CHAPTER 3

GENES, ENVIRONMENT, AND DEVELOPMENT

Learning Objective

• What do evolution and species heredity contribute to our understanding of universal patterns of development?

Species Heredity

• Genetic endowment that members of a species have in common– Reason that certain patterns of

development and aging are universal

• Examples– Walk and talk around 1 year of age– Sexual maturation from 12-14– Wrinkling of skin in 40s and 50s

Darwin’s Theory of Evolution

• Species heredity explained by evolutionary theory

• Main arguments of Darwin’s theory– There is genetic variation in a species– Some genes aid adaptation more than

others do– Adaptive genes passed on more frequently

• Evolution is about the interaction between genes and environment

Modern Evolutionary Perspectives

• Human development influenced by a shared species heredity that evolved through natural selection

• Human development also influenced by cultural evolution– Humans inherit a characteristically human

environment and learn methods of adapting to the environment

– Cultural evolution is based on learning and socialization

Learning Objectives

• What are the basic elements and processes of individual heredity, including genes, chromosomes, formation of a zygote, mitosis, and meiosis?

• What has the Human Genome Project contributed to our understanding of human development?

Individual Heredity – The Genetic Code

• Conception – new cell nucleus formed from genetic material of ovum and genetic material of sperm– New cell is the zygote

• Each parent contributes 23 chromosomes, 46 total, organized into 23 pairs– Chromosomes - threadlike bodies in

nucleus of each cell made up of genes

Individual Heredity – The Genetic Code

• Sperm and ova each have only 23 chromosomes because they are produced through the cell division process of meiosis – A reproductive cell (in ovary or in testis)

splits to form two 46-chromosome cells– The two cells split again to form a total of

four cells, each of which receives 23 chromosomes• In the female, results in three

nonfunctional cells and one ovum• In the male, results in four sperm

Individual Heredity – The Genetic Code

• The single-cell zygote becomes a multiple-cell organism through the process of mitosis– A cell (and each of its 46 chromosomes)

divides to produce two identical cells, each containing the same 46 chromosomes

– Following conception, through mitosis, the zygote divides into two cells, then into four cells, then into eight cells, etc.

• Mitosis continues throughout life

Individual Heredity – The Genetic Code

• Except for ova and sperm, all normal human cells contain copies of the 46 chromosomes received at conception– Both members of a chromosome pair

influence the same characteristics

• Chromosomes are strands of DNA– Deoxyribonucleic acid made up of

sequences of adenine, cytosine, guanine, and thymine (A, C, G, and T)

– Sequences are functional units - genes

The Human Genome Project

• Researchers mapped the sequence of the chemical units (A,C,G, and T) that make up the strands of DNA in a full set of human chromosomes

• Human genome consists of genes that serve as a template for the production of particular proteins and DNA that regulates the activity of the protein-producing genes

• 999 of 1,000 base chemicals are identical; only 1 of 1,000 accounts for human differences

Learning Objectives

• What factors account for genetic uniqueness?

• How genetically similar are twins?

• How is sex determined?

Genetic Uniqueness and Relatedness

• Genetic uniqueness of children of same parents due to crossing over, an aspect of meiosis– Before separation, pairs of chromosomes

line up; when they cross each other, parts are exchanged

– Crossing over increases the number of distinct sperm or ova an individual can produce

Genetic Uniqueness and Relatedness

• Genetic similarity of parent and child– An individual receives half her

chromosomes and genes from her mother and half from her father

• Genetic similarity of siblings– Siblings receive half their genes from the

same mother and half from the same father, but meiosis determines the genes actually received

– Siblings share half their genes, on average

Genetic Uniqueness and Relatedness

• Identical twins or monozygotic twins (or identical triplets, etc.) – Genetically the same– Result when one fertilized ovum divides to

form two or more genetically identical individuals

• Fraternal twins or dizygotic twins – Result from release of two ova and

fertilization by two sperm– As genetically alike as siblings

Determination of Sex

• Of the 23 pairs of chromosomes– 22 pairs - autosomes– 23rd pair - sex chromosomes

• In males, the 23rd pair consists of a long chromosome (X) and a short chromosome with fewer genes (Y)

• Females have two X chromosomes• Fathers determine a child’s sex

– If a sperm carries a Y chromosome, the XY zygote is a genetic male

– If a sperm carries an X chromosome, the XX zygote is a genetic female

• Caption: Chromosomes in each cell consist of strands of DNA

Learning Objective

• How are genes translated into physical and psychological characteristics?

