Chapter 3: Biology and Behavior Psych 2400

Heritability

a statistical estimate of the proportion of the measured variance on a trait among individuals in a given population that is attributable to genetic differences among those individuals -A crucial point to understand about heritability estimates is that they tell us nothing about the relative contributions of genetic and environmental factors to the development of an individual. Instead, they estimate how much of the variation among a given population of people is due to differences in their genes. -The implausibility of "liberal" or "evangelical" genes brings us back to a point we made earlier: despite the common use of the phrase, there are no genes "for" particular behavior patterns. As we have stressed, genes do nothing more than code for proteins, so they affect behavior only insofar as those proteins affect the sensory, neural, and other physiological processes involved in behavior. In addition, genome-wide association studies (GWAS), which are used in attempts to link specific DNA segments with particular traits, have revealed that genetic effects are cumulative. On their own, individual regions of chromosomes do not correlate with traits; it takes a combination of many genes, each with a small effect, to render a heritable trait (Plomin & Deary, 2015). -Part of the criticism stems from ways that the term "heritability" is often misinterpreted or misused by the public. One very common misuse involves the application of the concept of heritability to individuals, despite the fact that, as we have emphasized, heritability applies only to populations. In addition, a heritability estimate applies only to a particular population living in a particular environment. Consider the case of height. Research conducted almost exclusively with North Americans and Europeans—most of them White and adequately nourished—puts the heritability of height at around 90%. -A related, frequently misunderstood point is that high heritability does not imply immutability. The fact that a trait is highly heritable does not mean that there is little point in trying to improve the course of development related to that trait. -Finally, recall the equal environments assumption: the claim, underlying many behavior genetics designs, that MZ twins share environments that are equally similar as those shared by DZ twins. If this assumption is not correct—that is, if MZ twins are both genetically more similar and have more similar environments than DZ twins—the confounding of similarity of genes and environments would bias heritability estimates. Traits would appear to be more heritable than they actually are. -genome-wide complex trait analysis (GCTA) takes advantage of actual genetic resemblance across large groups of individuals, rather than estimates of genetic resemblance based on family relationships (Rietveld et al., 2013). By measuring actual genetic similarity, it is possible to tease apart aspects of genes and environment that are confounded within families (e.g., Plomin, 2014). -GCTA has also allowed researchers to determine whether the same genes are implicated in measures of a particular trait across development.

Homosexuality: Nature or Nurture?

a strong childhood predictor of homosexuality later in life? - choosing toys that are typical of the opposite sex -The more older brothers that a male has the greater the chance is that he will be gay. -Why are twins important to researchers when exploring factors that contribute to sexual orientation? - Because twins that show different sexual orientations share many, if not all, of the same genetics and presumably share the same prenatal and environmental influences. These twins contradict much of the research that points to a single factor contributing to sexual orientation.

Glial Cells

cells in the brain that provide a variety of critical supportive functions - formation of myelin sheath

Neurons

cells that are specialized for sending and receiving messages between the brain and all parts of the body, as well as within the brain itself -Sensory neurons transmit information from sensory receptors that detect stimuli in the external environment or within the body itself. Motor neurons transmit information from the brain to muscles and glands; and interneurons act as intermediaries between sensory and motor neurons.

event-related potential (ERP)

changes in the brain's electrical activity that occur in response to the presentation of a particular stimulus

Spine

formations on the dendrites of neurons that increase the dendrites' capacity to form connections with other neurons

Newborn screening

tests used to screen newborn infants for a range of genetic and non-genetic disorders

Francis Galton

very close relatives of an eminent man were more likely to be high achievers themselves than were less close relatives.

Genetics and Early Prenatal Development

Each human person begins as a single cell, the zygote. This single cell is formed from the fusion of two cells: • A sperm − the male reproductive cell, which is produced in one or two testicles; • An ovum − the female reproductive cell, vastly larger than the sperm, which is produced in one of two ovaries. The amazing result of this fusion is a tiny entity that could fit through the eye of a needle. In spite of its size, the zygote contains a detailed blueprint for the growth of the full human being. In this activity, we will take a closer look at the earliest biological mechanisms that make human development possible. A zygote contains 46 chromosomes, 23 from the father and 23 from the mother. Imagine these 46 chromosomes as separate volumes of an instruction manual called How to Build a Human Body. This set of 46 volumes is arranged in 23 pairs and contains all the necessary instructions for making the future baby similar to, yet different from, all the rest of us. Each chromosome consists of a long double-stranded molecule of deoxyribonucleic acid (DNA). These DNA molecules contain thousands of genes, short segments of DNA that are arranged like beads on a string and are the basic units of heredity. To continue with our instruction manual analogy, you can think of these separate genes as pages in each of the 46 volumes. -You should explain that the genes are composed of the chemical DNA. Thousands of genes reside on each chromosome. The male parent and the female parent each contribute 23 chromosomes yielding 23 pairs total. The 23 pairs of chromosomes are present inside of the zygote. -The zygote begins by duplicating each chromosome's DNA. With enough chromosomes to form two identical sets, the zygote separates and splits down the middle to form two identical cells. In a similar way but at different times, these "daughter" cells divide over and over again. The zygote and every new cell of the body contain the developing individual's unique set of 23 pairs of chromosomes. Therefore, a complete set of building instructions, carried on coiled strands of DNA, are replicated every time the cell divides, so each new cell always has a copy of the genetic code. -Your response should be 46 volumes, or 23 pairs of volumes. Each new daughter cell will have the same, complete set of instructions as did the parent cell has -By about the twelfth day of pregnancy, the ball of cells has embedded itself in the mother's uterine wall, and the cells now can acquire nutrients from her bloodstream. At this point, the new human being's DNA instructs each cell to grow by building proteins out of amino acids, a process called protein synthesis. The proteins will function both as the "bricks and mortar" and as the chemical "messengers" of the body. Depending on the kinds of proteins grown inside these miniature chemical factories, each cell multiplies and gives rise to a specific type of tissue, such as brain tissue, heart tissue, skin tissue, and so on. As these tissues grow, a human body begins to take shape and the developing organs begin to function. So, in order to build a body, the DNA must accomplish two tasks: • to replicate every time the cell divides and • to synthesize proteins as needed for growth and repair of cells, tissues, and organs. -The zygote and every new cell of the body contain the individual's unique set of 23 pairs of chromosomes, which function as the complete instructions for building and maintaining the body. • These instructions are carried on coiled strands of DNA and are replicated every time that the cell divides. Therefore, each new cell contains a copy of the individual's genetic blueprint. • DNA and the cell's chemical factory allow each cell to grow, multiply, and give rise to various tissues and organs as the human body develops. It is amazing to think that the entire process of DNA replication and protein synthesis is going on in every cell throughout the body for the individual's entire life. -Each cell of the body contains 23 pairs of chromosomes.

