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College aantekeningen Nature vs. Nurture
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Lecture notes of the course Nature vs. Nurture of the Minor Brain & Mind at VU Amsterdam
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Nature vs NurtureLecture 1 Overview genetics field
The key research question in behavior genetics is: Why do people show variation with
respect to a phenotype? What then causes this variation and is this due to genes or
environment?
The main goal is to understand why people differ in many traits. Those reasons need to be
found in either nature or nurture:
Nature: Our genetics determine our behavior. Our personality traits and abilities are
in our “nature”
Nurture: Our environment, upbringing and life experience determine our behavior.
We are “nurtured” to behave in certain ways
Both nature and nurture separate as the interaction between the two influence our
individual differences.
If a trait is heritable, then genetic relatedness should predict phenotypic relatedness. That is,
two individuals that share more of their genetic background, should also be more alike on
the trait.
We have a natural experiment called twin studies. This is the most often used experimental
design to separate the effect of genes and environment from each other. Two kinds of twins
can be born and these twins can be used for scientific purposes to dissect the relative
contribution of genes and environment to the traits that we study in twins. This is because
the two kinds of twins differ in the number of genes that they actually share with each other.
This is then also the only difference between the two.
Monozygotic twins (MZ, identical) Dizygotic twins (DZ, fraternal)
Share 100% of their genes Share on average 50% of their genes
100% of their shared environment 100% of their shared environment
0% of non-shared environment 0% of non-shared environment
MZ twins share 100% of their genes, while DZ twins share on average 50%. If a trait is
heritable and no influence of shared environment, MZ twins will correlate about 2x as strong
as DZ twins.
A lot of twin studies have been conducted. However, there is still a huge debate on the
relative influence of genes and environment. In order to resolve this, we need more reliable
estimates and an overview across age, sex, population. Another debate that is going on is
what is the nature of genetic influences. Do all genes that contribute to a trait all have their
independent effect or do these genes interact with each other. This is the additive vs. non-
additive debate. Another debate is how do we determine causal mechanism. If we know that
the relative contribution of a gene is very high and the influence of the environment is very
low, how do we find the causal mechanisms.
,A meta-analysis is done of all the twin studies that had ever been done between 1900-2012.
A couple important statistics were extracted: N (sample size) & r (twin correlation) for MZ,
DZ, h2 (estimates of heritability), c2 (effect of the shared environment). It was listed separate
for males and females across different age groups and across different populations. Standard
trait classification was used, using the ICF (international classification of traits that are not
necessarily hereditary diseases) and ICD10 (international classification of disease).
In the meta-analysis that looked into the heritability of all the human traits based on twin
studies, it was found that about 50% of every trait that you can measure is heritable and the
other 50% is due to the environment. Here, the influence of c 2 is relatively small.
Furthermore, the majority of traits are consistent with a model where all genetic variance is
additive.
The heritability is the proportion of trait variance that can be attributed to genetic variance.
It tells something about the extent to which observed individual differences can be traced
back to genetic differences. If a trait is heritable, people from the same family are
phenotypical alike. Heritability doesn’t mean that there is nothing you can do, because of
genetic influences, so it’s not a predisposition. It doesn’t lead to a mechanic inside of why
someone gets sick and someone else doesn’t. It is the variation in genes that underlie trait
differences between individuals.
If something is heritable you look at the DNA for the causal variants that lead to the
disease/personality.
There were two important discoveries in the field of the human genome. The structure of
DNA was discovered by Watson and Crick in 1953 and the sequencing of the human genome
(3 x 109 base pairs) by the Human Genome Project in 2002.
Each cell contains 23 sets of chromosomes, carrying the heritable blueprint of life.
Chromosomes are transmitted from parents to offspring. Each parent contributes 23 single
chromosomes via a process called meiosis and subsequent fertilization. Each single
chromosome is a DNA molecule, which consists of sequences of nucleotides (‘ACGT’). The 23
chromosomes together contain ~3 x 109 nucleotides. 99.9% of DNA sequence is the same
between individuals and approximately 106 sites differ between individuals, resulting in
phenotypic differences. So, only 0.1% is different between individuals.
Genetics variations (called ‘SNPs’ for single nucleotide polymorphism) can occur in a gene
(protein coding, regulatory region, exonic, intronic), but also outside the genes (regulatory,
or of currently unknown function). Genetic variations can be harmless (change in
phenotype), harmful (causing diabetes, cancer, Huntington’s disease and hemophilia), latent
(dependent on other factors) or silent.
