for a single-cell zygote. They are asked to use different colors to indicate the mother’s and father’s contributions and to identify the sex chromosomes with an arrow and label. The purpose of the activity is to actively engage students in processing some of the material at the beginning of Chapter 2 and to stimulate questions about the mechanisms of genetic transmission and about how the gene exerts its influence. 56 Hereditary Influences on Development ACTIVITY 2-3 UNDERSTANDING GENETIC TRANSMISSION This activity relates to the text material on transmission of characteristics through single-gene-pair inheritance. Present two problems to help students understand the possible outcomes that can occur from various parental genotypes. Problem 1: ask how a couple who is intellectually normal could have a child with PKU and potential cognitive deficits (to illustrate the possible outcomes of two heterozygous parents). Problem 2: ask whether the offspring of a woman who is a carrier for cystic fibrosis and normal male are at risk for cystic fibrosis (to illustrate the possible outcomes of one heterozygous and one homozygous parent, neither of whom displays the trait). ACTIVITY 2-4 SCARR AND MCCARTNEY’S PASSIVE, EVOCATIVE, AND ACTIVE GENOTYPE/ENVIRONMENT CORRELATIONS Coordinates with Lecture 2-3 and Handouts 2-3a and 2-3b. This activity can be used as a cooperative group activity or an individual take-home activity. Either before or after discussing Scarr and McCartney’s theory of genotype/environment interactions, ask students to generate examples from their own lives of each type of gene influence discussed by Scarr: passive, evocative, and active. Then have them discuss whether the example they have given is consistent with the developmental trends in the influence of each of the three types of influence shown in Figure 2.16, or ask them to discuss why one would expect the developmental trends in Figure 2.16. Use Handouts 2-3a and 23b for brief definitions of passive, evocative, and active influences. Consider projecting Figure 2.16 using a document camera or through PowerPoint. Display it during the pair/small-group activity or after to serve as a focus for the discussion. Scarr, S. & McCartney, K. (1983). How people make their own environments: A theory of genotype/environment effects. Child Development, 5, 424–435. ACTIVITY 2-5 USING THE INTERNET: PATTERNS OF GENETIC TRANSMISSION Ask students to use the Internet to learn more about the principles of genetic transmission. This activity provides students with problem sets and tutorials that expand their knowledge of monohybrid crosses and sex-linked inheritance. Then have them apply those to a genetically transmitted behavioral or physical characteristic of their choosing (e.g., cystic fibrosis, muscular dystrophy, phenylketonuria, Tay-Sachs, hemophilia, diabetes, Graves disease, schizophrenia, depression, Alzheimer’s disease, Huntington’s chorea, etc.), or a chromosomal disorder (e.g., Down syndrome, Fragile X syndrome, supermale syndrome) and find out the following: 1. How is the disorder transmitted, how severe is it, and what is the likelihood of occurrence in relatives? 2. Can the abnormality be detected prenatally? 3. Is there any known treatment for the disorder, and, if so, what does it involve? Students may want learn more about a disorder that has occurred in their own family. When discussing this topic, some points to make include the following. 57 Chapter 2 Point A: Not all offspring of a parent who displays or carries a genetic disorder will display or carry the disorder. Point B: Not all genetically predisposed individuals will display a characteristic (ever or in all contexts). For example, one may have an identical twin with schizophrenia or diabetes but may not display the same disorder him/herself. For many disorders, the concordance rate in identical twins has actually been found to be quite low. How can this be explained? Gottesman and Bertelsen (1989) and Scarr (1992) argue that a genotype may be unexpressed under some environmental conditions and overexpressed in others. Some environmental conditions, particularly stressful ones, are more likely to trigger the expression of genotypes for abnormal behavioral and physical disorders. Scarr argues that, if the environment is outside the speciesnormal boundaries, expression of the genotype for a disorder is more likely to occur. Abnormal behavior or a physical disorder may continue because an important component of their environment (often the parents’ unhealthy responses) remains consistent, eliciting abnormal behavior. Note: in his book Leaving Home (1980), one of Haley’s main theses was that, to help an addicted, “mad,” or apathetic youth begin to engage in productive behavior that will eventually allow for his/her leaving home and leading an independent life, parents must demand normal behavior. He argued that often the parents’ marital problems “hook” a child into behaving abnormally as a way of holding the marriage and family together. Haley described therapy strategies designed to help parents work together as a unit to provide the environmental catalyst for normal and increasingly independent behavior. Haley’s therapy orientation is very compatible with the theoretical position that Scarr takes in her presidential address and the 1992 Child Development article—variations in phenotype may be expressed in different contexts by the same individual. This orientation is a particularly appealing one because it does not “condemn” an individual who carries the genotype for a disorder or who has been