Identical twin video

- raised from infancy in two different households -identical twos who are separated at birth make life choices that seem to be made by genetically prescribed predispositions -Thrill-seeking - most prominent personality characteristic

1. Parent's Genotype - Child's Genotype

-Relation 1 involves the transmission of genetic material—chromosomes and genes—from parent to offspring. -Thus, every individual has two copies of each gene, one on the chromosome inherited from the father and one on the chromosome from the mother. Your biological children will each receive half of your genes, and your biological grandchildren will have one-quarter ( just as you have half your genes in common with each of your biological parents and one-fourth with each biological grandparent).

3. Child's Environment - Child's Phenotype

-the impact of the environment on the child's phenotype. (Remember, the environment includes everything not in the genetic material itself, including the array of prenatal experiences) -a highly salient and important part of a child's environment is the parents' relationship with the child—the manner in which they interact with him or her; the general home environment they provide; the experiences they arrange for the child; the encouragement they offer for particular behaviors, attitudes, and activities; and so on. Less obvious is the idea that the environment parents provide for their children is due in part to the parents' own genetic makeup. These types of gene-environment correlations are observed frequently in the study of child development. Parents' behavior toward their children (e.g., how warm or reserved they are, how patient or short-fused) is genetically influenced, as are the kinds of preferences, activities, and resources to which they expose their children.

Brain damage and recovery

As noted previously, because of its plasticity (especially early in life), the brain can become rewired—at least to some degree—after suffering damage. Children who suffer from brain damage thus have a better chance of recovering lost function than do adults who suffer similar damage. By contrast, adults who sustain the same type of brain damage undergo no such reorganization of language functions and may have a permanent loss in the ability to comprehend or produce speech. -It is not always true, however, that the chance of recovery from early brain injury is greater than it is for later injury. Likelihood of recovery depends on how extensive the damage is and what aspect of brain development is occurring at the time of the damage -Furthermore, even when children appear to have made a full recovery from an early brain injury, deficits may emerge later. -we can generalize that the worst time to suffer brain damage is very early, during prenatal development and the 1st year after birth, when neurogenesis is occurring and basic brain structures are being formed. Damage at this point may have cascading effects on subsequent aspects of brain development, with potentially wide-ranging negative effects. In contrast, when brain damage is sustained in early childhood—that is, when synapse generation and pruning are occurring and plasticity is highest—the chances for the brain's rewiring itself and recovering lost function are best.

Brain development: Early childhood

Brain growth is not as rapid during early childhood as it was during infancy, but critical changes do occur during early childhood that allow more rapid thought, more coordinated behavior, and better planning and goal setting. These abilities allow a child to acquire speaking fluency, better control of posture and movement, fantasy play, and effective problem-solving. The rapid growth of the brain continues beyond birth up to the second birthday by which time the brain has attained 80 percent of its adult size. Most of this growth is due to the increase in dendrites and synaptic connections. Synapses are removed (synaptic pruning) as the brain becomes more fine-tuned, but overall, more synapses are added than are removed. After the second birthday, the brain grows more slowly in size due to the closing of the gaps (sutures) between the separate skull parts. This growth process concludes between ages 6 and 8. The process of myelination is largely complete by a child's third birthday, but it continues at a much lessened pace through childhood, adolescence, and beyond. Myelination occurs when a fatty insulation (myelin) grows on the connecting fibers between neurons allowing for faster and more efficient communication within the brain. When brain researchers talk about "white matter," they are referring to myelinated nerve bundles. When researchers talk about "gray matter," they are referring to the unmyelinated brain cell bodies. One brain area that grows and rapidly myelinates during early childhood is the corpus callosum, a band of fibers that connects the left and right sides (hemispheres) of the brain. Because children do not have a mature corpus callosum during early childhood, the two sides, or hemispheres, of the brain do not communicate as well as they will after the brain matures a bit more. This lack of coordination of the cerebral hemispheres during early childhood is one reason why some of the behaviors of young children appear clumsy, wobbly, and slow. Any activity that demands greater coordination, balance, or speed demonstrates the immaturity of the corpus callosum and resulting lack of hemisphere coordination in children between ages 2 and 6. These activities, among others, may include: standing on one leg, hopping and skipping, running or sprinting, or performing gymnastics. -The prefrontal cortex is the last part of the human brain to fully myelinate and to reach maturity. The term prefrontal means "in front of the front." It is the very front part of the brain's outer layer known as the cerebral cortex. The prefrontal cortex is critical for reflective thought, planning, and control of impulsive behavior. At age 3 or 4, myelination that occurs in the prefrontal cortex allows for better impulse control and improvements in the ability to sustain attention, both of which are necessary for formal education to begin. -Given the high levels of brain growth during early childhood, it may not surprise you that a preschool child's brain uses a tremendous amount of energy. The young child's level of brain activity is near its peak, and the brain metabolizes nutrients at a higher-than-adult level until the child reaches age 10 or so. After that time, brain activity steadily declines until about age 20. This increase in brain metabolism during early childhood may reflect a highly active process of remodeling of synaptic connections as well as myelination of areas critical for reflective thinking, planning, and impulse control. Although the brain has matured in important ways, the brain in early childhood is still relatively immature. It is no surprise that children at this age are full of ideas and questions, but they are not quite ready for school. As you can well imagine, experience as well as brain maturation are necessary for the young child to sit in one place for an hour, to scan a page of print, to listen and think before talking, to remember important facts for more than a few seconds, and to control impulse and emotional behavior. -Myelination causes faster and more efficient communication within the brain.

