PSYCH – Week 14 Online Readings
Week 14: Genetics and Intelligence
Focus Question: How is the heritability of intelligence connected with the heritability of
other behavioural and human factors we have studies so far?
How are Behaviours and Other Traits Inherited?
Behaviour has been viewed as arising from one of two possible origins: either you
were born with it (nativism – from the genes) or you gained it through experience
(empiricism – from the environment). This is the naturenurture debate. However, genes
interact with the environment at every level so, to an extent, all behaviour results from
both nature and nurture.
It is important for psychologists to have an understanding of genetics, because
many psychological processes and disorders have genetic underpinnings. For example,
metal disorders like schizophrenia or alcohol abuse (some people are more susceptible).
Psychologists who study behaviour genetics specialize in the relationship between
genetics and behaviour. This subdiscipline of psychology was founded by John Scott,
John Fuller, and William R. Thompson, who studied the genetic basis of personality traits
in dog breeds during the 1950s.
Genetic material, or DNA, is organized into structures called chromosomes that
are in the nucleus of every cell. A human has 23 pairs of choromosomes – 22 are called
autosomes and the 23 pair is made up of the X and Y sex chromosomes. Males have a
XY pair; females have XX.
DNA (deoxyribonucleic acid): Genetic material of all organisms that makes up
chromosomes; resembles a twisted ladder, with strands of sugar and phosphates
connected by rungs made from nucleotide molecules of adenine, thymine, guanine, and
Sperm and egg cells are produced through a process called meiosis, which results
in each cell having only half of each pair of chromosomes. When sex cells combine
during reproduction, the half pairs combine to give the resulting cell a complete set of 23
pairs of chromosomes. Of particular importance, the egg can have only an X chromatid
[:One of two identical halves of a replicated chromosome] (because the mother had two
Xs), but any given sperm can contain either an X chromatid or a Y chromatid (because
the father had both X and Y chromosomes). Therefore, the sperm’s makeup determines
the sex of the offspring.
Genes and Alleles Genes can be defined as regions of chromosomes that encode (transfer genetic
information from DNA to proteins) particular proteins.
The point on the chromosome where a particular gene is located is called its locus
(plural: loci). Because chromosomes come in pairs, the gene on one chromosome of the
pair may or may not be the exactly the same as the gene on the other chromosome of the
pair at the same locus. When genes at the same locus on the two chromosomes are the
same, they are said to be homozygous at that locus. If not, they are heterozygous.
Homozygous: Each parent contributes the same allele for a particular gene.
Heterozygous: Each parent contributes different alleles for a particular gene.
The pair of genes at a given locus are called alleles [:alternative forms of the same
gene] (p. 66 in the text). One is inherited from the father, the other from the mother. If the
alleles are different, one often has a dominant effect over the other. For example, the
allele for eye colour is dominant for brown eyes and recessive for blue eyes. Therefore,
for a person to exhibit blue eyes, both of his or her alleles must be for blue eyes. If one is
brown and the other is blue, he or she will be brown eyed, because the browneye allele is
The genes for blue are like blueprints for the production of
proteins that control the production of pigment. In this
example, your iris colour is determined by the amount of
pigment present in its cells, and your genes tell the cells
how much pigment to make through the proteins they
**Although we talk about how genes can influence behaviour, they don’t do so directly.
Instead, genes guide cells to generate proteins that cause our cells to form chemicals in
the body that are related to behaviour, so they are an indirect influence. A gene guides
cells to generate proteins related to growth, eye pigment or other functions. These
proteins operate the cellular machinery that allows organisms, including us, to live. As a
result, any genetic contribution to a trait (structural or behavioural) is the result of certain
types of proteins being manufactured by a given cell. Some genes perform multiple
functions, some only one.
The type of protein made depends not only on the structure of a gene but also on the
environment (for example, diet and exercise of individual). Traits that have some
heritability are passed on to subsequent generations by the passage of genes through
Heritability: The amount of variability in a given trait in a given population at a given
time due to genetic factors; measured as h2 and sometimes referred to by this measure
instead of the word.
