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Chapter 2 Lecture Notes and Behavior
Genetics
Meiosis
 Meiosis: This is division of the germ cells
(i.e., the male or female reproductive cells)
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The result is that, for a male, the sperm (or
egg) cells contain half the number of
chromosomes (N=23) that normal cells in
other parts of the body contain (N=46).
Meiosis results in sperm (or egg) cells that
are a more or less random collection of
one each of the chromosomes from Mom
and Dad;
Meiosis
Meiosis
 The figure includes crossover: half of the
m and d chromosomes interchange
genetic material with each other.
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These are indicated by mc and dc
respectively.
The female germ cell undergoes a similar
process, so the actual number of possible
genetic combinations between one man
and one woman is staggeringly high.
Meiosis for a male: His germ cells start out with 23 pairs
of chromosomes, one set from mom (m) and one set from
dad (d), with crossover for half of germ cells.
After Meiosis
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Meiosis for a female: Her germ cells start out with 23
pairs of chromosomes, one set from mom (m) and one set
from dad (d), with crossover for half of germ cells.
After Meiosis
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Definitions: Mitosis, Autosome
 Mitosis: Normal cell division in non-germ
cells.

Mitosis replicates the previous cell unless
there are errors.
 autosomes: The 22 paired non-sex
chromosomes.
Mitosis: Normal (non-sex cell)
division
Definition: Allele: an alternative form of a gene at a particular place
on a chromosome. Genes cannot be alleles if there are no alternative
forms of the gene.
 If the alleles are homozygous, the two
genes at the locus are the same.
 If the alleles are heterozygous, the two
genes at the locus are different.
Definitions: Co-dominant, Dominant,
Recessive
 Co-dominant: Both alleles at a locus are
expressed with equal force; e.g., the ABO
blood system: If you are AB, you express
both alleles
 Dominant: Only one allele at a locus is
expressed. The other (recessive gene) is
suppressed.
 Recessive: An allele that is suppressed in
the presence of a dominant gene. If the
two recessive alleles occur at a locus,
then the trait is expressed.
Definitions: Sex Chromosomes, Xlinkage
 Sex chromosomes:
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The 23rd pair of chromosomes determine sex: Males are
XY and females are XX.
Non-sex chromosomes are autosomes
 X-linkage: Genes carried on the X-chromosome.
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Since males only have one X chromosome, any
recessive gene on the X-chromosome will be expressed
in males.
In females, such recessive genes may be suppressed by
a dominant gene on her other X-chromosome.
X-linkage explains why males are prone to disorders
caused by recessive genes on the X chromosome.
Hemophilia is an X-linked disorder.
Definition: Modifier genes
 Modifier genes: Genes that influence a
trait indirectly.
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For example, a dominant gene affects
whether people can get early cataracts, but
modifier genes determine how serious the
cataracts are likely to be.
Often these modifier genes are located on
different chromosomes.
 Genes work together.
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Most traits studied by psychologists are
influenced by multiple genes (polygeny)
Most genes influence more than one trait
(pleiotropy).
Recessive Genes
 Recessive genes are hard to get rid of,
even if they are highly deleterious or even
lethal.
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That's because they are typically masked
by dominant genes.
Even if both parents are recessive for a
harmful gene like PKU, only 25% of their
offspring would be homozygous for the
trait.
Recessive and Dominant Genes:
PKU
C Dominant gene for curly hair
S Recessive gene for straight hair
Father
Mother
C
C C
C
C
Curly hair
C
S
Recessive - Straight
C
C
Curly hair
S
C
S
Recessive - Straight
Recessive and Dominant Genes
 What would the above figure look like if only the
mother was a carrier?
 What would it look like if one parent was
homozygous for the recessive gene?
 What would happen to all of this if PKU was
dominant instead of recessive?
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Some disorders are caused by dominant genes.
Huntington's chorea is a lethal gene, but it's effects
don't show up until age 35-40.
Why would this type of lethal gene survive in the
population?
Heterozygous genes
 Some recessives are actually beneficial if
the person is heterozygous for the
condition.
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The example for the latter is Sickle Cell
Anemia.
Sickle Cell Anemia benefits people living in
areas infested by malaria.
Heterozygotes are superior to either
homozygous condition.
Heterozygous genes: Sickle Cell
Chromosomal Abnormalities
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Down Syndrome
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Sex-Chromosome Anomalies:
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Turner Syndrome (XO pattern)
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Triple-X (XXX pattern)
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Klinefelter’s syndrome
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Fragile X syndrome
Down Syndrome
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Down syndrome is caused by nondisjunction of the 21st chromosome during
meiosis.
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As a result, the child has three 21st
chromosomes instead of the normal two.
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Down Syndrome is a genetic disorder but it
is not caused by having a harmful gene
Chromosomal disorders: Down
syndrome
Chromosomal disorders: Down
syndrome karyotype
Normal Karotype
BEHAVIOR GENETICS
 Behavior genetics challenges the behaviorist
perspective of John Watson who famously wrote:

