Chapter 4: Genes, Evolution, and Behavior
Chromosomes and Genes
Hippocrates suggested that semen contains not body parts, but some sort of design
for the formation of the offspring. This idea was confirmed by Gregor Mendel, a
monk whose research with garden peas in the 1860s marked the beginning of
modern genetic theory.
Heredity involves the passing on of specific organic factors. These factors might
produce visible characteristics in the offspring, or they might simply be carried for
possible transmission to another generation.
Genotype: the specific genetic makeup of an individual
Phenotype: the observable characteristics produced by that genetic endowment. It
can be affected by other genes and by the environment.
Chromosome: a tightly coiled molecule of DNA that is partly covered by protein.
The DNA portion of the chromosome carries the hereditary blueprint in units is
Alleles: alternative forms of a gene that produce different characteristics.
Dominant, Recessive and Polygenic Effects
o Genotype and phenotype are not identical, some genes are dominant
(display of gene) and some are recessive (characteristics will not show
o Polygenic transmission: a number of gene pairs combine their
influences to create a single phenotypic trait. It magnifies the number
of possible variations in a trait that can occur.
Mapping the Genetic Code
o A human has a bout 25, 000 genes, the same number as a fruit fly.
o 200 human genes may have arisen from genes that bacteria inserted
into our early ancestors.
Genetic Engineering: The Edge of Creation
o Recombinant DNA procedures: researchers use certain enzymes to
cut the long threadlike molecules of DNA into pieces, combine them
with DNA from another organism, and insert them into a host
organism. Inside the host, the new DNA combination continues to
divide and produce many copies of itself. Scientists produced human
growth genome by using this procedure.
o Gene knockout: to alter a specific gene in a way that prevents it from
carrying out its normal function. Eg, inserting genetic amterial that
will prevent neurons from responding to a particular NT and then
measure whether the animal’s ability to learn or remember is
affected. This helps to know the importance of particular transmitter
substance in relation to the behaviors of interest.
o Genetic engineering allows scientists to duplicate and alter genetic
material or to repair dysfunctional genes. Behavior Genetics Techniques
Behavior geneticisits are interested in studying how hereditary and
environmental factors combine to influence psychological characteristics.
Heritability coefficient: a statistical estimate of how much of the variability
within a group is due to genetic factors.
Heredity: the passage of characteristics from parents to offspring by way of
Heritability: how much of the variation in a characteristic within a population
can be attributed to genetic differences
You could calculate a heritability coefficient for each group and obtain
estimates for the importance of genetic factors in explaining individual
differences within each group, but your results could not explain differences
If a characteristic has higher concordance (co-occurence) in people who are
more highly related to one another, then this points to a possible genetic
contribution (even if the people have lived in different environments)
Adoption study, a person who has adopted early in life is compared on some
characteristic both with biological parents.
o if the adopted person is more similar to the biological parents, then a
genetic influence is suggested
o if the adopted person is more similar to the adoptive parents, then
environmental factors are more important
Twin studies: are one of the more powerful techniques used in behavior
o Monozygotic (identical) twins develop from the same fertilized egg, so
they are genetically identical.
o Dizygotic (fraternal) twins develop from two fertilized eggs, so they
share 50% of their genetic endowment, like siblings.
o Assuming that, if the identical twins are far more similar to each other
than are the fraternal twins, then a genetic factor is likely to be
o To test if its genetic factor or environmental factor: psychologists find
and compare sets of identical and fraternal twins who were separated
very early in life and raised in different environment.
o Behavior genetics studies also have demonstrated that environmental
factors interact with genetic endowment in important ways.
GENETIC INFLUENCES ON BEHAVIOR
Heredity, Environment, and Intelligence
Question: To what extent are differences in intelligence due to genetic factors,
and to what extent does environment determine differences in intelligence?
Genetic argument: if intelligence is totally determined by genes: any two
individuals with exactly the same genes would have identical test scores (like identical twins), but the actual data shows that test scores of identical twins
are higher than any other correlations.
The more genes people have in common, the more similar they are in IQ.
This strong evidence suggests that genes play a significant role in
Environment contributes significantly to intelligence as well
Question: how do heredity and environment interact to affect
Biological Reaction Range, the Environment, Personality, and Intelligence
-Reaction range: for a genetically influenced trait is the range of possibilities-the
upper and lower limits- that the genetic code allows.
-An individual inherits a range for potential intelligence that has upper and lower
-The diverse abilities measured by intelligence tests are undoubtedly influenced by
large numbers of interacting genes, and different combinations seem to underlie
-Studies of IQ gains associated with environmental enrichment and adoption
programs suggest that the ranges could as large as 15-20 points on the IQ scale.