random assortment

During gamete division, the 23 pairs of chromosomes are shuffled randomly, with chance determining which member of each pair goes into each new egg or sperm. This means that for each gamete, there are 223, or 8.4 million, possible combinations of chromosomes. Thus, when a sperm and an egg unite, the odds are essentially zero that any two individuals—even members of the same family—would have the same genotype (except, of course, identical twins).

Family Studies of Intelligence

Genetic influence is shown by generally higher correlations for higher degrees of genetic similarity. Most notable is the finding that identical (MZ) twins resemble each other in IQ more than do same-sex fraternal (DZ) twins. At the same time, environmental influences are reflected in the fact that identical twins are not identical in terms of IQ. Further evidence for an environmental role is that MZ twins who are reared together are more similar than those reared apart. -Surprisingly, the actual pattern is exactly the opposite: as twins get older, the degree of variance in IQ accounted for by their genetic similarity increases. In a study of 11,000 twin pairs across four countries, researchers found that the correlations in IQ between co-twins increased with age for MZ twins and decreased with age for DZ twins. These divergent patterns were observed first from childhood to adolescence, and again from adolescence to young adulthood -This surprising pattern of results—namely, that genetic influences on intelligence increase with age—is consistent with the idea that people actively construct their own environment: the phenotype-environment correlation (Relation 4) discussed earlier. As children get older, they increasingly control their own experiences, and their parents have less influence over their activities. -As children have more opportunities to shape their own environments based on their genetic propensities, the genetic effects of intelligence become more prominent.

Brain Development: Adolescence

In many respects, a person is at the peak of his or her physical capacities in the teenage years. Consider the number of outstanding teenage athletes in fields such as gymnastics, figure skating, or tennis. Many teenagers are happy and well adjusted. They look forward to the privileges and responsibilities of adulthood. While the teenage years may be a positive experience for some, adolescents are also at greater risk for drug and alcohol abuse, reckless driving, depression, and criminal behavior. How can we explain this rise in problem behaviors during adolescence? Some recent research suggests that brain development and hormonal changes may hold important clues for understanding why adolescence is so exciting, and yet at times, so difficult for teens and their families. The brain is still growing and changing during most of the teen years achieving its maximum size around the age of 18. At its peak, the brain weighs about 1.4 kilograms (about 3 pounds), which is about 4 times its weight at birth. Adolescence is a time of growth in the connections among neurons as well as a time of synaptic pruning and myelination. All of these developments act to further refine thoughts, actions, and behaviors and prepare the adolescent for adulthood. The brain centers for sensation and perception are well developed during infancy, but other areas of the brain take much longer to mature. The frontal lobe within the cerebral cortex (in particular, the prefrontal cortex) shows significant changes from mid-childhood through the early 20s. This area of the cortex is responsible for inhibiting impulses, focusing attention, and planning — in short, all of the behaviors that we associate with intelligence, responsibility, and emotional maturity. Maturation of the prefrontal cortex is the result of two important changes at the microscopic level: myelination, which is the addition of myelin to the connecting fibers of neurons, and pruning, which is the remodeling of synaptic connections. Both of these changes yield a brain that processes information more quickly and efficiently. Teen brains demonstrate a slower metabolism, which is another characteristic of maturity. A preschool child's brain burns calories at higher-than-adult levels (hypermetabolism) in order to build new connecting fibers, synapses, and myelin. As the process of synaptic pruning customizes the brain making it more efficient, the brain metabolism declines from age 10 years until about 20 years of age. Some recent studies have suggested that there is a change in the balance of activity between the limbic system, which is involved in motivated behavior and automatic emotional reactions, and the prefrontal cortex, which is responsible for deliberate and thoughtful behavioral control. The specific nature of the interaction of these brain centers is unknown, but it is safe to say, "the adolescent brain is a brain in flux . . ." (Spear, 200, p. 438). Along with the brain changes that occur during adolescence, a rise in the level of hormones in the bloodstream also occurs. Hormones produced by the adrenal glands that are believed to increase the overall excitability of the brain begin to rise around 7 years of age and reach a plateau in late adolescence. Increasing levels of hormones produced by the gonads, which are responsible for the bodily changes of puberty, are also related to slight increases in moodiness, aggressive behavior, and aggressive thoughts in both males and females (Spear, 2000). While there is good evidence for the idea that hormonal changes are related to behavioral changes, recent research has shown that this connection is a modest one at best. There is little support for the commonplace idea that risky teen behavior is caused by "raging hormones". What are we to make of these changes in the brain across adolescence? Researchers are careful to point out that "correlation is not causation." Just because the brain is changing, it does not necessarily mean that these brain changes cause the unique behaviors of adolescence. Researchers have offered some promising leads. The limbic system within the brain is involved in the way in which the brain experiences "reward" and "pleasure." The observed shift in the balance of activity between the limbic system and the prefrontal cortex may be responsible for the well-documented decline in "pleasure" and "happiness" that is observed in early adolescence. Some researchers have described the adolescent as having a "mini reward-deficiency syndrome" that leads some to seek increased levels of stimulation through interaction with peers, risky behavior, and drug or alcohol use. Some researchers stress that the roots of adolescent risk-taking and impulsive behaviors stem from teenagers' lack of relevant experiences and skills. Adults, after all, have many more years of experience with both the short and the long-term consequences of risky behavior. The metabolism of the brain decreases throughout childhood and adolescence, and the balance of activity appears to change between the structures in the brain's limbic system and the prefrontal areas of the cortex. We also know that adolescence is a time of increased interactions with peers, new experiences outside the home, and new demands for mature behavior. These new interactions and experiences help adolescents develop new skills for coping with increased stresses, but the growth of these skills takes time. -brain changes that occur in adolescence: (a) shifts in activity from the limbic system to the prefrontal cortex, (b) an increase in myelination of the frontal cortex, (c) remodeling (pruning) of synaptic connection, and (d) an overall decrease in metabolism. Your answer may also mention hormonal changes.