Genotype and Phenotype The term genotype refers to the genetic makeup of a trait; it is an organizms
genetic makeup. The term phenotype refers to how the trait is expressed (pg 62). With
eye colour, because the allele for brown eyes is dominant, a person who has one allele for
brown eyes and one for blue (described as ‘Bb” with B for brown and b for blue) will
have the same eye colour as someone whose alleles are both for brown eyes (BB). These
two individuals have the same phenotype but different genotypes. This is an example of a
Recessive traits show up phenotypically in subsequent generations when an
organism carrying the recessive trait breeds with another one carrying the recessive trait.
“It is important to recognize that every individual’s phenotype is produced by the
interaction of its genotype with the environment.” So both genes and the environment as
well as experience contribute to behaviour.
Labs have two separate genes for fur colour (it is polygenic: A trait that is influenced by
more than one pair of genes.) The “B” gene controls colour, while the “E” gene controls
the expression of B. The dominant “B” allele produces black fur. The recessive “b” allele
produces brown fur. With an E (dominant) allele, the fur will remain the colour that the
“B” or “b” allele coded for (black or brown). But if the dog has two “e” (recessive)
alleles, it will be golden regardless of the “B” or “b” allele!
Behaviour is rarely affected by singlegene mechanisms, but there are a few examples,
such as fearfulness in dogs and the FOXP2 gene in humans, though single gene
behaviours are usually disorders.
FOXP2 Gene in the KE Family
The FOXp2 gene resulted in individuals having difficulty with speech articulation. The
disorder was traced to a single gene on chromosome 7.
Most behaviours and disease states are not caused by a single gene. Many disorders have
a polygenetic basis – disorders run in families often, like schizophrenia or depression.
Fluid intelligence or fluid reasoning is the capacity to think logically and solve problems in novel
situations, independent of acquired knowledge. It is the ability to analyze novel problems, identify patterns
and relationships that underpin these problems and the extrapolation of these using logic. It is necessary for
all logical problem solving, e.g., in scientific, mathematical and technical problem solving. Fluid reasoning
includes inductive reasoning and deductive reasoning.
Crystallized intelligence is the ability to use skills, knowledge, and experience. It does not equate to
memory or knowledge, but it does rely on accessing information from longterm memory.
Crystallized intelligence is one’s lifetime or intellectual achievement, as demonstrated largely through one's
vocabulary and general knowledge. This improves somewhat with age, as experiences tend to expand one's
Concordance and Twins
Much evidence for the effects of genes on behaviour comes from studies examining
the relationship between genetic similarity and similarity in some trait (like intelligence). Often these studies compare identical twins to fraternal twins and siblings. Identical
(monozygotic) twins arise from one fertilized egg and have nearly identical genotypes.
Fraternal (dizygotic) twins, in contrast, arise from separate cells and are no more
genetically similar than any two siblings with the same parents.
If the rate of concordance (matching phenotypes between twins, or the expression of
similarity in traits or absence of traits by both twins) for a trait is higher for identical
twins than for fraternal twins, then that trait has a genetic component. A number of traits,
including intelligence and schizophrenia, are more likely to be concordant for identical
twins than for fraternal twins, thus we conclude they have a genetic basis.
Epigenetics: The study of heritable changes that occur without a change in the DNA
sequence (a mutation).
Exposure to external factors may cause changes in a group of cells or the
blueprint for those cells, and that change might continue for the life of the organism. EX:
stress, diet, behaviour, toxins, etc. At any point during human development, specific
signals could cause incremental changes in gene expression
patterns, ultimately creating differentiated cells.
Epigenetics can also describe lesscommon changes that
pass from one organism to its descendants through sexual
Differentiated cells: Lessspecialized cells whose profiles
or characteristics have, over time, grown increasingly different from and more specialized
than other cells of the same type (e.g., a singlecell zygote develops into a multicellular
Heritability describes the proportion of the observed variance in a behaviour that can be
attributed to genetic differences among individuals. You would need a statistical measure
that shows variance in a behaviour that can be attributed to genetic differences in a
Genetic influence is measured by a statistic called heritability, or h .
Heritability describes the proportion of the observed variance in a behaviour that can be
attributed to genetic differences among individuals. Thus, a trait that shows high
heritability necessarily varies within a population (e.g., intelligence, as opposed to
binocular vision), and this phenotypic variation can be attributed to genetic differences.
An inherited trait, one passed on through genes, can have low or high heritability,
but a trait that is not inherited always has, in principle, zero heritab