Chapter 2

Genes, Environment and Development

Modified: 2025-07-02 4:02 PM CDST

I. Evolution and Human Development (p. 35)

• A. Evolution and Human Development
  • 1. Virtually everyone develops in similar ways at similar ages
  • 2. Species heredity—genetic endowment members of a species have in common
• B. Darwin’s evolutionary theory (p. 36)
  • 1. Main arguments of Darwin’s theory
  • 2. There is genetic variation in a species (members of a species do not share all of the same genes)
  • 3. Some genes aid in adaptation more than others do (e.g., how to find food)
  • 4. Genes that aid in adaptation to the environment will be passed on more often than genes that do not—the principle of natural selection
  • 5. Kettlewell’s study of moths in England demonstrates natural selection principles
  • 6. Study of pollution and moth color
  • 7. Light-colored moths survived in rural areas with light-colored trees
  • 8. Changes in pollution led to changes in moth color
  • 9. Evolution not just about genes; rather, development is driven by interaction between genes and environment

Kettlewell (1959): Industrial melanism. Examined the pheonotypes of moths (Biston betularia) before and after industrialization and environmental cleanup. Discovered that black variants were more common when tree trunks were blackened by pollution and that white variants were more common after sooty smoke stacks were outlawed.

Hummingbirds: Evolution in action. As a result of the ubiquity of hummingbird feeders, Anna's hummingbirds in California have evolved longer beaks and expanded their range. Here is a case of enviromental change and an evolutionary response.

• C. Ethology seeks to understand the evolved behavior of various species in their natural environment.
• D. Evolutionary psychology is an application of evolutionary theory to understanding human thinking and behavior.
• E. Life History Strategies
  • 1. The slow life history strategy is more likely to be adopted when life is secure and predictable.
  • 2. The fast life history strategy is more likely to be adopted in stressful environments in which life is harsh and unpredictable—and possibly short: communities with high crime, poverty, and death rates, unstable families, limited work, and the like.
• F. Cultural evolution—we “inherit” from previous generations a characteristically human environment and tried and true ways of adapting to it, learning to adjust to it, and passing on what we know to the next generation
• 1. Most significant biological evolutionary legacy is powerful and flexible brain that assists in learning.

