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Chapter 3

PSY100H1 Chapter Notes - Chapter 3: Behavioural Genetics, Twin Study, Gregor Mendel

Course Code
Dan Dolderman

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Chapter 3 – Genetic and Biological Foundations
- researchers can now examine DNA to predict who will develop specific disorders and t understand how certain diseases
are passed from one generation to the next
- researchers have also identified genes that predispose people to be, among other things, outgoing, sociable, and intelligent
- researchers recently developed techniques that allow them to turn specific genes on and of
- changing the expression of a single gene can have profound effects even on complex behaviours such as social interaction
- research at the biological processes that occur within the brain and body to produce experience, thought, emotion,
memory, and behaviour
- by building across the levels of analysis, we plan to demonstrate how psychological scientists conduct research to
understand different questions related to mind, brain, and behaviour
What is the genetic basis of psychological science?
- within nearly every cell in the body is the genome for making the entire organism – it is the master recipe that provides
detailed instructions for everything from how to grow a gall bladder to where to place the nose on our face
- whether a cell becomes part of a gall bladder or a nose is determined by which genes are turned “on” or ‘off’ within the
cell, and this is determined by cues form outside the cell
- the genome provides the option, and the environment determines which option is taken
- the term genetics is typically used to describe how characteristics are passed along through inheritance, but it also refers
to those processes that are involved in turning genes off and on
- chromosomestructures within the cell body that are made up of genes
- the typical human has 23 pairs of chromosomes half of each pair coming form each parent
- genes in turn are components of a substance called DNA, which consists of 2 intertwined strands of molecules o sugar,
phosphate, and nitrogen
- the precise sequence of these molecules along the DNA strand specifies an exact instruction, through the production of
RNA to manufacture distinct proteins
- proteins, of which there are thousands of different types, are the basic chemicals that make up the structure of cells and
direct their activities
- gene – the unit of heredity that determines a particular characteristic in an organisms
- the process by which the gene produces RNA and then protein is known as gene expression, in that the gene is “switched
- the gene has not only recipes for how to produce specific proteins, but is instructions for when to produce them
environment determines all of this
- the environment during its development likely causes a gene sensitive to temp to be expressed or switched on
- gene provide options, and the environment determines which option is taken
- each cell in human body contains same DNA, cut cells become specialized for different tasks or for having different
effects when specific genes are expressed
- genetic research providing a compelling new understanding of the biological basis of psychological activity
Heredity involves passing along genes through reproduction
- first clues to the mechanisms responsible for heredity were discovered by the monk Gregor Mendel around 1866
- there must be discrete units, now referred to as genes, that exist in two versions – these two versions of a gene are known
as alleles
- if the alleles differ, one of them is dominant and the other is recessive
- dominant genes – a gene that is expressed in the offspring whenever it is present
- recessive – a gene that is expressed only when it is matched with a similar gene from the other parent
- genotype and phenotype
o the existence of dominant and recessive genes means that not all genes are outwardly expressed
o genotype – the genetic constitution determined at the moment of conception
o phenotype – observable physical characteristics that result form both genetic and environmental influences
- polygenic effects
o when there s a range of variability within a pop for certain characteristics, this indicates that the characteristic is
polygenic, that is, influenced by may genes, as well as the environment
o most human traits and diseases are polygenic
Genotypic variation is created by sexual production
- each ahs a specific combination of genes that differ in part due to random cell division that occurs prior to reproduction
- reproductive cells from each parent divide to produce gametes, egg and sperm cells, that contain only one half of each
pair of chromosome
- one half o each parent’s chromosomes join in a random fashion