Chromosomes

Molecules of DNA that transmit genetic information; chromosomes are made up of DNA ( in the cell body)

Brain development: Infants and Toddlers

Most of a newborn's organs are relatively immature at birth. For example, babies cannot digest solid food, and they have no control over either their bowels or their bladders. The brain, in particular, is quite immature at birth, and this is reflected in a newborn's short waking periods, poor eyesight, and limited control of posture and movement. There is good evidence that, indeed, "babies are born fetuses"! Compared to our closest primate species, we have less overall capacity at birth to act voluntarily (e.g., cling to mother); our bones are less developed; and the networks of brain cells (neurons) are still developing rapidly. In this sense, we are all born "premature." Most scientists believe that as our species developed a larger and more complex brain, childbirth had to occur before our brain could fully form. If the prenatal period was extended, the baby's head would have been too large to pass safely out of the mother's body at birth. At birth, a baby's brain has already attained 25 percent of its adult weight. In contrast, a baby's body weight is only 5 percent of its adult weight, so the newborn's brain is proportionally five times more developed than its body in terms of weight at birth. This rapid brain growth continues. The first two years, like the prenatal period, is a time of tremendous brain growth. By age two, the baby's brain has achieved 75 percent of its adult weight! The rest of the body also grows quickly, and it achieves 20 percent of its adult weight in the same amount of time. Most of the increase in brain weight over the first two years is due to transient exuberance, a proliferation of dendrites in the cortex. Remember, dendrites receive signals from connecting neurons across tiny gaps called the synapses. Dendrites show a fivefold increase in the first two years with as many as 15,000 new connections for each neuron. The vast neural networks created by the rapid growth of dendrites allow more flexible processing of information. As a result of maturation and experience, some of these connections will be lost due to synaptic pruning, or remolding, as the brain becomes more finely tuned. Another aspect of brain maturation is the development of myelin, the insulation that forms on the long outgoing axons of neurons. Myelin prevents neighboring cells from short-circuiting each other's activity, and it speeds up the transmission of signals. The process of myelination begins in the prenatal period and is largely completed by age three. However, the prefrontal cortex, the brain region most important for focused attention and goal setting, develops at a slower pace through childhood, adolescence, and even into the late 20s! Brain development continues at a rapid pace in the first two years. The brain reaches nearly 75 percent of its adult weight, and there are far more connections than will be present later on—leaving much room for "remolding" of the brain's connections in the years to come. While many areas of the cortex have matured through myelination, the areas most important for complex thinking (e.g., focused attention and goal-setting) will not function optimally until adolescence and the mid 20s.

3.3 Brain Development Review

Nature and nurture cooperate in the construction of the human brain. Some important brain structures include the neurons, which communicate with one another at synapses; the cortex, in which different functions are localized in different areas; and the cerebral hemispheres, which are specialized for different kinds of processing. The processes involved in the development of the brain include neurogenesis and synaptogenesis, followed by the systematic elimination of some synapses and the preservation of others as a function of experience. Two forms of plasticity contribute to the development of behavior. As a result of experience-expectant plasticity, the brain is shaped by experiences that are available to every typically developing individual in interaction with every species-typical environment. Through experience-dependent plasticity, the brain is also structured by an individual's idiosyncratic life experiences. Because of the importance of experience in brain development, sensitive periods exist during which specific experience must be present for normal development. Timing is also a crucial factor in the ultimate impact of brain damage.

Birds of a feather flock together

People like to spend time with others who are similar to them.

5. Child's Environment - Child's Genotype

That is, it is now known that although the structure of DNA remains "fixed" (mutations aside), certain epigenetic mechanisms, mediated by the environment, can alter the functioning of genes and create stable changes in their expression—and some of these changes can be passed on to the next generation.

Extra

The X chromosome carries roughly 1500 genes, whereas the much smaller Y chromosome carries only about 200. Thus, when a female inherits a recessive allele on the X chromosome from her mother, she is likely to have a dominant allele on the chromosome from her father to suppress it, so she will not express the trait in question. In contrast, when a male inherits the same recessive allele on the X chromosome from his mother, he likely will not have a dominant allele from his father to override it, so he will express the trait. Males are thus more likely than females to suffer a variety of sex-linked inherited disorders caused by recessive alleles on their X chromosome

3.2 Behavior Genetics Review

The field of behavior genetics is concerned with how development results from the interaction of genetic and environmental factors. Using the family-study methodology, behavior geneticists compare the correlations among individuals who vary in the degree of genetic relatedness and in similarity of their rearing environments. Contemporary methods like GCTA have allowed researchers to move beyond twin or adoption designs to consider actual genetic overlap among unrelated people. Heritability estimates indicate the proportion of the variance among individuals in a given population on a given trait that is attributable to genetic differences among them. Most behavioral traits that have been measured show substantial heritability; at the same time, heritability estimates reveal the close partnership of heredity and environment in development and the fallacy of considering the influences of nature and nurture as independent of each other.