II. Individual Heredity (p. 40)

• A. The Genetic Code
  • 1. Early genetic materials
  • 2. Zygote—cell created at conception
  • 3. Chromosomes—46 threadlike bodies (23 pairs, one from father and one from mother) containing the genes
  • 4. Meiosis—reproductive cell division in which one 46-chromosome sperm or ova splits into two 46-chromosome cells, and these split into two more cells (each with 23 chromosomes), resulting in one viable egg (and three nonfunctional in females) or four viable sperm in males
  • 5. Mitosis—cell division producing two identical cells (e.g., 46-chromosome cell splits into two 46-chromosome cells); continues throughout life
  • 6. Mitosis creates new cells that replace old cells
• B. The Human Genome Project
  • 1. Federally funded attempt to map out entire DNA sequence of all human chromosomes using supercomputers; completed in 2003
    2. 3.1 billion A, C, G, T molecules
  • 3. Only about 3% of genome consists of traditionally defined genes (those that are transcribed into RNA and serve as templates for the production of certain proteins)
  • 4. Remaining stretches of DNA consist of “junk genes” that regulate activity of genes
  • 5. More emphasis being placed on that 98% of DNA
  • 6. Humans share majority of genes with primates
  • 7. Some gene alleles have evolved in recent centuries (e.g., variation that makes humans tolerate lactose in milk driven by rapid spread of dairy farming in Europe)
• C. Genetic uniqueness and relatedness
  • 1. Each parent can produce more than 8 million genetically different sperm or ova, and any couple could produce 64 trillion babies without having two with identical genes
  • 2. Crossing over—exchanges in pairs of chromosomes before separating
  • 3. Identical twins (monozygotic)—one fertilized ovum splits to make two genetically identical individuals (1 in 250 births)
  • 4. Child shares average of about 50% of genes with each parent
  • 5. Non-identical twin siblings share about 50% of their genes
  • 6. Fraternal twins (dizygotic)—two eggs released and each fertilized by different sperm (1 in 125 births)
  • 7. Same genetic relationship as with any other sibling
  • 8. Tend to run in families
  • 9. More common today due to increased use of fertility drugs and in vitro fertilization
  • 10. Individuals share some genes with all kin members
• D. Determination of sex
  • 1. Twenty-two of 23 chromosomes (autosomes) are similar in males and females.
  • 2. Twenty-third pair are the sex chromosomes.
  • 3. A male child has one long chromosome called an X chromosome and a shorter companion with fewer genes called a Y chromosome.
  • 4. Females have two X chromosomes
• E. From Genotype to Phenotype
  • 1. Genotype—genetic makeup one inherits
  • 2. Phenotype—actual characteristics based on genetics and environment
  • 3. Genes responsible for production of chemical substances (e.g., melanin, which impacts iris color)
  • 4. Genetically-coded proteins guide formation of neurons, influencing potential intelligence and personality
  • 5. Genes influenced by biochemical environment surrounding genes and behavior of individual genetic “blueprint” written in erasable pencil (not indelible ink)
  • 6. Genetic expression—activation of particular genes in particular cells at particular times in the life span
  • 7. A gene is influential only if it is “turned on”
  • 8. Genetic expression can be affected by environmental factors like diet, stress, toxins, and parenting
• F. Mechanisms of Inheritance
  • 1. Single gene-pair inheritance—characteristic influenced by only one pair of genes (one from mom and one from dad)
  • 2. Gregor Mendel—19th-century monk and pioneer in inheritance research
  • 3. Noticed patterns in cross-bred strains of peas
  • 4. Called one dominant, as it was likely to show up in later generations
  • 5. Dominant genes—if even only one gene inherited, will produce the effect
  • 6. Recessive genes—need one gene from both parents to produce effect
  • 7. Tongue curling dominant
  • 8. Two tongue-curling parents could have a no-curling child if both pass on recessive gene (chances of this are 25%)
  • 9. Incomplete dominance—individual expresses a blend of dominant and recessive traits
  • 10. Child of one parent with dark skin and the other parent with light skin who has light brown skin
  • 11. Co-dominance—neither gene in pair is dominant or recessive
  • 12. AB blood type is mix of A and B blood types
• G. Sex-Linked Inheritance—trait influenced by gene on sex chromosomes
  • 1. Most are X-linked (rather than sex-linked) as most attributes are associated with genes on only the X chromosome
  • 2. Color blindness (more common in males) is sex-linked
  • 3. Boy who inherits defective X chromosome from mom and no color vision gene on Y will be color blind
  • 4. Girl who inherits defective X chromosome may inherit normal color vision gene on other X chromosome and have normal color vision
  • 5. Hemophilia—genetic disorder resulting in deficiency in blood’s ability to clot; sex-linked disorder
  • 6. More common in males as it is associated with a recessive gene on X chromosome
• H. Polygenic Inheritance—most human characteristics determined by multiple genes
  • 1. Polygenic traits—characteristic influence by multiple pairs of genes
  • 2. The characteristic impacted by polygenetic traits (e.g., weight, intelligence, depression) tends to be distributed in the population in a bell-shaped or normal curve
• I. Mutations and Copy Number Variations
  • 1. Change in structure or arrangement of one or more genes that produce new phenotype
  • 2. Hemophilia may have been introduced to royal families of Europe by Queen Victoria
  • 3. Environmental hazards (e.g., radiation) can increase odds of mutations
  • 4. Some mutations beneficial
  • 5. Sickle-cell disease—sickle-shaped white blood cells protects from effects of malaria
  • 6. Does more harm than good in individuals living in non-malaria environments
  • 7. Copy number variations
  • 8. Instances in which part of the genome is either deleted or duplicated
  • 9. More extensive than a mutation
  • 10. Can extend over a large stretch of DNA
  • 11. Can either be inherited or arise spontaneously
  • 12. Significantly increase the risks of a number of polygenic disorders involving the nervous system
• J. Chromosome Abnormalities
  • 1. Child receives too many or too few chromosomes (about 1 in 160 born with more or less than 46 chromosomes)
  • 2. Down syndrome
  • 3. Trisomy 21 (three 21st chromosomes)
  • 4. Physical (e.g., distinctive eye folds, short, stubby limbs) and mental (some degree of intellectual disability) impact
  • 5. In some parts of the world, over half of these infants die due to heart defects, but in United States and other wealthy nations, many with Down syndrome are living into middle age (many show premature signs of aging including Alzheimer’s disease)
  • 6. Both mother and fathers can contribute to odds of having child with Down syndrome
  • 7. Odds 1 in 733 (but increase with parents’ age, especially maternal age of 35 or higher)
  • 8. As the result of more exposure to environmental hazards, aging ova and sperm more likely to be abnormal
  • 9. Delaying parenthood until 30s or 40s increases risk of chromosomal abnormalities
  • 10. Sex chromosome abnormalities
  • 11. Turner syndrome (about 1 in 3,000 females)
  • 12. Female with a single X chromosome (XO)
  • 13. Physically small, cannot reproduce, stubby fingers, lower than average spatial and math skills
  • 14. Klinefelter syndrome (about 1 in 200 males)
  • 15. Male with an extra X chromosome (XXY)
  • 16. Tend to have long limbs and, at puberty, they may show feminine characteristics such as enlarged breasts
  • 17. Fragile X syndrome
  • 18. Most common hereditary cause of intellectual disability
  • 19. One arm of X chromosome nearly detached (thus the term fragile)
  • 20. Results in intellectual disability, some form of cognitive impairment, and autism in some
  • 21. More common in males
  • 22. Too many repeated gene sequences lead to problem in formation of connections between neurons in brain
  • 23. Those who carry the gene but do not have enough repeating sequences to have full-blown fragile X, may develop tremors or have problems with infertility or early menopause in middle age
• K. Genetic Diseases and Their Diagnosis
  • 1. Genetic counseling assesses risk concerning potential for genetic problems
  • 2. Counselors report percent probability of having a child with a disorder
  • 3. Carriers do not have the disease but can transmit the gene for it to their children.
  • 4. Common prenatal screening procedures used by counselors include ultrasound, amniocentesis, chorionic villus biopsy, maternal blood sampling, preimplantation genetic diagnosis (Application 2.1 Prenatal Detection of Abnormalities)
  • 5. Prenatal diagnosis
  • 6. Ultrasound—visual image of fetus
  • 7. Amniocentesis—sample of amniotic fluid analyzed for genetic material and other problems
  • 8. Chorionic villus sampling—extract hair cells from chorion surrounding fetus and check for genetic defects
  • 9. Maternal blood sampling—check fetal blood cells that entered mom via placenta
  • 10. Preimplantation genetic diagnosis—allow conception via in vitro fertilization, check DNA of first cells