symptomatic at one point of his/her life to always being “abnormal.” If abnormal or stressful environmental situations become normalized (e.g., by the parents reconciling or making the decision to live separate lives), so should the youth’s behavior and health. The perspective on genotype/environment interaction espoused by Gottesman and Bertelsen and by Scarr helps our understanding of why there is so much individual variability in when and whether individuals display abnormal behavior or physical symptoms. It is typically when stressors reach a sufficient level that the abnormal pattern is released/expressed. For example, some individuals with a genetic predisposition for Graves disease (a hyperthyroid condition) may display symptoms in the late elementary years, during adolescence, or, more commonly, between 20 and 40 years of age. Others might not display the symptoms until much later in life. For each, the emergence of symptoms tends to follow a period of exposure to abnormally high environmental stress (e.g., family conflict, unusually high job stress, an unsupportive teacher at school, etc.). Resources Gottesman, I. I. & Bertelsen, A. (1989). Confirming unexpressed genotypes for schizophrenia. Archives of General Psychiatry, 46, 867–872. Haley, J. (1980). Leaving home: The therapy of disturbed young people. New York: McGraw-Hill. Scarr, S. (1992). Developmental theories for the 1990s: Development and individual differences. Child Development, 63, 1–19. ACTIVITY 2-7 APPLYING GOTTESMAN’S RANGE OF REACTION NOTION 58 Hereditary Influences on Development As a lead-in or conclusion to a presentation of Gottesman’s range of reaction principle (e.g., Lecture 2-1), ask students to turn to Figure 2.15 in the text. This figure presents the hypothetical reaction ranges for the intellectual performances of three children. Ask students to answer the following questions. Question 1: Suppose that you are a teacher and have been given IQ information on a new student, Jamie. You have been told that his IQ score is 100. Assume that you have no additional information about Jamie at the moment, that is, you know nothing about the home environment, past educational experience, language spoken in the home, etc. What can be concluded about the child’s genotype/genetic potential with respect to IQ test performance? Which student in Handout 2-1 is Jamie most similar to genetically? (Note: it is not possible to determine whether Jamie is more similar to Juan or Tony in Handout 2-1 knowing only the IQ score. Have students try to articulate scenarios under which Jamie would resemble each of the three children.) Question 2: Now suppose that you have been told that Erica has an IQ of 75 and Ben has an IQ of 100. Can we confidently conclude that Erica is genetically inferior to Ben with respect to intellectual potential, knowing only the IQ scores? Explain your answer using the Gottesman model. (Note: students should conclude that they cannot draw the conclusion that Erica has lower genetic potential in the absence of information about the favorableness of the environment these children have been reared in. If Erica has been reared in a very restricted environment and Ben in a highly enriched one, it is within the realm of possibility that Erica actually has the greater potential.) ACTIVITY 2-8 ETHICS OF EMBRYO DESIGN http://ublib.buffalo.edu/libraries/projects/cases/genetics/genetic_selection.html A case study from the collection through the National Center for Case Study Teaching in Science, this dilemma case, originally developed by Julia Omarzu for a course in developmental psychology, is based on the true story of Jack and Lisa Nash, whose daughter Molly was born with a rare genetic disorder, Fanconi anemia. By having another child with specific genetic markers, the Nashes hoped to cure Molly using stem cells from the new baby’s umbilical cord blood. Students learn about developmental disorders and consider the ethical issues of genetic manipulation and fertility treatments. Questions for the case include both factual information and critical thinking about ethics. The questions can be modified to fit the needs of the individual class. 59 Chapter 2 WEB RESOURCES How Cells Divide: Mitosis versus Meiosis: http://www.pbs.org/wgbh/nova/baby/divi_text.html This companion site to the Nova program, 18 Ways to Have a Baby, provides a flash presentation on the processes of mitosis and meiosis. Human Genome Project: http://www.ornl.gov/sci/techresources/Human_Genome/education/education.shtml The Education page of the official site for the Human Genome Project offers images, webcasts, and presentations suitable for use in the classroom. Journey into DNA: http://www.pbs.org/wgbh/nova/genome/dna.html# This companion site to the Nova program, Journey into DNA, allows the visitor to examine DNA on 15 different levels. Learn.Genetics: http://learn.genetics.utah.edu/content/begin/traits/ This website from the University of Utah’s Genetic Science Learning Center has a wealth of information and teaching resources and interactive learning objects on topics related to genetics. American Association for Klinefelter Syndrome Information and Support: http://www.aaksis.org National Fragile X Foundation: http://www.fragilex.org/html/home.shtml Trip-lo X Syndrome: http://www.triplo-x.org Turner Syndrome Society of the U.S.: http://www.turnersyndrome.org/ 60 Hereditary Influences on Development VIDEO SUGGESTIONS Birth Defects: Causes and Prevention (Films for the Humanities and Sciences, 10 min., 1990) Describes both genetic and environmental causes of birth