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- the fertilized cell, known as a zygote, contains 23 pairs of chromosomes, ½ of each pair form the mother and the other
half from the father
- the 2 chromosomes in the 23rd pair are the sex chromosomes, denoted X and Y due to their shape
- females have two X chromosomes, whereas ales have XY
- calculating all possible combinations of the 23 chromosomes indicates that this can result in some 8 million possible
outcomes, the zygote is one of the 64 trillion possible combinations
- the net outcome is that a unique genotype is created at the moment of conception, and this accounts for the genetic
variation in the human species
- the zygote grows when the chromosome duplicates and then the cell divides into two new cells with an identical
chromosome structure
- cell division is the basis of life cycle and is responsible or growth and development
- errors in this process leads to mutations, most of which are benign and have little influences
- a genetic mutation ma produce a selective advantage or disadvantage in terms of survival or reduction
- mutations that lead to abilities or behaviours that are advantageous to an organisms ma spread through the gene pool
because those who vary the gene are more likely to survive and reduce
- there are ethnical issues related to genetic testing, such as whether the info should be available to employers and
insurance companies
Genes affect behaviour
- the study of how genes and environment interact to influence psychological activity is known as behavioural genetics
- behavioural genetics has made important contributions to the biological revolution, providing info about the extent to
which biology influences mind, brain, and behaviour
- any researcher that suggests that ability to perform retain behaviours are based in biology is controversial
- a concern of psychological scientists is that the extent to which all of these characteristics are influenced by nature and
nurture, by genetic makeup and the environment
- people are born essentially like undeveloped photographs – the image is already captured, but the way it eventually
appears can vary based on the development process
- the basic picture is there form the beginning
- behavioural genetics methods
o behavioural geneticists use two basic methods to assess the degree to which traits are inherited – twin studies and
adoption studies
o twins studies compare similarities between different types of twins to determine the genetic basis of specific
o monozygotic twinsalso called identical twins – twin siblings who result form one zygote splitting into two,
and therefore share the same genes
o dizygotic twins – fraternal or nonidentical twins – twin siblings who result form two separately fertilized eggs
o adoption studies compare the similarities between biological relatives and adoptive relatives
o adopted siblings have similar home environments but different genes
o the assumption is that similarities among adopted siblings have more to do with environment than genes
o growing up in the same household has relatively little influence on many traits, such as personality
o one interesting behavioural genetic study is to compare monozygotic twins who have been raised together or
raised apart
o this would seem the ideal way to test the relative contributions of gene and the environment
o some critics have argued that most of the adopted twins were raised in relatively similar environments, in part
issues adoption agents try to match the child to the adoptive home
o some evidence suggests that twins raised apart are more similar than wins raised together
o this would likely occur if parents encouraged individuality in twins raised together in emphasizing different
strengths and interests
o continued research using the methods of behavioural genetics is needed to provide additional insight into the
relative influence of heredity and environment on individual behaviour
- understanding heritability
o heredity is the transmission of characteristics from parents to offspring by means of genes
o heritability – a statistical estimate of the fraction of observed measure of the overall amount of difference
among people in a pop that is caused by differences in heredity
o heritability refers to differences in a certain trait among individual, not to the trait itself
o the heritability for a trait depends on variability within a pop
o although there is genetic variation among us, the vast majority of genes work nearly identically in all people,
and, indeed, across may species of other animals
o the pop used to estimate total variation can affect the estimate obtained for heritability
o the more diverse the pop, the lower the estimate of heritability

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o this occurs because of the increased variability that come from diversity and because the estimates of the genetic
variation do no consider much diversity
o the important point is that heritability is an estimate that is not precise and that can be affected by a number of
o it helps behavioural geneticists in clarifying the interaction between he environment and genes, so that they can
understand the circumstances in which genes operate
Social and Environmental Contexts influence genetic expression
- social context can affect genetic constitution
- low levels of MAO (monoamine oxidase) have been implicated in aggressive behaviours
- the gene for Mao comes in 2 forms, one of which leads to higher levels of Mao and one of which lads to lower levels
- genes and the social context interact to affect the phenotype
- Sandra Scarr proposed a theory of development that stresses the interactive nature of genes and environment
- According to Scarr, early environments influence young children, but children’s genes also influence the experience they
- Genes predispose certain behaviours that elicit different responses, and these subsequent interactions then shape the
- Because genes and social contexts interact, it can be very difficult to separate their independent effects
Gene Expression can be modified
- a variety of gene manipulation techniques are employed to enhance or reduce the expression of genes, or even to insert
genes into a different animal species
- animals with genetic modifications are then compared with animals without such modifications to text theories about the
genes’ function
- these gene manipulation techniques have led to dramatic increases in understanding how gene expression influences mind
and behaviour
- changing even a single gene can lead to dramatic changes in behaviour
- efforts to manipulate genes in humans have so far proven difficult and in some cases even dangerous
- there is new hope for altering gene expression with a method that focuses on RNA rather than on DNA
- gene expression involves DNA sending instructions via RNA to create specific proteins
- although RNA was once reviewed as a little more than a messenger, recent dramatic discoveries have indicated that RNA
might be more important than ever imaged
- RNA interference (RNAi), prevents gene expression by silencing the DNA instructions to make proteins
- The hope of being able to control such gene expression opens the possibility of bold new advances in our understanding
of how genes influence mind and behaviour
How does the nervous system operate?
- many of the genes in the human genome direct the development and ongoing functions of the nervous system, a
communication network that serves as the foundation for all psychological activity
- comprising billions of specialized cells known as nerve cells, the nervous system takes in a variety of info form the
eternal world, evaluates that info, and subsequently produces behaviours or makes bodily adjustments to adapt to the
Neurons are specialized for communication
- neurons – the basic unit of the nervous system that operates through electrical impulses, which communicate with other
neuron through chemical signals – neurons receive, integrate, and transmit info in the nervous system
- neurons differ from most other cells because they are excitable – they operate through electrical impulses and
communicate wit other neurons through chemical signals
- they have three functions – to take in information from neighboring neurons (reception), integrate those signals
(conduction), and pass signals to other neurons (transmission)
- neurons com in a wide assortment of shapes and sizes, but they typically share four structures regions that assist the
neuron’s communication functions
- first region – dendrites
o dendrites – branchlike extensions of the neurons that detect info from other neurons
o increase the neuron’s receptive field
o detect chemical signals from neighbouring neurons
- second region – cell body
o cell body – in the neuron, where info from thousands of other neurons is collected and processed
o the site of metabolism and genetic action
o contain DNA, the recipe for action for the neuron
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