"Give me a dozen healthy infants, well-formed, and
my own specific world to bring them up in and I'll
guarantee to take any one at random and train him
to become any type of specialist I might select — a
doctor, lawyer, artist, merchant-chief and, yes,
even into beggar-man and thief, regardless of his
talents, penchants, tendencies, abilities, vocations
and race of his ancestors."
Heredity-Environment
Interactions
 Debate in human development:

Role of heredity and maturational factors
Role of learning and experience
vs.

After 1926, Watson and behaviorists flourish

Contemporary psychologists see interaction of
both in development – genes help shape
environment and environments influence
genes
BEHAVIOR GENETICS
 1.) BEHAVIOR GENETICS STUDIES IDIOGRAPHIC
DEVELOPMENT: What combination of genetic and
environmental on average affect where people are on the
normal curve.
BEHAVIOR GENETICS
 2.) BASIC CONCEPTS:
PHENOTYPE = OBSERVABLE OR MEASURABLE
CHARACTERISTICS

HAIR COLOR, IQ
 GENOTYPE = GENETIC COMPLEMENT OF
PERSON

HAVING A RECESSIVE GENE FOR COLOR
BLINDNESS
 POLYGENY: MANY GENES INFLUENCE A TRAIT
BUT NO ONE GENE HAS A MAJOR EFFECT
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HUNDREDS OF GENES INFLUENCE IQ AND
PERSONALITY;
Few, if any cause more than 1% of the variation
GENOTYPE → ENVIRONMENT
INTERACTIONS: Active
 GENOTYPE → ENVIRONMENT correlations: WAYS THAT
GENETIC TENDENCIES BECOME CORRELATED WITH
PARTICULAR ENVIRONMENTS.
 1.) ACTIVE GENOTYPE → ENVIRONMENT correlation
(NICHE-PICKING):
 CHILD SEEKS OUT ENVIRONMENTS AS A RESULT OF
GENETIC INFLUENCES
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CHILD WITH A SENSATION SEEKING TEMPERAMENT
The active genotype-environment interaction probably
increases in importance as the child grows older. Why?
GENOTYPE → ENVIRONMENT
INTERACTIONS: Evocative
 2.) EVOCATIVE GENOTYPE →
ENVIRONMENT correlation: CHILD
EVOKES ENVIRONMENTS AS A RESULT
OF GENETIC INFLUENCES

CHILD WITH A DIFFICULT
TEMPERAMENT EVOKES NEGATIVE
RESPONSES IN CAREGIVERS; CHILD
WITH A SUNNY DISPOSITION GETS
POSITIVE RESPONSES.
GENOTYPE → ENVIRONMENT
INTERACTIONS: Evocative
 2.) EVOCATIVE GENOTYPE → ENVIRONMENT
correlation: CHILD EVOKES ENVIRONMENTS AS A
RESULT OF GENETIC INFLUENCES
 Children with violent natural parents and children with
non-violent natural parents are adopted into separate
families, and the adoptive families are compared.
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Children with violent natural parents have adoptive parents
who use harsh discipline.
Children with non-violent natural parents have adoptive
parents who use mild discipline.
This "environmental" effect is the result of an evocative
genotype → environment effect: Aggressive, difficult
children evoke harsh parenting.
GENOTYPE → ENVIRONMENT
INTERACTIONS: Passive
 3.) PASSIVE GENOTYPE ENVIRONMENT
correlation:
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CHILD IS PASSIVE RECIPIENT OF
ENVIRONMENTS WHICH FIT WITH
HIS/HER GENOTYPE.
INTELLIGENT PARENTS HAVE CHILD
WITH GENETIC POTENTIAL FOR
INTELLIGENCE;