-**intellectual growth depends not only on genetic endowment and environmental
advantage, but also on interests, motivation, and other personal characteristics
that affect how much we “apply ourselves”, or take advantage of our gifts and
Behavior Genetics and Personality
o Personality differences could be traced to differences in brain
development or function-eg,. A relationship b/w neuroticism and a
gene allele that increases the action of the NT serotonin, as has a
relationship b/w novelty seeking and a single gene allele that decreases
the action of the NT dopamine.
o Five Factor Model, believe that individual differences in personality can
be accounted for by variation along five broad personality dimensions
or traits known as the Big Five:
Extraversion-Introversion (sociable, outgoing vs quiet,
Agreeableness (cooperative, helpful, good-natured vs
antagonistic, uncooperative, suspicious)
Conscientiousness (responsible, goal-directed,
dependable vs undependable, careless, irresponsible)
Neuroticism (worrying, anxious, emotionally unstable
vs, well-adjusted, secure, calm)
Openness to experience (imaginative, artistically
sensitive vs, unreflective, lacking in intellectual
curiosity) o Studies indicate that b/w 40-50% of the personality variations among
ppl are attributable to genotype differences. Genetic factors account for
a significant amt of personality difference.
o Monozygotic twins are more similar to each other than are dizygotic
twins. However, this issue is complicated by the possibility that
identical twins may also have more similar experiences than fraternal
o To compare personality traits in identical and fraternal twins who
either were raised together or reared apart
Allow us to divide the total variation among individuals
on each personality trait into 3 components:
1) variation attributable to genetic factors
2) variation due to a shared family environment among
those reared together
3) variation attributable to other factors: unique individual
studies show that identical twins are far more similar in personality
traits than are fraternal twins, and it makes little difference whether
they were reared together or in different adoptive families. Family
environment had little influence on personality differences in
The individuals’ unique experience, such as school experiences, social
interactions, and individual learning experiences, was an important
factor and accounted for 36-56% of the variation in individual
Attitudes also have an inherited component.
Genes control the development and function of physical
characteristics-vestibular system, the inner ear, and other structures
that give us our sense of balance.
Genetic factors relevant for personality interact with the environment
by predisposing an individual toward particular types of activities
because of genetically influenced difference in brain activity.
Genetic influence has also been reported for a tendency to abuse
alcohol, a variety of personality disorder dimensions, seasonal mood
changes, anxiety and novelty seeing.
Environment and individual’s unique experiences are equally
EVOLUTION AND BEHAVIOR
No behavior by any organism can occur in the absence of biologically based
mechanisms that receive input from the environment, process the information, and
respond to it.
In human, these inborn mechanisms allow us to learn remember, speak a language,
perceive certain aspects of our environment at birth, respond with universal
emotions, and bond with other humans. Evolutionary psychologists also believe that important aspects of social behavior
(aggression, se roles, mate selection) are the products of evolved mechanisms (did
not evolve). What evolves are genetically produced physical structures that interact
with the demands of the environment to produce a behavior.
Evolution of Adaptive Mechanisms
Is a change over time in the frequency with which particular genes and the
characteristics they produce occur within an interbreeding population
Mutations: random events and accidents in gene reproduction during the
division of cells.- they help to create variation within a population’s physical
characteristics, making evolution possible
Darwin’s landmark contribution was in specifying the process by which
species change over time as they adapt to environmental demands.
Characteristics that increase the likelihood of survival and ability to
reproduce within a particular environment will be more likely to be
preserved in the population and therefore will become more common in the
species over time.
Natural selection acts as a set of filters, allowing certain characteristics of
survivors to become more common and those of nonsurvivors to become less
For natural selection to work, individual variation must be present in a
relevant species characteristic.
Adaptions: the products of natural selection
o Allow organisms to meet recurring environmental challenges to their
survival, thereby increasing their reproductive ability.
Tool use, bipedal locomotion, and social organization put new selection
pressures on many parts o the body. But the greatest pressure was placed on
the brain structures involved in the abilities most critical to the emerging
way of life: attention, memory, language, and thought.
Brain tripled in size, and the most dramatic growth occurred in the parts of
the brain that are the seat of the higher mental processes.
Today’s human brain is not as big as Neanderthal’s brain. Human capabilities
are not solely determined by the brain; cultural evolution is also important in
the development of adaptations. Culture provides important environmental
input to evolutionary mechanisms.
Domain-specific adaptations: designed to solve a particular problem, such
as selecting a suitable mate, choosing safe foods to eat, voiding certain
environmental hazards, detecting cheating and deception in others, and
forming cooperative alliances with other ppl. This mechanisms suggest that
the human mind is not a general, all-purpose problem solver but rather a
collection of specialized and somewhat independent modules that