Review 3.1 Nature and Nurture

The five relations shown in Figure 3.1 depict the complex interplay of genetic and environmental forces in development: (1) The course of children's development is influenced by the genetic heritage they receive from their mother and father, with their sex determined solely by their father's chromosomal contribution. (2) The relation between children's genotype and phenotype depends in part on dominance patterns in the expression of some genes, but most traits of primary interest to behavioral scientists are influenced by multiple genes (polygenic inheritance). (3) As the concept of norm of reaction specifies, any given genotype will develop differently in different environments. A particularly salient part of children's environment is their parents, including their parents' own genetic makeup, which influences how parents behave toward their children. (4) Children's own genetic makeup influences how they select and shape their own environment and the experiences they have in it. (5) Conversely, children's experiences can change their genetic expression through epigenetic mechanisms.

sensitive periods

There are a few sensitive periods when the human brain is especially sensitive to particular kinds of external stimuli. It is as though a time window were temporarily opened, inviting environmental input to help organize the brain. Gradually, the window closes. The neural organization that occurs (or does not occur) during sensitive periods is typically irreversible.

mutation

a change in a section of DNA -mutations are random, spontaneous errors; others are caused by environmental factors. Most are harmful. Those that occur in germ cells can be passed on to offspring; many inherited diseases and disorders originate from a mutated gene. -however, a mutation makes individuals more viable, that is, more likely to survive—perhaps by increasing their resistance to some disease or by increasing their ability to adapt to some crucial aspect of their environment. Such mutations provide the basis for evolution: a person with the favorable mutated gene is more likely to survive long enough to produce offspring, who, in turn, are likely to possess the mutated gene, thus heightening their own chance of surviving and reproducing. Across generations, these favorable genes proliferate in the gene pool of the species.

Cell body

a component of the neuron that contains the basic biological material that keeps the neuron functioning

corpus callosum

a dense tract of nerve fibers that enable the two hemispheres of the brain to communicate

Phenylketonuria (PKU)

a disorder related to a defective recessive gene on chromosome 12 that prevents metabolism of phenylalanine -a disorder related to a defective recessive gene on chromosome 12. Individuals who inherit this gene from both parents cannot metabolize phenylalanine, an amino acid present in many foods (especially red meats) and in artificial sweeteners. If they eat a normal diet, phenylalanine accumulates in the bloodstream, causing impaired brain development that results in severe intellectual impairment. However, if infants with the PKU gene are identified shortly after birth and placed on a stringent diet free of phenylalanine, intellectual impairment can be avoided, as long as the diet is carefully maintained. Thus, a given genotype results in quite different phenotypes—cognitive disability or relatively normal intelligence—depending on environmental circumstances.

myelin sheath

a fatty sheath that forms around certain axons in the body and increases the speed and efficiency of information transmission -The importance of myelin is highlighted by the severe consequences that can arise from disorders that affect it. For example, multiple sclerosis is a disease in which the immune system attacks myelin, interfering with neuronal signaling and producing varying degrees of physical and cognitive impairment. Myelin is also implicated in mental illness: individuals with schizophrenia show disruptions in white matter that have been linked to multiple genes that regulate myelination -Glial cells also play a key supportive role in promoting brain health. They function as neural stem and progenitor cells during brain development, and some subsets of these cells continue to do so into adulthood. When the brain is injured, certain types of glial cells react by rapidly increasing in numbers, protecting the brain and potentially aiding in regeneration

John Stuart Mill

according to Mill, Galton's subjects rose to eminence more because of environmental factors than hereditary ones.

4. Child's Phenotype - Child's Environment

active child theme—the child as a source of his or her own development. -They are active creators of the environment in which they live in two important ways. First, by virtue of their nature and behavior, they actively evoke certain kinds of responses from others. The second way in which children create their own environment is by actively selecting surroundings and experiences that match their interests, talents, and personality characteristics.

Norm of reaction

all the phenotypes that can theoretically result form a given genotype in relation to all the environments in which it can survive and develop -for any given genotype developing in varying environments, a range of outcomes would be possible

frontal lobe

associated with organizing behavior; the one that is thought responsible for the human ability to plan ahead -"executive," is involved in cognitive control, including working memory, planning, decision making, and inhibitory control.

Synesthesia

blending of different types of sensory inputs

Genome

each person's complete set of hereditary information (p. 12); the complete set of genes of any organism (p. 94)

Heritability (Environmental Effects)

estimating heritability automatically estimates the proportion of variance not attributable to genes. Because heritability estimates rarely exceed 50%, a large contribution from environmental factors is usually indicated. -Behavior geneticists try to assess the extent to which aspects of an environment shared by biologically related people make them more alike and to what extent non-shared experiences make them different. The most obvious source of shared environment is growing up in the same family. Shared-environment effects can also be inferred when twins or other relatives are more similar on some trait than would be expected on the basis of their genetic relatedness. -Behavior geneticists' investigations of the effects of non-shared environments arise from the recognition that even children who grow up in the same family do not have all their experiences in common—either inside or outside the family. Indeed, most effects of environment in behavior genetics designs are not shared by children within the same family (Plomin et al., 2016). This may be due to aspects of the family structure.

Environment

every aspect of an individual and his or her surroundings other than genes

eminence

fame or recognized superiority, especially within a particular sphere or profession

Regulator genes

genes that control the activity of other genes -The activation or inactivation of one gene is always part of a chain of genetic events. When one gene is switched on, it causes another gene to turn on or off, which has an impact on the status of yet other genes. Thus, genes never function in isolation. Instead, they belong to extensive networks in which the expression of one gene is a precondition for the expression of another, and so on. (Thalidomide) -a given gene can function multiple times in multiple places during development. All that is required is that the gene's expression be controlled by different regulator genes at different times.

