Chapter 6 Genetics.
- the researchers looked for genetics and environmental differences between the youths who
were depressed and those who were not. The amazing thing is that they found- nothing!
- They found no impact of either genes or environment can rates of depression.
genotype: the genetic makeup that codes for a specific trait
Both genes and environment affect our personalities
behavioural genetics: the study of the genetic and environmental contributions to individual
differences in personality and behavior.
behavior genetics answers question how do genes and environment work together to make us
the kind of people we are.
Nature and Nurture as Allies
-the best characterization of nature-nurture issue is that nature and nurture transact. That is, genes and
environment can work separately, together or they may influence one another
genotype-environment interaction: combination of nature and nurturance
-yet, another possibility is that it may be impossible to separate the effects of genes from the effects of
genotype-environment correlation: when a genotype is exposed differently to an environment; when
personality affects the environment people find themselves in.
genotype- environment correlations may be passive, reactive or active. When we cannot
separate the affect or impact of genes from the environment.
phenotype: the manifestation of the genes; the observable physical or psychological trait which is
coded by the genes.
phenotype= genotype + environment+ gene environment correlation + gene environment interaction
Genes and Environment as Co-actors
Heritablility (2) is the amount of observed individual differences in same characteristics that can be
accounted for genetic differences. heritability refers to differences across a group or population of people, not to a specific person.
with heritability, we cannot know what contributes more to the personality of a particular
person, but we can know how much genetics and environment each account ofr differences in a
particular personality characteristic of this particular sample of people.
heritability refers to the inheritance of a particular trait in a particular population at a particular
time, so sometimes heritability estimates differ depending on the exact sample and methods
used by researchers.
environmentality (e2) estimates the extent ot which observed individual differences can be traced in
any way to individual differences in environment.
- together, heritability, environmentality and measurement error accounts for the differnces we see
among people on a given characteristic.
- the greater the heritability of a characteristic the less the environmentality. And the greater the
environmentality, the less the heritability.
Shared and Non-shared environment
environment referred only to any part of the phenotype not accounted for by genes, however,
these days researchers are trying to zero in on and specifically identify the exact aspects of an
environment that account for differences in personality among a group of people.
shared environment includes aspects of the family environment that are generally same for all children
in a household, including physical, psychological and social aspects.
physical aspects - the type of dwelling (eg. apartment vs house and its layout)
- the number of books/computers at home
- the presence of a video game system
psychological aspects - eg. home atmosephere, parenting practices, the quality of
sibling interactiosn, psychopathology( alcoholism, drug use,
depression) in home.
social variables - eg. socioeconomic status, family structure, education attainment
parents, an urban or rural setting and religion.
non-shared environment includes experiences that relatives have which makes hem different from one
-this may include:
unique experiences within the family ( being the eldest child, being the only boy,
spacing of siblings, differential parental treatments.)
outside the family (peers, teachers, sports, hobbies) -when it comes to personality, most of the environmental influence ends up being of the non-shared
- when family members resemble one another it is more due to the heredity than to shared
environment. That is, children growing up in the same family are not any more similar than children
growing up in different families.
- certainly environment is important; however, the important aspects of the environment for personality
development do not appear to be shared by family members
- it may be that family environment takes children different from one another, or it may be that
researchers are looking tooo broadly and have not identified specific aspects of the environment that
-also parents may handle children differently depending on their personality, creating a unique
environment for each child.
we estimate that heritability of a characteristic by seeing if people who have similar genes show
-if the characteristic has a strong genetic component then we would expect identical twins who
hare 100% of their genes to be more similar than strangers in that charcteristics.
We can see this more clearly in physical characteristics, but it is harder to see the effects of
genes and environment in personality.
monozygotic (MZ) twins: identical twins who have exact genetic duplicates of each other. This
happens when a fertilized egg, the zygote, split into two (or sometimes more) identical parts that
each go on to develop a fetus; they share 100% common genetic makeup.
dizygotic (DZ) twins: fraternal twins occur when two zygotes develop in utero at the same time. DZ
twins result from the fertilization of the different eggs by wo different sperm cells, hence they are
- essentially fraternal twins are no more alike than ordinary siblings, sharing about 50% of
- although twins may look alike, the only way to be sure if they are identical (MZ) or fraternal (DZ) is to
conduct a gentic test (of course, if the twins are of different sexes, then we should know that they must
be fraternal twins).
h2 = 2 (rMZ - rDZ ) - a second way of estimating heritability is to compare identical twins who have been raised in separate
- called MZA twins ( monozygotic twins raised apart)
-if such twins score similarly in a trait such as extroversion, then we know extroversion has
a strong genetic component.
h2= rMZA - second formula for estimating heritability
equal environment assumption applies only to similar treatment (environmental affects) that is related
to the specific characteristic under study.