III. Studying Genetic and Environmental Influences (p. 49)

• A. Basics on genetics
• B. Behavioral genetics—study of the extent to which genetic and environmental differences are responsible for a given trait
  • 1. Impossible to give a specific percentage about how much the environment (or genetics) contributes to a specific trait
• C. Heritability—estimated proportion of trait variability attributable to genes
  • 1. To say that intelligence is heritable is to say that differences in intelligence between people are to some degree attributable to differences in genetic endowments
• D. Twin, Adoption, and Family Studies
  • 1. Twins studies
    • 2. Compare identical twins (share 100% of genes) and fraternal twins (share an average of 50% of genes) raised together and reared apart
    • 3. Criticisms of this approach include problem of shared prenatal environment and more similar treatment of identical twins vs. fraternal twins
  • 4. Adoption study
    • 5. Similar environments and different genes
    • 6. If adopted children like adoptive parents, characteristic said to be due to experience, but if they are not like adoptive parents, characteristics said to be due to genetic factors
  • 7. Criticisms of the approach
    • 8. Maternal prenatal environment could also impact development
    • 9. Must correct for tendency of adoption agencies to place children in above-average environments
  • 10. Family studies
    • 11. Compare various members within family (e.g., half-siblings, unrelated siblings from stepfamilies)
• E. Estimating Influences
  • 1. Concordance rates—percent of pairs in which, if when one has trait, so does the other
  • 2. Plomin and colleagues’ behavioral geneticists estimated impact of three factors on emotionality
  • 3. Genes
  • 4. Some support for heritability of emotionality
  • 5. Shared environmental influences—common work or home experience
  • 6. Very weak support for influence on emotionality
  • 7. Nonshared environmental influences—unique experiences not shared by other family members (e.g., differential treatment by parents, different life crises)
  • 8. Support for the impact of unique experiences on emotionality
  • 9. Failure to find a strong shared environmental influence on a behavior does not mean family influences are unimportant
  • F. Molecular Genetics
    • 1. Analysis of particular genes and their effects
    • 2. Identify how much multiple genes contribute to polygenetic traits
    • 3. Selected Behavioral Genetics Findings

