PSY100H1 Chapter 3: Genetic and Biological Foundations
2 Aug 2010
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Chapter 3: Genetic and Biological Foundations
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 mater recipe that
provides retailed instructions for everything from how to grow a gall bladder to where to place the nose on
your face.
• The term genetics is typically used to describe how characteristics, such as height, hair colour, and weight, is
passed along through inheritance, but it also refers to those processes that are involved in turning genes off
and on.
• Within each cell are chromosomes, which are structures made up of genes.
• The typical human has 23 chromosomes, half of each pair coming from each parent.
• Proteins, of which there are thousands of different types, are the basic chemicals that make up the structure
of cells and direct their activities.
• A gene is a segment of DNA that is involved in producing proteins that carry out specific tasks.
• The process by which the gene produces RNA and then protein is known as gene expression, in that the gene is
“switched on”
• Genes provide options, and the environment determines which option is taken.
• Gene expression not only determines the physical makeup of the body but also remains important throughout
life and is involved in all psychological activity.
• It is the gene expression that allows us to learn, to see, and to fall in love.
Heredity Involves Passing along Genes through Reproduction
• The first clues to the mechanisms responsible for heredity were discovered by the monk Gregor Mendel around
1866.
• Mendel developed an experimental technique called selective breeding for studying genetics.
E.g. Mendel used a flower with 2 types of colours to control which plants were bred with which other plants.
! Mendel cross-pollinated two types of plants to see which colour flowers would be produced.
! Mendel deduced that there must be discrete units, now referred to as genes, which exist in two
versions.
• The two versions of genes are known as alleles.
• If alleles differ, one is dominant and the other is recessive.
• Dominant genes are expressed (become apparent) whenever they are present.
• Recessive genes are expressed only when they are matched with a gene from the other parent.
Genotype and Phenotype
• The existence of dominant and recessive genes means that not all genes are outwardly expressed.
• The genotype is the genetic constitution of an organism, the actually genetic makeup that is
determined at the moment of conception.
• The phenotype is the observable physical characteristics of an organism that result from both genetic
and environmental influences.
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• The environment can also affect the phenotype; for instance, good nutrition leads to increased physical
size and a suntan changes the colour of the skin.
E.g. Example of environmental influence on the phenotype is phenylketonuria, a disorder in which
infants are unable to break down and enzyme contained in dairy and other products, such as the
sweetener aspartame.
Polygenic Effects
• When there are a range of variability within a population for certain characteristics, such as height,
this indicates the characteristic is polygenic, that is influenced by many genes, as well as the
environment.
• Most human traits and diseases are polygenic.
Genotypic Variation is created by Sexual Reproduction
• A fertilized cell, known as a zygote, contains 23 pairs of chromosomes, one half of each pair from the mother
and the other half from the father.
• The two chromosomes in the 23rd pair are the sex chromosomes, denoted X and Y due to their shapes.
• Females have two X chromosomes, whereas males have one X and one Y chromosomes.
• A zygote is one of 64 trillion possible combinations.
• Cell division is the basis of the life cycle and is responsible for growth and development.
• Sometimes there are errors in this process, which lead to mutations, most of which are benign and have little
influence.
Genes Affect Behaviour
• The study of how genes and environment interact to influence psychological activity is known as behavioural
genetics.
• People are born essentially as undeveloped photographs: the image is already captured, but the way it
eventually appears can vary based on the development process.
Behavioural Genetics Methods
• Behavioural geneticists use two basic methods to assess the degree to which traits are inherited: twin studies
and adoption studies.
• Twin studies compare similarities between different types of twins to determine the genetic basis of specific
traits.
• Monozygotic twins, also called identical twins are the result of one zygote dividing into two, each having the
same chromosomes and the genes they contain.
• Dizygotic twins, sometimes called fraternal or non-identical twins, are the result of two separately fertilized
eggs and are no more related than any other pair of siblings.
• Adoptive studies compare the similarities between biological relatives and adoptive relatives.
• Identical twins are more likely to be similar whether they were raised together or not.
• It is clear that twins share similarities other than just behaviour quirks, such as intelligence and personality.
Understanding Heritability
• Heredity is the transmission of characteristics from parents to offspring by means of genes.
• Heritability is a statistical estimate of the portion of observed variation in a population that is caused by
differences in heredity.
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• Variation is the measure of the overall amount of difference among people.
• It is important to understand that heritability refers to differences in a certain trait among individuals, not to
the trait itself.
• Heritability refers to populations, not individuals. The heritability for a trait depends on variability within a
population.
• Estimates of heritability are concerned only with the extent that people differ in terms of their genetic
makeup.
• The population used to estimate total variation can affect the obtained for heritability.
• The more diverse the population, the lower the estimate of heritability.
• Heritability is an estimate that is not precise and that can be affected by a number of factors.
Social and Environmental Contexts Influence Genetic Expression
• Differences in a gene that controls a certain enzyme (monoamine oxidase, or MAO) may be important in
determining susceptibility to the effects of maltreatment.
• Low MAO has been implicated in aggressive behaviours.
• The gene MAO comes in two forms: one of which leads to higher levels of MAO, and one which leads to lower
levels of MAO.
• Children who were mistreated and have low levels of MAO were more likely to be convicted of a violent crime
than those with the high-activity gene.
• Sandra Scarr proposed a theory of development that stresses the interactive nature of genes and environment.
• Early environments influence young children, but children’s genes also influence the experiences they receive.
• Children exposed to the same environment interpret and react to it in different ways.
• Genes predispose certain behaviours that elicit different responses, and these subsequent interactions then
shape the phenotype.
• 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 inset genes into a different animal species.
• Changing even a single gene can lead to dramatic changes in behaviour.
• Changing the expression of one gene leads to the expression of a cavalcade of other genes, which ultimately
influences even complex behaviours.
• Genes seldom work in isolation to influence mind and behaviour; rather, the complex interaction among
thousands of genes gives rise to the complexity of human experience.
• Gene expression involves DNA sending instructions via RNA to create specific proteins.
• In 1998, researchers discovered that injecting a slightly modified form of RNA into worms led to a dramatic
shutdown in gene expressions in the very genes that were associated with producing that RNA in the first place.
• The modified RNA is broken down by other gene expression processes into tiny segments called small RNA.
• This process now referred to as RNA interference (RNAi), prevents gene expression by silencing the DNA
instructions to make proteins.
• RNAi may be used as therapy for a variety of diseases and disorders ranging from cancer, to AIDS, to
schizophrenia.
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Document Summary
Switched on : genes provide options, and the environment determines which option is taken, gene expression not only determines the physical makeup of the body but also remains important throughout life and is involved in all psychological activity. It is the gene expression that allows us to learn, to see, and to fall in love. Heredity involves passing along genes through reproduction: the first clues to the mechanisms responsible for heredity were discovered by the monk gregor mendel around, mendel developed an experimental technique called selective breeding for studying genetics. Mendel used a flower with 2 types of colours to control which plants were bred with which other plants. mendel cross-pollinated two types of plants to see which colour flowers would be produced. mendel deduced that there must be discrete units, now referred to as genes, which exist in two versions: the two versions of genes are known as alleles.
