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Behaviour genetics in psychology at the VU (minor)
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Lecture 1A: History and MendelGenes and environment always play a role in complex traits. A large part of the
environmental influences are not random, we choose the environment
History
Hippocrates Believed that humans reproduce through an egg that swims through the
(460-375 BC) body (female of men) and picks up the gemmules (tiny pieces of every
aspect of your body) and put that in the egg, and this egg grew out to a
new human being
Pythagoras Believed that the father supplies the essential characteristics (“form”)
and the mother supplies the material building blocks
Aristotle Believed children were made from “purified blood from the testes”
(semen) and menstrual blood
Antonie van Developed the theory of preformationism: we all come from miniature
Leeuwenhoek versions of ourselves. Sperm contain complete performed individuals
en Nicolaas called “homunculus”
Hartsoeker ➔ Benefits: Adam had every person already in his sperm?
➔ Flaws: They don’t have empirical evidence, how do they create
women?, you will need infinite human sperm
Jean-Baptiste Focussed on inheritance of acquired characteristics: he said that use
de Lamarck and disuse of traits creates changes in life. However this would mean
(1744-1829) that in the end we all look the same, because we have the same
struggle to survive
Charles Darwin said that many more individuals of each species are born that
Darwin can survive, thus there is a struggle for existence. This led to the theory
of natural selection: any being, if varies slightly in any manner profitable
to itself, under the complex and sometimes varying conditions of life,
will have a better chance of surviving, and thus be naturally selected.
➔ Theory of EVOLUTION: all species arose through the natural selection of small,
inherited variations that increase the individual’s ability to survive and reproduce.
➔ Charles Darwin’s theory of Pangenesis: Contributions to egg or sperm from every
part of the body. He also implied blending inheritance. This led to the Blending
Theory.
Francis Galton Looking at humans, and realized that there is some inheritance of
(1822-1911) intelligence/ability. He made a pedigree (family tree) to show that high
scientific ability runs in the family.
→The 300 most distinguished men (1883): Francis looked at 1000
men from 300 families and made a family tree. He found that the
further away (genetically speaking), the less you look alike, he thus
realized that you look more like your parents and sisters than your
uncle and aunts.
General Cognitive ability: An IQ test can demonstrate genetic
relationships between people.
1
,Gregor Mendel Gregor Mendel mixed wrinkled peas with smooth peas and found that
(1822-1884) the new generations' (pure breed) plants were all smooth (generations
F1). Then he repeated this with the new generations and found that
75% smooth and 25% wrinkled (generations F2). THis 3:1 ratio
repeated itself again and again. This pattern shows that plants contain
two separate elements (now referred to as alleles) that existed in
different versions. In determining an offspring’s features, some of these
versions would be dominant and others would be recessive. In pea
plants, wrinkled paes are recessive, so wrinkled peas occur only when
both alleles are wrinkled types.
- Allele: this is one specific variant of a gene
Mendel’s first law= Law of Segregation: A recessive element is
expressed only if both elements are recessive, while a dominant
element is expressed when one or two are present.
● Dominant Allele (S): an allele that produces the same
phenotype in an individual regardless of whether one or two
copies are preset
● Recessief allele (s): an allele that produces its phenotype when
two copies are present
● Punnett square: a square diagram that is used to predict the
genotypes of a particular cross or breeding experiment.
Everything with capital R makes smooth pee, only homozygoot
rr makes wrinkled pee.
Mendel’s second law: Law of independent assortment: The
inheritance pattern of one trait will not affect the inheritance pattern of
another.
→ most important about Mendel’s second law are it’s exceptions.
● The law holds for:
○ Genes on different chromosomes
○ Genes that are far apart from the same chromosome
● The law does not hold for:
○ Linkage: Genes that are close together on the same
chromosome which inhibits crossing
■ When two genes are very close to each other on
the same chromosome they won’t cross. So,
instead of finding all four types of F2 offspring,
you only do two types: dominant for both A and
B and recessive for both A and B.
■ The chance for crossing is a function of the
distance between two loci, the closer the loci’s
the lower the chance. Crossing-over happens on
average once for every chromosome during
meiosis.
● linkage: Loci that are close together on a
chromosome and thus inherited together
within families
● recombinations: a process that occurs
during meiosis in which chromosomes
exchange parts
● Linkage analysis: Identify the location of
a gene on a particular chromosome
■ Centigram (map unit): the distance between two
loci estimated by the number of recombinations
per 100 gametes.