Translation and Expression of the Genetic Code

• Environmental influences combine with genetic influences to determine how a genotype is translated into a phenotype– Genotype

• The genetic makeup a person inherits– Phenotype

• The characteristics or traits that are expressed

– Gene expression• Activation of particular genes in particular

cells at particular times; guided by genetic influences and affected by environmental factors

Learning Objectives

• What are the mechanisms by which traits are passed from parents to offspring?

• What is an example of how a child could inherit a trait through each of these three mechanisms?

Mechanisms of Inheritance:Single Gene-Pair Inheritance

• Single gene-pair inheritance– Pattern described by Gregor Mendel– Human characteristics influenced by one

pair of genes (one from mother, one from father)• A dominant gene trait will be expressed• A recessive gene trait will be expressed

only when the gene is paired with another recessive gene for the trait

• Caption: Can you curl your tongue?

Mechanisms of Inheritance:Sex-Linked Inheritance

• Sex-linked characteristics are influenced by single genes located on sex chromosomes– Actually X-linked because most attributes

are associated with genes on the X– Y chromosomes are smaller and have

fewer genes to serve as counterpart or to dominate• Example: if a boy inherits a recessive

color-blindness gene on the X, there is no color vision gene on the Y to dominate the color-blindness gene

Mechanisms of Inheritance:Sex-Linked Inheritance

• A female who inherits a color-blindness gene usually has a normal color-vision gene on her other X chromosome that can dominate the color-blindness gene– If a female is to be color-blind, she must

inherit the gene on both Xs

• Hemophilia is another condition that illustrates the principles of sex-linked inheritance

• Caption: X-linked inheritance

Mechanisms of Inheritance:Polygenic Inheritance

• Traits such as height, weight, intelligence, personality, and susceptibility to cancer and depression are polygenic– Influenced by multiple pairs of genes

interaction with environmental factors

• Many degrees of expression are possible in polygenic traits– Traits tend to be distributed in the

population according to the normal curve

Learning Objectives

• How do genetic mutations occur?

• How do mutations affect development?

• What are the most common chromosomal abnormalities?

• When chromosomal abnormalities occur, how do they affect development?

Mutations

• Mutation– A change in gene structure or arrangement

that produces a new phenotype

• May be harmful or beneficial depending on their nature and the environment – Example: sickle-cell disease

• Can be inherited by offspring

Chromosomal Abnormalities

• Chromosomal abnormalities occur when there are errors in chromosome division during meiosis– The ovum or sperm will have too many or

too few chromosomes– Chromosomal abnormalities are the main

source of pregnancy loss

Examples of Chromosomal Abnormalities

• Down syndrome or trisomy 21– 21st chromosomes = 3– Children have distinctive physical

characteristics and typically are classified as having some degree of mental retardation

– Associated with older age of parents

Examples of Chromosomal Abnormalities

• Chromosomal abnormalities that involve a child receiving too few or too many sex chromosomes

– Consequence of errors in meiosis or damage from environmental hazards

– Turner syndrome – a female born with a single X chromosome (XO)

– Klinefelter syndrome – a male born with one or more extra X chromosomes (XXY)

– Fragile X – one arm of the X is only barely connected

• Caption: Photo of X chromosome with fragile X problem

Learning Objectives

• What methods are used to screen for genetic abnormalities?

• What are the advantages and disadvantages of using such techniques to test for prenatal problems?

• What are some abnormalities that can currently be detected with genetic screening?

Genetic Diagnosis and Counseling

• Genetic counseling– A service that helps people understand

and adapt to the implications of genetic contributions to disease

– Carriers do not have the disease but can transmit the gene for it to their children

Issues in Genetic Diagnosis and Counseling

• Sickle-cell disease– Sickle-shaped blood cells cluster together

and distribute less oxygen through the circulatory system

– If parents are carriers of the recessive gene for sickle-cell disease• Have 25% chance of having a child with

sickle-cell disease• Have a 50% chance of having a child

who will be a carrier

Issues in Genetic Diagnosis and Counseling

• Huntington’s disease– Associated with a single dominant gene– Strikes in middle age and disrupts the

nervous system– Child of a parent with Huntington’s disease

has a 50% chance of developing the disease

– Discovery of gene for Huntington’s on chromosome 4 led to a test that can reveal if a person has inherited the gene

Learning Objectives

• What methods do scientists use to discern the contributions of heredity and environment to physical and psychological traits?

• What are the strengths and weaknesses of these methods?

• How do scientists estimate the influences of heredity and environment to individual differences in traits?

• How do genes, shared environment, and nonshared environment contribute to individual differences in traits?