PARENTS ALSO PROVIDE A GREAT DEAL OF
INTELLECTUAL STIMULATION WHICH
MESHES WITH THE CHILD'S GENETIC
POTENTIAL.
GENOTYPE → ENVIRONMENT
correlations: Passive
 3.) PASSIVE GENOTYPE ENVIRONMENT
correlation:
P=parent
C=Child
g=genes
e= environment
P
g
e
C
Shared and Unshared Environmental
Influences
 SHARED ENVIRONMENTAL INFLUENCES:
ENVIRONMENTAL INFLUENCES SHARED BY
CHILDREN IN THE SAME FAMILY.

EXAMPLE: CHILDREN IN SAME FAMILY GO TO
SAME SCHOOL, HAVE SAME ALCOHOLIC
MOTHER OR AFFECTIONATE MOTHER, ETC
 UNSHARED ENVIRONMENTAL INFLUENCES:
DIFFERENT CHILDREN RECEIVE DIFFERENT
ENVIRONMENTS; OR DIFFERENT CHILDREN
RESPOND TO THE SAME ENVIRONMENT
DIFFERENTLY;

EXAMPLE: DIFFERENT BIRTH ORDER, SEX
DIFFERENCES, PEER RELATIONSHIPS
Shared and Unshared Environmental
Influences
 UNSHARED ENVIRONMENTAL INFLUENCES: DIFFERENT
CHILDREN RECEIVE DIFFERENT ENVIRONMENTS; OR
DIFFERENT CHILDREN RESPOND TO THE SAME
ENVIRONMENT DIFFERENTLY;
 EXAMPLE: DIFFERENT BIRTH ORDER, SEX
DIFFERENCES
 According to Plomin et al. (2001), unshared environmental
influences are typically far more important than shared
environmental influences.
 Notice that the unshared environment idea is linked to the
active child concept (Why?) and is used to explain the fact
that adoptive siblings and even biologically related siblings
are typically not very similar.
 As the text says, 'Clearly, researchers in individual
differences can no longer assume a homogeneous home
environment for all siblings; be alert to this fact when you
read the reports and conclusions of such studies.'
Reaction Range: The range of phenotypic expression for
a genotype depending on different environments of
different quality.
Canalization
 CANALIZATION: THE GENETIC RESTRICTION OF
A PHENOTYPE TO A SMALL NUMBER OF
DEVELOPMENTAL OUTCOMES, PERMITTING
ENVIRONMENTAL INFLUENCES TO PLAY ONLY
A SMALL ROLE IN THESE OUTCOMES;
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Genes restrict the extent to which the environment
can influence the phenotype.
A highly canalized phenotype is not much
influenced by environmental influences.
A weakly canalized phenotype is open to
environmental influences.
Canalization
Behavior Genetics Methods: Adoption
Studies
 Adoption Studies: Comparison of adopted
children to natural and adoptive families.
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Correlations with natural family indicate
genetic influence;
correlations with adoptive family indicate
environmental influences.
Assumption: environment before adoption
does not systematically affect the behavior
studied.
Behavior Genetics Methods: Adoption
Studies
 Findings: Adopted children's IQ scores are
correlated with those of their biological parents,
indicating genetic influence.
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Their IQ scores are also correlated with their
adoptive parents at age 7, indicating environmental
influence.
However, at age 18, there is no correlation between
adopted children and their adoptive parents after
18 years of living together!!!
Not on test: Notice that, again, as noted in Table 24 on p. 61, there is a distinction between beneficial
effects of adoption on the average IQ of adoptees
while nevertheless, the adoptees remain more
highly correlated with their biological parents.
 Correlations are independent of the mean.
Behavior Genetics Methods: Adoption
Studies
 The findings of adoption studies are
influenced by the range of the subject
population.
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Researchers try to study children adopted
into a wide range of environments so that
they will not underestimate the effects of
environmental differences.
In general, a wider range, especially if it
included abusive, highly stressful
environments, would usually result in a
lower estimate of genetic influences. Why?
Behavior Genetics Methods: Twin
Studies
 Monozygotic (MZ) (identical) twins: Twins
that share the same sperm and egg.
 Dizygotic (DZ) (fraternal) twins: Twins that
have different sperm and egg.

