Chapter Six: The Biological Domain: Genetics & Personality
The Human Genome
Genome: refers to the complete set of genes an organism processes
o The human genome contains between 20,000 and 30,000 genes; all located on 23 pairs
Although the number of genes humans possess is similar to the number of genes estimated for
mice and worms, the manner in which human genes get decoded into proteins turns out to be
far more variable than other species
These protein-coding games, making up roughly 2% of human genome, are only part of the story
o Many parts og the other 98% of the DNA in the human chromosomes used to be
chalked up as genetic junk because scientists believed that these parts were functionless
residue that served no purpose
o Parts of these chunks of DNA have an impact on humans, potentially affecting
everything from a person’s physical size to personality
Controversy About Genes and Personality
Why might the study of genes and personality be controversial? Ideology, eugenics, others..
o Ideology: if individuals differ in thrill seeking, e.g. care caused by specific genes, then
does this mean that we should not hold juvys responsible for stealing cars for joy rides?
If scientists trace a behaviour patter or personality trait to a genetic component,
some people worry that such findings might lead to pessimism about the
possibilities for change
o Eugenics: is the notion that we can design the future of the human species by fostering
the reproduction of persons with certain traits and by discouraging the reproduction f
persons without those traits
Many people worried that findings from genetic studies might be used to
support programs intend to prevent some individuals from reproducing or even
worse to bolster the cause of those who would advocate that some people be
eliminated in order to create a “master race”
o Even though some observed differences between people can be due to genetic
differences, this does not mean that the environment plays no role in modifying the
trait. (e.g. height, 90% genetic differences & 10% environment)
Behavioural genetics are typically not content simply with figuring out the percentage of
variance due to genetic and environmental causes
o Percentage of variance: refers to the fact that individuals vary, or are difference from
each other and this variability can be portioned into % that are due to different causes
o Also interested in the correlation between genes and environment interact and
correlate with each other What is Heritability? (and what methods do geneticists use to get their answers?)
Heritability is a statistic that refers to the proportion of observed variance in a GROUP of
individuals that can be accounted for by genetic variance.
o Describes the degree to which genetic differences between individuals cause differences
in an observed property, such as height, extraversion or sensation seeking
o It provides useful information in identifying the genetic and environmental
determinants of personality
Heritability has a formal definition: the proportion of phenotypic variance that is attributable to
o Phenotypic variance: refers to observed individual differences (height, weight,
o Genotypic variance: refers to individual differences in the total collection of genes
possessed by each person.
o Heritability of .50 means that the environmental component is .50
o Heritability of .20 means that the environmental component is .20
The environmental contribution is defined in a similar way
o The percentage of observed variance in a GROUP of individuals that can be attributed to
environmental (nongenetic) differences called environmentally
Misconceptions About Heritability
One misconception about heritability is that it can be applied to a single individual, it can’t. it
refers only to differences in a sample or population, not an individual.
Another is that it is constant. Heritability always depends on both the range of genetic
differences in the population and the environmental differences that population.
Last misconception is that heritability is an absolutely precise statistic. It is actually an estimate
of the percentage of phenotypic differences due to genetic differences.
Nature-Nurture Debate Clarified
Nature-nurture debate- the arguments about whether genes or environments are more
important determinants of personality
o At the level of an individual: no nature-nurture debate, every individual contains a
unique constellation of genes
o At the level of the population: we can disentangle the influence of genes and
Behavioural Genetic Methods
Selective breeding- studies of Humans’ Best Friend Selective breeding occurs by identifying the dogs that possess the characteristic and having
them mate only with other dogs that also possess the characteristic
o Could be physical, behavioural
Family studies: correlate the degree of genetic relatedness among family members with the
degree of personality similarity and capitalize on the fact that there are known degrees of
genetic overlap among family members.
o Parents 50% genes shared with children, siblings share 50%, grandparents &
grandchildren 25% as well as uncles aunts niece and nephew. First cousins only share
12.5% of genes.
if a personality characteristic is highly heritable then family members with greater genetic
relatedness should be more similar to each other than are family members with less genetic
Family members who share the same genes also typically share the same environment, e.g.
brother and sister shy because parents raised them that way.
Twin studies: estimate heritability by gauging whether identical twins, who share 100
percent genes are more similar to each other than are fraternal twins, who only share 50
percent of genes.
Identical twins (monozygotic MZ twins), come from a single fertilized egg (or zygote) which
divides into two at some point during gestation. No one knows why they divide, they just
do. 100% of genes shared.
Fraternal twins (dizygotic DZ twins), because they come from two eggs that were separately
fertilized. Can be the same sex or opposite sex, whereas identical will be same. 50% genes
one simple method to calculate heritability: double the difference between the MZ
correlation and DZ correlation (look @ book for formula)
o rmz- refers to the correlation coefficient computed between pairs of monozygotic
twins, and rdz- refers to the correlation