Genetic and Environmental Influences: Behavioral Genetics

• Behavioral genetics – Study of the extent to which genetic and

environmental differences contribute to differences in traits

– Use heritability estimates

• The proportion of all the variability in the trait within a large sample that can be linked to genetic differences among individuals

• Variability that is not associated with genetic differences is associated with environmental and experiential differences

Studying Genetic and Environmental Influences: Experimental Breeding

• Selective breeding– Attempting to breed animals for a particular

trait to determine whether the trait is heritable

• Tryon bred maze-bright rats to demonstrate the influence of genetics upon maze-learning ability

• Other animal breeding studies showed genetic contributions to activity level, emotionality, aggressiveness, etc.

Studying Genetic and Environmental Influences: Twin, Adoption, and Family

Studies• Twin studies

– Studies of twins, both identical and fraternal, reared apart and together

• Adoption studies– Are adopted children more similar to

biological parents or to adoptive parents?

• Family studies – Studies of siblings with different degrees of

genetic similarity and varying environments

Estimating Influences

• Concordance rates

– Statistical calculations to estimate the degree to which heredity and environment account for individual differences in a trait of interest

– A trait is heritable if the concordance rates are higher for more genetically related than for less genetically related pairs of people

• Correlation coefficients

– Used when a trait (e.g., intelligence) can be present in varying degrees

Estimating Influences

• From correlations reflecting the degree of similarity between twins, behavioral geneticists can estimate the contributions to individual differences in emotionality from– Genes– Shared environmental influences

• Common experiences– Nonshared environmental influences

• Experiences unique to the individual

Molecular Genetics

• Analysis of genes and their effects– Useful in identifying the multiple genes that

contribute to polygenic traits– Example: Alzheimer’s disease

• Twin studies show heritability but the roles of specific genes have not been clarified

Learning Objectives

• How do genes and environments contribute to individual differences in intellectual abilities, personality and temperament, and psychological disorders?

• What do researchers mean when they talk about the heritability of traits?

• Which traits are more strongly heritable than others?

Findings from Behavioral Genetics Research – IQ

• Correlations highest in identical twins– Heritability of IQ is about .50

• Genetic differences account for 50% of variation in IQ and environmental differences for 50%

– Genetic endowment appears to gain importance from infancy to adulthood as a source of individual differences in IQ

– Shared environmental differences become less significant with age

Findings from Behavioral Genetics Research – Temperament and Personality

• Temperament – Tendencies to respond in predictable ways

that serve as the building blocks of personality• Buss and Plomin (1984) reported average

correlations of .50 - .60 between temperament scores of identical twins but scores for fraternal twins not much greater than zero– Conclusion: living in the same home does not

generally make children more similar in personality

– Shared environment influences are important but nonshared influences are more important

Findings from Behavioral Genetics Research – Psychological Disorders

• Schizophrenia

– Concordance rate for schizophrenia in identical twin pairs is 48% and for fraternal twins the rate is 17%

– 90% of children who have one parent with schizophrenia do not develop schizophrenia

• This means that environmental factors contribute significantly

– People inherit predispositions to develop disorders, not the disorder per se

Heritability of Different Traits

• Traits that are strongly heritable include– Physical characteristics – eye color, height,

weight– Physiological functions – measured brain

activity, reactions to alcohol, level of physical activity, susceptibility to certain diseases

• A trait that is moderately heritable– General intelligence

• Traits that are less heritable– Aspects of temperament and personality,

susceptibility to many psychological disorders

Influences on Heritability Estimates

• Characteristics of the sample studied– Age, environmental factors

• Environmental factors– Socioeconomic status

• Caption: Correlations between the traits of identical twin raised apart in to Minnesota Twin Study

Learning Objectives

• What is an example that illustrates the concept of a gene-environment interaction?

• What are three ways that genes and environments correlate to influence behavior?

• What are the major controversies surrounding genetic research?

Gene-Environment Interactions

• The effects of genes depend upon the nature of the environment and how the individual responds to the environment– Often takes a combination of high-risk

genes and high-risk environment to trigger psychological problems

Gene-Environment Correlations

• Three kinds of gene-environment correlations– Passive – the home environment that

parents provide their children is correlated with the children’s genotypes

– Evocative gene-environment correlations

• Child’s genotype evokes certain types of reactions from others

– Active gene-environment correlations

• Children’s genotypes influence the kinds of environments they seek

Genetic Influences on Environment

• Notion that people’s genes are correlated with and possibly influence their life experiences– Measures of environment are heritable

• Especially family environments of children

– Evidence that those who have closest genetic similarity are also similar in the environments they experience and in their perceptions of those environments

Controversies Surrounding Genetic Research

• Accomplishments in the study of genetics have led to debates about procedures, public policy, and ethical issues– Reproductive technology, cloning, gene

therapy, stem cell research

• Some researchers may have overstated the importance of genes and underestimated the importance of family in development