Genetically they are no more alike than any
other two siblings.
Behavior Genetics Methods: Twin
Studies
 Environmental influences are indicated if MZ twins and DZ
twins have about the same correlations for a trait.
 Genetic influences are indicated if MZ twins are
substantially more similar than dizygotic twins.
 Assumption: Environmental influences do not tend to make
MZ twins more similar than DZ twins.
 This would occur if parents treat MZ twins more
similarly than DZ twins because, for example, it's cute to
have them dress alike and if this procedure actually
made their personalities or IQ more similar.
 One way to get around this is to study MZ twins reared
apart, especially if they are reared in radically different
environments.
Behavior Genetics Methods: Twin
Studies: Table 2.5, p. 64
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Correlations for IQ scores
MZ twins reared together
MZ twins reared apart
DZ twins reared together
Siblings reared together:
Parent and Child:
Foster parent and child at age 7:
 at age 18:
Siblings reared apart:
 Cousins:
.86
.79
.60
.47
.40
.31
.00
.24
.15
THE DEGREE OF GENETIC INFLUENCE INCREASES AS
CHILDREN APPROACH ADULTHOOD.
Genetic Influences are stronger in adulthood than among children.
 1.) CORRELATIONS OF MZ TWINS STAY
HIGH WHILE CORRELATIONS FOR DZ TWINS
DECLINE
 IQ CORRELATIONS FOR MZ AND DZ TWINS
AGE
MZ
DZ
6 MO
.75
.72
12 MO
.68
.63
24 MO
.81
.73
36 MO
.88
.79
4 YR
.83
.71
6 YR
.86
.59
8 YR
.83
.66
15 YR
.88
.54
Identical Twins’ Intelligence Test Scores for MZ over the
First Two Years; scores for DZ twins would be less
similar to each other.
THE DEGREE OF GENETIC INFLUENCE INCREASES AS
CHILDREN APPROACH ADULTHOOD.
Genetic Influences are stronger in adulthood than among children.
 2.) ADOPTION STUDIES
a.) CORRELATION BETWEEN ADOPTIVE PARENTS AND
ADOPTED CHILDREN DECLINES FROM .35 TO ZERO
 AGE 7: r=0.35
AGE 17: r=0.00
 b.) IQ OF ADOPTED CHILDREN DECLINES TO CLOSE TO
THE IQ OF NATURAL PARENTS
 AGE 7: IQ OF ADOPTED CHILDREN = 110
 AGE 17: IQ OF ADOPTED CHILDREN = 95
 EXPLANATION: ACTIVE GENOTYPE → ENVIRONMENT
INTERACTION (NICHE-PICKING) BECOMES MORE
IMPORTANT AS CHILDREN GET OLDER.
Abusive Environments Lower IQ
 CONCLUSION: DIFFERENT ENVIRONMENTS WITHIN THE
"NORMAL" OR "AVERAGE" RANGE DO NOT HAVE MUCH
INFLUENCE ON IQ.
 HOWEVER, ABUSIVE ENVIRONMENTS CAN AND DO AFFECT IQ.
AVERAGE VS ABUSIVE ENVIRONMENTS
Minor Gain from
Better Normal
Environments
IQ score
_____________________________________________
Abusive “Good enough” Average
Enriched
Abusive Environments
Dramatically lower IQ
Quality of Environment
Is heritability higher at the low end of
the socioeconomic status scale?
 Text: A recent study by Turkheimer suggests less
heritability (genetic influence) at the lower end of
the socioeconomic scale, and much higher
heritability at the higher ends.
 The idea is that in better environments, the vast
majority of variation is caused by genetic
variation.
 But in poor environments, more variation is
caused by bad environments.
 However, other studies have not found this effect
except in clearly abusive environments.
Rank Order versus Average Effects
of Adoption (not on test)
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One classic adoption study showed that adopted children often averaged
20 or more IQ points higher than their biological mothers.
Because in this study the adoptive parents tended to be more highly
educated and more socially and economically advantaged than the
biological parents, this result was probably due to the more stimulating
home environment that the adoptive parents provided.
But note also that, despite this environmental influence on development,
individual differences seemed still to be substantially influenced by
genetic inheritance.
The rank ordering of the children's IQ scores more closely resembled that
of their biological mothers than that of their adoptive parents.
The children whose biological mothers had the lowest IQ scores were
likely to have lower IQ scores than the children whose biological mothers
scored higher.
Thus, although the absolute level of intellectual development was
apparently boosted by the environmental influences provided by the
adoptive parents, individual differences among the adopted children in
intellectual performance—that is, their relative standings in this regard—
appeared to stem more from their biological inheritance than from the
increased intellectual stimulation provided in their adoptive homes.
Rank Order versus Average Effects
of Adoption (not on test)
CHILDREN ADOPTED NEAR BIRTH
120
111
115
129
125
Adoptive Parents: Average IQ= 120
114
112
110
108
106
Adopted Children: Average IQ = 110
94
92
90
88
86
Natural Parents: Average IQ = 90
On average, adopted children gain 20 IQ points, but individual
differences are affected by natural parents’ IQ
Rank Order versus Average Effects
of Adoption (not on test)
 TESTED AT AGE 7:
 ENVIRONMENTAL VARIATION IS IMPORTANT:


AVERAGE OF AC (110) > AVERAGE OF NP (90);
THIS IS AN AVERAGE EFFECT OF ADOPTION OF 20 IQ
POINTS.

STUDIES TEND TO SHOW THAT THIS AVERAGE EFFECT
WASHES OUT AS THE CHILDREN APPROACH
ADULTHOOD.
 GENETIC VARIATION IS IMPORTANT:
1.) AVERAGE OF AC (110) < AVERAGE OF AP (120)

2.) POSITIVE CORRELATION BETWEEN AC AND NP
Interactions between Genes and
Environments: Gottlieb’s model
Interactions between Genes and
Environments
 Interactions between genes and environments
modeled by Gottlieb's bi-directional model.

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Note that the environmental effects on the genes
refer to turning the genes on or off, not to actually
modifying the genes.
In the mallard duck, a 'genetically governed
preference' for the sounds of other ducks is
modified by exposing them to different sounds
before they are born.
The idea would be that a different gene would be
turned on, not that the duck's genes are changed.
Interactions between Genes and
Environments
 Interactions between genes and environments
modeled by Gottlieb's bi-directional model.

Tennis and testosterone: Loss affects
 behavior (slumping posture)
 psychological state (low self-esteem)
 neural activity in certain parts of the brain;
 the genes for testosterone are turned off.

But the genes haven't been changed by these
events.
 When the person later wins a match, the genes may
turn on again.
Genes sometimes determine whether
environment has an influence
 The developmental stage of the child
affects whether the environment has an
influence



Example: Critical periods for the effects of
teratogens on the fetus.
Environmental influences are dependent
on timing.
Genetic systems determine when the
environment may have an influence.
Genes sometimes determine whether
environment has an influence
 The developmental stage of the child
affects whether the environment has an
influence
 PKU: Effects of dietary intervention on
PKU depend on when the intervention is
attempted. Earlier intervention is more
effective.
Diet and Intelligence in PKU
children
“Heritability factors”
 Heritability: A statistical estimate of the contribution of genetic
differences to phenotypic differences in traits, such as
intelligence or personality.
 Traits with high heritability are mainly influenced by genetic
differences:

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
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Most of the differences we see among people are the result of
genetic differences.
Low heritability is the opposite: Most of the differences we see
among people are the result of environmental differences.
Heritability may be different for different groups and under
different environmental conditions.
Heritability may change with age. (In general, genetic influence
becomes stronger with age.)
What would happen to heritability if everyone was raised in the
same environment?