Gene Anomalies

genetic disorders can originate from extra or missing chromosomes, so too can they result from extra, missing, or abnormal genes. One intriguing instance is Williams syndrome. This rare genetic disorder involves a variety of cognitive impairments, most noticeably in spatial and visual skills, but relatively less impairment in language ability This condition has been traced to the deletion of a small section of approximately 25 genes on chromosome 7. Some individuals, however, have a smaller deletion; in those cases, the degree of impairment is decreased, suggesting a clear relationship between the number of genes deleted and the resulting phenotype

Chromosomal anomalies

genetic disorders originate with errors in germ-cell division that result in a zygote that has either more or fewer than the normal complement of chromosomes. Most such zygotes cannot survive, but some do. Down syndrome most commonly originates when the mother's egg cells do not divide properly, and an egg that is fertilized contains an extra copy of chromosome 21. - probability of such errors in cell division increases with age, thus the incidence of giving birth to a child with Down syndrome is markedly higher for women older than 35. (

Prenatal testing

genetic testing used to assess the fetus's risk for genetic disorders

carrier genetic testing

genetic testing used to determine whether prospective parents are carriers of specific disorders

parietal lobe

governs spatial processing as well as integrating sensory input with information stored in memory and with information about internal states.

Heterozygous

having two different alleles for a trait

Homozygous

having two of the sam allele for a trait

endophenotypes

intermediate phenotypes, including the brain and nervous systems, that do not involve overt behavior -although every cell in your body contains copies of all the genes you received from your parents, only some of those genes are expressed. At any given time in any cell in the body, some genes are active (turned on), while others are not.

Lobes

major areas of the cortex associated with general categories of behavior

Synapses

microscopic junctions between the axon terminal of one neuron and the dendritic branches or cell body of another -In this communication process, electrical and chemical messages cross the synapses and cause the receiving neurons either to fire, sending a signal on to other neurons, or to be inhibited from firing. The total number of synapses in the human brain is staggering—hundreds of trillions—with some neurons having as many as 15,000 synaptic connections with other neurons.

family study

modern behavior genetics research -In order to examine genetic and environmental contributions to a given trait or characteristic, behavior geneticists first measure that trait in people who vary in terms of genetic relatedness—parents and their children, identical and fraternal twins, non-twin siblings, and so on. Next, they assess how highly correlated the measures of the trait are among individuals who vary in the degree to which they are genetically related -Finally, behavior geneticists compare the resulting correlations to see if they are (1) higher for more closely related individuals than for less closely related people, and (2) higher for individuals who share the same environment than for individuals who do not. -One is the twin-study design, which compares the correlations for identical (monozygotic, or MZ) twins with those for same-sex fraternal (dizygotic, or DZ) twins. As you will recall, identical twins have 100% of their genes in common (though the expression of these genes is affected by epigenetic factors over the course of development, as discussed in the preceding section), whereas fraternal twins are only 50% genetically similar ( just like non-twin siblings). For twins who grow up together, the degree of similarity of the environment is generally assumed to be equal. -equal environments assumption, the claim is that both types of twins shared the same womb, were born at the same time, have lived in the same family and community, and are always the same age when tested. Thus, with different levels of genetic similarity and essentially equal environmental similarity, the difference between the correlations for the two types of twins is treated as an index of the importance of genetic factors. If the correlation between identical twins on a given trait or behavior is substantially higher than that between fraternal twins, it is assumed that genetic factors are responsible for the difference. -the adoptive twin study—compares identical twins who grew up together versus identical twins who were separated shortly after birth and raised apart, If the correlations for twins reared apart are similar to those for twins reared together, it suggests that environmental factors have little effect. Conversely, to the extent that the correlations between identical twins who grew up in different environments are lower than those for identical twins who grew up together, environmental influence is inferred.

DNA (deoxyribonucleic acid)

molecules that carry all the biochemical instructions involved in the formation and functioning of an organism

Axon

neural fibers that conduct electrical signals away from the cell body to connections with other neurons

Dendrites

neural fibers that receive input from other cells and conduct it toward the cell body in the form of electrical impulses

Gregor Mendel

observed distinct patterns of inheritance in the pea plants that he cross-bred in his monastery garden. Some aspects of these inheritance patterns were later discovered to occur in all living things,

James Watson, Rosalind Franklin, and Francis Crick

observed distinct patterns of inheritance in the pea plants that he cross-bred in his monastery garden. Some aspects of these inheritance patterns were later discovered to occur in all living things, (1950s)

Dominant recessive patterns

occurring only when an individual has two recessive alleles for the condition. There are thousands of such recessive-gene disorders, including phenylketonuria (PKU) (discussed on page 103) and sickle-cell anemia (discussed below), as well as Tay-Sachs disease, cystic fibrosis, and many others. Disorders that are caused by a dominant gene include Huntington disease (a progressive and always fatal degenerative condition of the brain) and neurofibromatosis (a disorder in which nerve fibers develop tumors). -In some cases, a single gene can have both harmful and beneficial effects. One such case is sickle-cell disease, in which red blood cells are sickle-shaped rather than round, diminishing their capacity to transport oxygen. This disease, which can be debilitating and sometimes fatal, affects about 1 of every 500 African Americans. It is a recessive-gene disorder, so individuals who are homozygous for this trait (inheriting two sickle-cell genes, one from each parent) will suffer from the disease. -individuals who are heterozygous for this trait (carrying one normal and one sickle-cell gene) have some abnormality in their blood cells but usually experience no negative effects.

association areas

parts of the brain that lie between the major sensory and motor areas and that process and integrate input from those areas -that lie in between the major sensory and motor areas. The parts of the human brain that have evolved to be most enlarged compared with other species are also those that grow the most as children develop: prefrontal, parietal, and temporal cortices (e.g., Kaas, 2013). Other brain regions, including sensory and motor areas, expanded less during the evolution of the human brain.