defects. Brave New Babies (Penn State Audio-Visual Services, 48 min., 1982) An introduction to genetic engineering. Different Strokes: Nurture and Human Diversity (Insight Media, 30 min. 2006) Discusses the ways in which nature and nurture affect an individual’s growth, ranging from prenatal environment to peer groups to cultural factors. Frontiers of Science: Genetic Testing—Clinical and Ethical Issues (Penn State Audio-Visual Services, 101 min., 2001) Explores the role of genetic counseling in understanding the impact of DNA technology on patients. Heredity and Environment: Beginnings of a Baby (Insight Media, 29 min., 2011) Describes basic mechanisms of heredity and discusses the joint role of heredity and environment in determining human characteristics and development. Phenylketonuria (Films for the Humanities and Sciences, 29 min., 2000) Explores diagnostic methods, including the Guthrie test. The biochemistry and metabolic pathways are traced, and interviews are provided with both dietary experts and patients about the complications of dealing with PKU in pregnancy. Prenatal Testing: A Mixed Blessing (Films for the Humanities and Sciences, 51 min., 1995) Follows four couples through the process of an initial screening indicating some risk for Down syndrome, the decision to have an amniocentesis or not, and the final decisions they made. The Human Genome: Traits, Disorders, and Treatments (Insight Media, 30 min. 2005) Examines genetic disorders such as sickle-cell anemia, Huntington’s disease, and hemophilia. Explores polygenic inheritance and environmental influences on gene expression. To Build Our Future (Penn State Audio-Visual Services, 60 min., 1982) A film on genetic counseling. 61 Chapter 2 HANDOUTS Handout 2-1: Understanding Gottesman’s Range of Reaction Notion Handout 2-2a and 2-2b: Understanding PKU and Understanding Tay-Sachs Handout 2-3a and 2-3b: Scarr’s Triarchic Theory of Individual Differences and Three Ways Genotype Influences Environment Handout 2-4: King and Eleftheriou Deer Mice Study: Different Responses to the Same Experience 62 Handout 2-1: UNDERSTANDING GOTTESMAN'S RANGE OF REACTION NOTION Text Figure 2.15 Handout 2-2a: UNDERSTANDING PKU NORMAL PHYSIOLOGY normal diet containing phenylalanine functioning + enzyme phenylalanine hydroxylase essential amino acid tyrosine normal brain ABNORMAL PHYSIOLOGY normal diet containing phenylalanine + (lack enzyme) pyruvic acid + (lack enzyme) no pyruvic acid buildup mental disability THERAPY diet low in phenylalanine no mental disability Handout 2-2b: UNDERSTANDING TAY-SACHS Q: What is the likelihood that offspring of parents who are both carriers of the Tay-Sachs gene will be born with Tay-Sachs? A: Chances are one in four for each pregnancy. Father Normal Tay-Sachs Normal Mother Tay-Sachs Q: What is the mechanism of influence of the recessive gene for Tay-Sachs? A: Lack of enzyme (HEX A) buildup of fat (ganglioside/GM2) in cells Handout 2-3a: SCARR’S TRIARCHIC THEORY OF INDIVIDUAL DIFFERENCES The genetic uniqueness of individuals results in the creation of unique environmental experiences for and by these individuals. 1. Genotype influences environment in three ways: Passive gene influences Evocative gene influences Active gene influences 2. With development, genotype/environment influences become decreasingly passive and increasingly active. 3. Genetic differences become more potent determiners across development as individuals become increasingly capable of and responsible for making their own environments. In the Scarr model, variations in environment are believed to be best understood as products of parental and child characteristics, rather than as causes of their unique characteristics. Based on Scarr, S. (1992). Child Development, 63, 1–19. Handout 2-3b: THREE WAYS GENOTYPE INFLUENCES ENVIRONMENT 1. Passive Genotype/Environment Correlations: Athletic parents provide athletic opportunities (environmental influence) and contribute athletic genes to child (genetic influence). Examples: 2. Evocative Genotype/Environment Correlations: Attractive, happy children receive more favorable reactions than less attractive or moodier children. Examples: 3. Active Genotype/Environment Correlations: Individuals select activities that are compatible with their genetic predispositions Examples: Handout 2-4: KING AND ELEFETHERIOU DEER MICE STUDY (DIFFERENT RESPONSES TO THE SAME EXPERIENCE) Experimental Environment (daily handling days 3–25) Genotype 1: Gracilis (Gs) Genotype 2: Bairdii (Bs) Control Environment (no handling) Genotype 1: Gracilis (Gs) Genotype 2: Bairdii (Bs) Question 1: Would the two subspecies differ in their reactions when assessed in an open-field situation? In a learning situation? Results: Control Bs were much more active and learned faster than the control Gs, indicating a “baseline” subspecies difference. Control Gs crossed fewer squares on the open-field task than Control Bs Control Gs learned at a considerably slower rate than Control Bs Question 2: Would handling differentially affect the two subspecies’ reactions in an open-field situation? On a learning task? Results: Effects were in opposite directions for the two subspecies. Handled Bs crossed more squares and learned at a slower rate than Control Bs Handled Gs crossed fewer squares and learned at a faster rate than Control Gs Conclusion: The response to a particular experience may differ with genotype (i.e., not everyone responds the same way to the same treatment or practices). WHY? Because genetic makeup sets limits on the way/extent to which environment affects behavior.