2
, ○ X-linked inheritance: This concerns sex-dependent
transmission and is explained through color blindness.
■ Red-green colorblindness is caused by a
recessive allele on the X chromosome. Males
only have one X → more chances to be color
blind than women we need two alleles.
Mendel’s disorders
➔ Mendelian disorders: disorders that follow mendelian inheritance patterns
★ Huntington’s disease (Dominant allele)
○ This is an inherited brain disorder causing very fast worsening of the physical
cognitive and emotional self. The risk of HD in the offspring is 50% due to one
dominant allele.
○ Because it develops relatively old, people often have already reproduced,
therefore the dominant disease does not die out. There is no cure and no
effective treatment.
★ PKU - phenylketonuria (recessive allele)
○ PKU: this is a metabolic disorder caused by deficiency of the lover enzyme
phenylalanine hydroxylase (PAH). Unlike HS, PKU is due to the presence of
two recessive alleles. For offspring to be affected, they must have two copies
of the PKU allele.
○ Carriers: Those offspring with only one copy of the OKU allele (are not
afflicted with the disorder, but pass it onto offspring.
○ PKU prevents normal metalization of phenylalanine (Phe) which is present in
eg protein rich foods → disease is completely manageable by a certain low
protein diet. Damage occurs to the central nervous system leading to mental
retardation.
○ 1 in 50 individuals are carriers of one PKU allele.
★ Scurvy
○ Scurvy is a disease coming from vitamin C deficiency. The deficiency leads to
symptoms of weakness, anemia, gum disease and skin problems. Scurvy is a
disorder caused by a single gene in humans.
○ HOWEVER, it is not a genetic disorder in humans because there is NO
genetic variation for it in humans. Everyone has that defective gene. BOth
PKU and scurvy are disorders treated by diet, but only in PKU it makes sense
to care about inheritance and Mendel’s Laws.
○ Hardy-weinberg equilibrium: the frequencies of alleles and genotypes do
not change across generations unless forces such as natural selection or
migrations change them,
■ This rule is the basis for a discipline called population genetics,
whose practitioners study forces that change allelic frequencies.
→ Some traits are just more dominant than other traits: a big nose is more dominant than a
little nose and dark hair is more dominant than light hair
3
, Qualitative vs quantitative traits (still about Mendel)
● Qualitative traits: traits that are influenced by a single gene. These traits follow a
simple pattern of inheritance and the phenotypes have distinct categories
(present/absence). Traits expression is unaffected by the environment. (example
(huntington's, PKU, CF)
● Quantitative traits (polygenic/complex traits): traits influenced by multiple genes.
These traits do not follow a simple pattern of inheritance. The phenotype is
expressed on a continuum and the trait expression may be altered by the
environment. These traits are often nominal distributed in a bell shaped curve
(example: alzheimer, autism, depression but also IQ)
● Behavioral traits: are also quantitative/polygenic traits that do not follow the
mendelian inheritance pattern. Such as: ADHD, anxiety/depression, autism,
personality, wellbeing, happiness, intelligence, ready ability → normal distribution.
Beyond Mendel’s law
➔ Schizophrenia: the complex disorder is familial; meaning that the risk for
schizophrenia increases with genetic relatedness. Heritability is relatively high, in
order from high to low it is; height, schizophrenia, anxiety disorder, depression.
● Morbidity risk estimate of lifetime expectancy: the chance of being affected during an
entire lifetime
● Relative risk: chance to develop a disorder when a family member has the disorder
● correlation: an index of resemblance that ranges from .00 (no resemblance) to 1.0
(perfect resemblance).
● Concordance/discordance: presence/absence of a particular condition in two family
members/ discordance means one has it, one not. Concordance means both have it
or both don’t have it.
● Liability-threshold model: a model that assumes that dichotomous disorders (have
it or not) are due to underlying genetic liabilities that are normally distributed. The
disorder appears only when a threshold of liability is exceeded.
Lecture 2A: Introduction to human genetics
Building blocks
● From a physics/chemistry angle:
○ We consist of atoms and atoms together are → molecules
○ A particular combination and arrangement of molecules → life
○ Differences in molecular composition → organism
Complexity
● Describing behavior by physics/chemistry laws is very difficult
○ Physics are unable to describe the interaction between more than two
particles
○ One van view psychology as a derived scientific field due to complexity
● Remember that science often relies on models
○ Even in physics, although its models often are really precise
Summarized; Living matter
● Highly structured
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