Heritable

refers to an characteristics or traits that are influenced by heredity

Multifactorial

refers to traits that are affected by a host of environmental factors as well as genetic ones To fully answer Galton's question, behavior geneticists try to tease apart genetic and environmental contributions by taking advantage of the differences observed among a population of people or other animals. Two premises underlie this endeavor: 1. To the extent that genetic factors are important for a given trait or behavior, individuals who are genotypically similar should be phenotypically similar. In other words, behavior patterns should "run in families": children should be more similar to their parents and siblings than to second- or third-degree relatives or unrelated individuals. 2. To the extent that shared environmental factors are important, individuals who were reared together should be more similar than people who were reared apart.

polygenic inheritance

result from interactions among multiple inherited genes, often in conjunction with environmental factors. Among the many diseases in this category are some forms of cancer and heart disease, type 1 and type 2 diabetes, and asthma. Psychiatric disorders, such as schizophrenia, and behavior disorders, such as attention-deficit hyperactivity disorder, also involve numerous genes. -inheritance in which traits are governed by more than one gene

Genes

sections of chromosomes that are the basic unit of heredity in all living things (each gene is a segment of DNA that is the code for the production of particular proteins.) - only make up 2% o the human genome -Much of the rest of our genome—once thought to be "junk" DNA—turns out to play a supporting role in influencing genetic transmission by regulating the activity of protein-coding genes.

Sex lined inheritances

single-gene conditions are carried on the X chromosome and are much more common in males. (Females can inherit such conditions, but only if they inherit the culprit recessive alleles on both of their X chromosomes.) Sex-linked disorders range from relatively minor problems, like male-pattern baldness and red-green color blindness, to very serious problems, including hemophilia and Duchenne muscular dystrophy.

cerebral cortex

the "gray matter" of the brain that plays a primary role in what is thought to be particularly humanlike functioning, from seeing and hearing to writing to feeling emotion -The folds and fissures that are apparent in Figure 3.7 form during development as the brain grows within the confined space of the skull; these convolutions make it possible to pack more cortex into the limited space. -cortex plays a primary role in a wide variety of mental functions, from seeing and hearing to reading, writing, and doing arithmetic to feeling compassion and communicating with others.

Recessive

the allele that is not expressed if a dominant allele is present

dominant

the allele that, if present, gets expressed

Plasticity

the capacity of the brain to be affected by experience -less information needs to be encoded in the genes. This economizing may, in fact, be a necessity: the number of genes involved in the formation and functioning of the nervous system is enough to specify only a very small fraction of the normal complement of neurons and neural connections. In addition, if brain structures were entirely hard-wired, organisms would be unable to adapt to their postnatal environment. To complete the final wiring of the brain, nurture joins forces with nature. -One kind involves the general experiences that almost all infants have just by virtue of being human. This form of plasticity is referred to as experience-expectant. The second kind, referred to as experience-dependent, involves specific, idiosyncratic experiences that children have as a result of their particular life circumstances—such as growing up in the United States or in the Amazon rain forest, experiencing frequent cuddling or abuse, being an only child or one of many siblings, and so on.

Sex chromosomes

the chromosomes (X and Y) that determine an individual's gender -Because a female has only X chromosomes, the division of her gametes (germ cells) results in all her eggs having an X. However, because a male is XY, half his sperm contain an X chromosome and half contain a Y. For this reason, it is always the father who determines the sex of offspring: if an X-bearing sperm fertilizes an egg, a female (XX) zygote results; if an egg is fertilized by a Y-bearing sperm, the zygote is male (XY). It is the presence of a Y chromosome—not the fact of having only one X chromosome—that makes an individual male. -gene on the Y chromosome encodes the protein that triggers the prenatal formation of testes by activating genes on other chromosomes. Subsequently, the testes produce the hormone testosterone.

Myelination

the formation of the myelin around the axon of neurons that speeds and increases information processing abilities -The myelinated portions of axons are white, leading to the term white matter, and lie below the gray matter (cell bodies) at the surface of the cortex. As noted earlier, a crucial function of myelin is to increase the speed of neural conduction. Myelination begins deep in the brain and moves upward and outward into the cortex. This process occurs rapidly for the first few months after birth, slows somewhat during toddlerhood, and continues slowly into young adulthood (e.g., Dubois et al., 2014). The various cortical areas thus become myelinated at very different rates, possibly contributing to the different rates of development for various behaviors.

Genotype

the genetic material an individual inherits

Level of antisocial behavior observed in young men as a function of the degree to which they had been maltreated in childhood.

the importance of a combination of environmental and genetic factors leading to antisocial outcomes—suffering abusive treatment as a child and possessing a particular variant of MAOA, an X-linked gene known to inhibit brain chemicals associated with aggression. Young men who had a relatively inactive version of the MAOA gene, and who had experienced severe maltreatment, grew up to be more antisocial than other men. More concretely, 85% of the maltreated group with the relatively inactive gene developed some form of antisocial behavior, and they were almost 10 times more likely to be convicted of a violent crime. -The important point here is that neither factor by itself (possessing the inactive MAOA gene or being abused) predisposed boys to become highly aggressive; the higher incidence of antisocial behavior was observed only for the group with both factors.

Temporal Lobe

the lobe of the cortex that is associated with memory, visual recognition, and the processing of emotion and auditory information

occipital lobe

the lobe of the cortex that is primarily involved in processing visual information

synaptic pruning

the normal developmental process through which synapses that are rarely activated are eliminated - 40% of synaptic superfluity is eliminated -This pruning occurs at different times in different areas of the brain. You can see from Figure 3.8 that synapse elimination in the visual cortex begins near the end of the 1st year of life and continues until roughly 10 years of age, whereas synapse elimination in the prefrontal area shows a slower time course. -The brain undergoes substantial changes during adolescence, including a wave of overproduction and pruning akin to that in the first years of life (Giedd et al., 1999; Gogtay et al., 2004). Although the amount of white matter in the cortex shows a steady increase from childhood well into adulthood, the amount of gray matter increases dramatically starting around 11 or 12 years of age. The increase in gray matter proceeds rapidly, peaks around puberty, and then begins to decline as some of it is replaced by white matter (see Figure 3.9). The last area of the cortex to mature is the dorsolateral prefrontal cortex, which is vital for regulating attention, controlling impulses, foreseeing consequences, setting priorities, and other executive functions. It does not reach adult dimensions until after the age of 20, and synaptic pruning continues until individuals are in their 30s (Petanjek et al., 2011). Some subcortical areas, such as the thalamus, show similarly protracted developmental trajectories (Raznahan et al., 2014).