Since there are no environmental differences, all of the differences
in the population would be due to genetic differences.
This means the heritability would be 1.00—all of the variation
would be explainable by genetic differences between individuals.
“Heritability factors”
 Not on test: Heritability is a proportion of the
entire variation that is due to genetic variation
and therefore ranges between 0 and 1.
 A heritability of 1 would mean that all of the
variation is genetic, a heritability of 0 would mean
that all the differences are caused by different
environments.
 A heritability of .5 is common for many traits and
means that about half of the variation is the result
of people having different genes and about half is
the result of their being in different environments.
Environmentalists should like
Behavior Genetics
 Psychologists value behavior genetics not
just because it reveals genetic influences,
but because it also tells us lots about
environmental influences.
 Examples:


passive, evocative, and active (niche
picking) genotype → environment
interaction
shared versus unshared environmental
influences.
Table 2-4
 #2: strong genetic effects do not rule out environmental
influences.



There may be strong correlations between biological relatives for,
say IQ (indicating relatively high heritability), but children may still
show a general rise in IQ levels as a result of adoption (indicating
environmental influence).
We can fix PKU by changing a child’s diet. We can improve
people’s vision even if they have a genetic tendency to myopia.
#4: Genetic influences increase with age rather than decrease.
 #5: Genes affect developmental change, and not all genes are
turned on at birth:

Many genes, like the genes for puberty and higher cognitive
processing, are not turned on until later in development.

Puberty, baby teeth, gray hair, and Piaget's stages are influenced
by genes turning on and off during development.
Temperament
 Temperament is a person's typical mode of response to the
environment.
 Temperament is an early version of personality, and
includes things like activity level, excitability, positive
emotion, etc.
 Temperament is usually used in describing infants and
young children, but it's linked to personality in adults.
 The work of Thomas and Chess is famous but mainly
outdated.
 Difficult temperament: Children with difficult
temperament sleep and eat irregularly, are easily upset
by new situations and experience extremes of fussiness
and crying.
 Linked with behavior problems in older children.
Temperament: Rothbart’s Model
 Positive affect: Smiling, laughter, etc. (FFM



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
Extraversion)
Irritable distress: Irritability, fussiness or anger
and distress at limitations on her behavior.
Fearful distress: indexed by negative reactions to
new situations (FFM Neuroticism)
Activity level: Child's tendency to be more or less
active.
Attention span/persistence: concentration, focus
on task, distractibility (FFM Conscientiousness)
Rhythmicity: Predictability or regularity of child's
behavior patterns.
Racial differences in temperament
 Chinese, Japanese, American Indian
babies are calmer, easier to console, more
able to quiet themselves after quieting =
better inhibitory ability.
 This influences peer relations and school
performance.
Difficult Temperament
 Difficult infants tend to have more behavioral
problems later, including conduct disorder
(aggressiveness), and poor school performance.
 This could be for two reasons:
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(1) characteristics of the child (malleability);
(2) the responses such a child evokes from the
environment.
The latter is a possible example of the evocative
genotype-environment interactions.
What kinds of responses do you think difficult children
would elicit from their mothers?
Temperament: Final points
 Genetic influences on temperament increase during early
childhood.
 Goodness of fit: The degree to which a child's temperament is
matched by her environment.
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The more effectively parents accept and adapt to the child's
unique temperament, the better the fit.
What might be a good fit for a child with a difficult temperament?
For a child with a fearful temperament?
 Genetic differences influence a wide variety of traits related to
temperament and personality:
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Emotionality
Activity level
Sociability
Fears and anxieties
Attention span and persistence
Morality and respect for authority