IV. The Heritability of Different Traits (p. 53)

• A. Intellectual Abilities
  • 1. Overall heritability of IQ around .50
  • 2. Evidence for impact by genetics, shared, and nonshared environments
  • 3. Pairs of family members reared together have more similar IQ than those reared apart
  • 4. Fraternal twins, especially those with similar family experiences who grew up at the same time, tend to be more alike than siblings born at different times
  • 5. IQs of adopted children related to those of their adoptive parents
  • 6. Shared environment influences tend to make people more similar while unique nonshared experiences make them different
  • 7. Influence of genes becomes greater with age until adulthood
  • 8. With age, IQ’s estimated heritability increases for identical twins
  • 9. With age, IQ’s estimated heritability decreases for fraternal twins
  • 10. Shared environmental influences become less significant with age
  • 11. IQs of adopted children more strongly correlated with biological and adoptive parents
  • 12. IQ levels of children (including adopted children) can be improved if they are raised in a stimulating home environment early in life
  • 13. Genes largely account for intellectual stability from early adulthood to late middle age
  • 14. Genetic influences on intelligence still strong in old age
  • 15. Heritability may diminish in old age as the result of disease and nonshared environmental experience
• B. Temperament and Personality
  • 1. Temperament—set of tendencies concerning emotional reactivity, activity, and sociability
  • 2. Findings indicate that living in the same home does not make children more similar in personality

V. Gene–Environment Interplay (p. 56)

• A. Gene–Environment Interactions
  • 1. Genes do not determine anything but rather provide potential
  • 2. Predisposition to depression results in somewhat higher probability of having depression
  • 3. Individuals with two high-risk variants for depression more vulnerable to depression than those with protective variant only if they experience multiple stressful events
  • 4. Multiple stressful events tend to not result in schizophrenia in those with protective genes
    5. Often takes combination of high-risk genes and a high-risk environment to trigger many psychological problems
• B. Diathesis–stress model—psychological disorder results from an interaction of a person’s predisposition or vulnerability to problems and the experience of stressful events
• C. Differential susceptibility hypothesis—some people’s genes make them more reactive than other people to environmental influences, whether good or bad ones
• D. Gene–Environment Correlations
  • 1. Gene–environment interactions tell us that people with different genes react differently to the experiences they have.
  • 2. Gene-environment correlations say that people with different genes experience different environments.
• E. Passive gene–environment correlations—parents’ genes influence the environment they provide for children, as well as the genes the child receives
• F. Evocative gene–environment correlation—child’s genotype evokes certain reactions
• G. Active gene–environment correlation—child’s genotype influences the environment that they seek
• H. Implications for gene–environment correlations
  • 1. Behavioral geneticists are discovering that measures of environment are heritable.
  • 2. Genes influence how parents, peers, and others treat children.
  • 3. Environmental influences then contribute to the individual’s development.
• I. Epigenetic Effects on Gene Expression
  • 1. Epigenetic effects
  • 2. Environmental factors affect whether or not particular genes in particular cells are expressed
  • 3. Might explain differences in identical twins
  • 4. Controversies surrounding genetic research
  • 5. Gene therapy—involves substituting normal genes for the genes associated with a disease or disorder6. Developmentalists question the idea of trying to separate the influences of genes and environment on differences between individuals

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