Phenotype

the observable expression of the genotype, including both body characteristics and behavior

Synaptogenesis

the process by which neurons form synapses with other neurons, resulting in trillions of connections (p. 119) -begins prenatally and proceeds very rapidly both before birth and for some time afterward. Note that both the timing and rate of synapse production vary for different cortical areas; synapse generation is complete much earlier in the visual cortex, for example, than in the frontal area. As with myelination, the differential timing of synapse generation across areas of the brain likely contributes to the developmental timing of the onset of various abilities and behaviors.

Crossing over

the process by which sections of DNA switch from one chromosome to the other; crossing over promotes variability among individuals -some of the chromosomes that parents pass on to their offspring are constituted differently from their own.

experience-dependent plasticity

the process through which neural connections are created and reorganized throughout life as a function of an individual's experiences

experience-expectant plasticity

the process through which the normal wiring of the brain occurs in part as a result of experiences that every human who inhabits any reasonably normal environment will have -the normal wiring of the brain is in part a result of the kinds of general experiences that have been present throughout human evolution, experiences that every human with an intact sensory-motor system who inhabits a reasonably normal environment will have: patterned visual stimulation, voices and other sounds, movement and manipulation, and so forth -brain can "expect" input from these reliable sources to fine-tune its circuitry; synapses that are frequently activated will be strengthened and stabilized, and those that are rarely activated will be "pruned." Thus, our experience of the external world plays a fundamental role in shaping the most basic aspects of the structure of our brain. -benefit of experience-expectant plasticity is that, because experience helps shape the brain, fewer genes need to be dedicated to normal development. Another is that the brain is better able to recover from injury to certain areas because other brain areas can take over the function that would have been performed by the damaged area. The younger the brain when damaged, the more likely recovery is. -The downside of experience-expectant plasticity is that it is accompanied by vulnerability. If for some reason the experience that the developing brain is "expecting" for fine-tuning its circuits does not occur, whether because of inadequate stimulation or impaired sensory receptors, development may be compromised.

neurogenesis

the proliferation of neurons through cell division -begins 42 days after conception (in humans) and is virtually complete by the midway point of gestation (Stiles & Jernigan, 2010). Thus, most of the roughly 100 billion neurons you currently possess have been with you since before you were born. Notably, however, we do continue to generate new neurons throughout life. During bouts of learning, for example, neurogenesis occurs in the hippocampus, a brain region important for memory processes (Gould et al., 1999). Neurogenesis does not always occur, however: it can be inhibited by stress (Mirescu & Gould, 2006). This pattern of results suggests that neurogenesis later in life is not fixed and predetermined but is instead adaptive, increasing under rewarding conditions and decreasing in threatening environments (e.g., Glasper, Schoenfeld, & Gould, 2012). -After their "birth," neurons begin the second developmental process, which involves migration to their ultimate destinations—typically outwards from the center of the brain toward the developing neocortex. Some neurons are pushed along passively by the newer cells formed after them, whereas others actively propel themselves toward their ultimate location. Early in gestation, the brain is very small so the distances traveled are quite short. But as the brain grows, neurons require guides, in the form of a special kind of glial cell (radial glial cells) that provides scaffolding for neurons, to correctly find their destinations. -Once neurons reach their destination, cell growth and differentiation occur. Neurons first grow an axon and then a "bush" of dendrites (refer back to Figure 3.6). Thereafter, they take on the specific structural and functional characteristics of the different structures of the brain. Axons elongate as they grow toward specific targets, which, depending on the neuron in question, might be anything from another neuron in the brain to a bone in the big toe. The main change in dendrites is "arborization"—an enormous increase in the size and complexity of the dendritic "tree" that results from growth, branching, and the formation of spines on the branches. Arborization enormously increases the dendrites' capacity to form connections with other neurons.

2. Child's Genotype - Child's Phenotype

the relation between one's genotype and one's phenotype. Keep in mind that phenotypes include both physical characteristics, such as height and eye color, and behavioral characteristics, such as temperament and intelligence. Genes also influence unobservable aspects of the phenotype that influence behavior, most notably, our brain and nervous systems.

Behavior Genetics

the science concerned with how variation in behavior and development results from the combination of genetic and environmental factors

cerebral lateralization

the specialization of the hemispheres of the brain for different modes of processing -For example, most aspects of speech and language are lateralized to the left hemisphere in humans, with a similar asymmetry observed for communicative signals in nonhuman species from mice to primates (Corballis, 1999). However, contrary to popular belief, the data do not support the idea that people are left brain-dominant or right brain-dominant; individuals do not tend to have a general preference to use one hemisphere over the other (e.g., Nielsen et al., 2013).

Epigenetics

the study of environmental influences on gene expression that occur without a DNA change -Epigenetic factors can help explain why identical twins do not have identical pathways through life: different environments can alter gene expression in subtle ways across developmental time. These stable changes in gene expression that are mediated by the environment involve processes including methylation, which silences gene expression. Differences in experience over the course of development are reflected in differences in methylation levels. Consider identical twin pairs at age 3 and at age 50.

cerebral hemispheres

the two halves of the cortex; for the most part, sensory input from one side of the body goes to the opposite hemisphere of the brain -For the most part, sensory input from one side of the body goes to the opposite side of the brain, and the motor areas of the cortex control movements of the opposite side of the body.

regulator gene defects

thought to originate from defects in regulator genes, which, as discussed on page 100, control the expression of other genes. For example, a defect in the regulator gene that initiates the development of a male can interrupt the normal chain of events, occasionally resulting in a newborn who has female genitalia but is genetically male.

Alleles

two or more different forms of a gene -alleles of a given gene influence the same trait or characteristic (e.g., eye color), but they contribute to different developmental outcomes (e.g., brown, blue, hazel, gray eyes).