Twins – equal environment, equal things – but it might get them differnet personality
the assumption that identical twins are not treated more alike than fraternal twins, an
assumption behind the double-the-difference between MZ and DZ twins reared apart measuere
assumption of reprentiveness: the assumption that identical twins are typical of the population on the
specific characteristic under investigation, an assumption behind double-the-difference between MZ
and DZ twins reared about measure of heritability.
Many people can be apply to population – you generale that results to everyone , twin are different with
the issue of common low birth weight/ prematurity of twins- problematic of reprentativeness.
-rMZA method of estimating heritability also has its limitations
researchers assume that adopted families of each twin are different from each other; twins may
be more alike on a certain characteristic due to selective placement during the adoption process
and not their genetics. (selective placement: the assumption that adopted families of MZ twins
raised apart are different fro each toher, that the identical twins were not purposely placed in
similar environments, an assumption behind the rMZA measure of personality; selective
placement makes it impossible to see the effect of genetics apart from the effect of
environment because it confounds the two).
adoption studies of MZA twins also assume that families who adopt are the same as families
who do not adopt.
Selective placement- twins raised apart are different from each other
Heritability of Common Personality Characteristics
A solid finding in the research – one that has been well replicated across many samples and for
both self-report and other report—is that virtually all individual differences in human behavior including cognitive abilities, personality, social attitudes, psychological interests, and
psychopathology are moderately heritable.
The heritability of personality traits ranges from .40 to .60 and is the same for both men and
Shared environment typically accounts for very little variation, whereas non-shared
environment accounts for a great deal.
The variance in personality traits typically breaks down like this:
Observed differences in personality traits= 40% Genetics + 0% Shared Environment + 40%
Non-shared environment + 20% Error
So even after we account for the 40 to 60% of variation in most personality traits that come
from genetics, there is still plenty of variance left to be explained.
According to FFM model, human personality can be described using five broad categories of
traits: neuroticism, extraversion, openness, agreeableness, and conscientiousness.
Identical twins are very similar in the five factor traits, even if they are raised apart, indicating a
moderately strong genetic component to these traits. Indeed, the heritability of all five factors is
in the .41 to .50 range, indicating that about 41 to 50% of the variation in these traits is due to
But identical twins that were raised together have the highest correlations and indeed, even
fraternal twins who are raised together look a bit like each other even though fraternal twins
share only about 50% of their genes on average. This suggests that there is a moderate effect of
the environment too.
The environment- both shared and nonshared- accounts for about 47 to 53% of the variance in
The shared environment accounts for only about 8 to 17% of the variation in these traits. Other
researchers have found similar results when looking at the individual facet traits that make up
each of the five factors.
Although each of the five factors have moderate heritability – genetics accounted for about 35
to 60% of the variance in self-reported traits – there was a great effect of nonshared
environment on these traits. In fact, shared environment accounted for the smallest amount of
variance in both self-reports and peer reports.
Then and Now: The Science of Genetics
-Johann Mendel (1822-1884) : the person who started the whole field of genetics; the founder of
-He studied the variation in plants and noticed that sometimes characteristics of plants seemed to skip a
generation. He meticulously planted and recorded details of his pea plants and counted their offspring
instead of merely generalizing the results as earlier researchers have done.
- First, each parent plant passes on one form of the gene (Mendel used the word element) for a given
characteristic to its offspring, who get two forms of the gene, one from each parent. -allele: alternative forms of the same gene which occur in pairs, one inherited from each birth parent.
- these two alleles can either be the same or different. When the alleles are different, one
characteristic will be dominant over the other. However, both alleles will be passed on to the next
For example, say a pea plant with wrinkled peas breeds with a plant that has smooth peas. The next
generation will each get some combination of alleles for smooth and wrinkly peas. If smoothness is the
dominant characteristic, then the peas will look smooth, but the plants will still carry and pass on the
recessive wrinkly pea trait to the next generation.
This concept of dominance explains the pattern of seeds Mendel observed in successive
generations. In fact, such inheritance pattern where one trait dominates over another is called
Mendel’s experiments with peas.
About 99.9% of the human DNA sequence is the same for each and every one of us, it’s that .1%
that does differ that makes us unique individuals.
Epigenetics: the study of how the environment changes the function of genes without changing
the genes themselves.
o EX: - when we come from India our genes are different , this environment effects our
genes , its not changing our genes but it has an impact
Gene: a sequence of DNA that codes for a specific trait; genes are composed of coding regions
called exons (that part of a gene which codes for a specific trait) and noncoding regions called
introns (that part of the gene which does not code for a specific trait but may orchestrate the
functioning of nearby genes in direct response to the environment).
The ends of allies – introns ( they get cut off) ( JUNK) The middle part of allie – exons ( they always stay in the middle )
Of the 3.3 billion base pairs of DNA in the human genome, only about 2 to 3% are functioning
The remainder of the DNA- nearly 2 meters of it- was once thought to do nothing because it
occurs outside genes. This so-called junk DNA is actually turning out to be