Monogenic disorders are disorder influenced by only one gene (one mutation). The effect of
having this mutation is quite strong, as it directly results in a disease such as Huntington’s
disease. Most genetic causes here are already known Polygenic disorders/traits are
influenced by multiple genes, each of small effect. Very hard to find out which genes are
,responsible as they are caused by multiple genetic and environmental factors with possible
interaction. The genetic causes here are mostly unknown.
Finding genetic variants that are linked to disease will help us to get more biological insight
and help us in designing treatments.
Candidate gene studies (1980s-1990s) compare cases to controls by genotyping them and
see how often a certain variant occurs in both groups. Based on this, you can say to which
extent a certain trait is related to a certain disease. Certain candidate genes were researched
a lot that were thought to have a link with certain diseases. These genes were easy to
genotype, so the testing of these genes was very practical. However, the conclusion was that
most of these genetic associations were false positives. Therefore, it is important to look for
replication in gene studies in the 90s!
Genome-wide association studies (GWAS) (2006-current) allowed us to genotype multiple
genetic variants at the same time in an efficient way. Microarrays can now contain more
than 1 million tagging SNPs covering the genome in high density. For each SNP we can
compare the allele frequency across cases and controls and conduct a statistical test for a
difference in frequency. This results in a Manhattan plot.
GWAS found that many complex traits are highly heritable. GWAS have detected some
genetic variants, but this is only a fraction of genetic variance (<2%). The majority of human
complex traits are probably caused by thousands of genes of very small effects, so huge
sample sizes are needed. These huge sample sizes have been achieved, so GWAS results are
reliable and many genes are now discovered.
With so many genetic results comes a new responsibility. New questions raise such as ‘How
are genetic results being used?’, ‘What are the ethical challenges?’ and ‘What is the
responsibility of researchers?’.
GWAS provides statistically associated variants and genes, but does not reveal how it works.
Current GWAS results for complex traits are difficult to interpret, which complicates the
formulation of hypotheses that can be tested in functional experiments.
There are four issues that complicate making sense of GWAS results:
1. GWAS hits for polygenic traits mostly outside genes, or non-coding genetic regions,
with likely regulatory functions that are currently unknown
2. GWAS hits for polygenic traits that have small effects
3. SNPs are correlated which complicates pinpointing ‘the’ causal SNP
4. There are 100s of genes involved in polygenic traits – a single gene will not provide
the whole picture
The GWAS results are integrated with information on functional consequences of risk
variants. These risk variants are then linked to genes and tested for converging biological
functions of associated genes. It then shows the enrichment of the expression of associated
genes in specific cell types.
, Lecture 2 Short historical overview Nature vs Nurture
Nature are the genes and hereditary factors:
Physical appearance
Personality characteristics
Nurture are the environmental variables:
Childhood experiences
How we were raised
Social relationships
Surrounding culture
The main question is: Why do we differ and to what extend is this variation due to genetic
factors and to environmental factors? And why are we similar?
The debate can be divided into two sides. To what extent play genes and environment a role
in shaping behavior?
Nature Nurture
Behavior is caused by innate characteristics Behavior is determined by environment –
what people learn, observe, experience
All behaviors are present from conception Behavior results from interaction with
environment
Genes provide the blueprint for behavior: Tabula rasa: born as a blank slate, an empty
some expressed at birth, some vessel, waiting to be filled up with
programmed to “emerge with age” experiences
Deterministic (probabilistic) Everything is possible given the right
environment
Deterministic (probabilistic)
Hippocrates was the founder of modern medicine and one of the first for thinking how
human behavior came about and how it is the product of biology. He hypothesized that
there were 4 body fluid types and that a mix of these 4 determined the type of person you
became.
John Locke introduced the idea of tabula rasa and that we are all born “equal”. This would
mean that the individual differences in knowledge between people originate from their
experiences and that all behavioral traits are formed by nurture.
Charles Darwin can be seen as one of the founding fathers of genetics. He was a homo
universalis: naturalist, geologist, biologist, etc. He studies how the form of finches’ beaks
adapted to their environment and how these adaptations were inherited by future
generations. He also wrote about how humans evolved and coined the hypothesis that
humans might be descendants from apes.
Sir Francis Galton became interested in genetics and was inspired by ‘The Origin of Species’
of his uncle. He wanted to study how behavior ran in family and he was interested in these
hereditary human abilities. He researched the question ‘Is the offspring of ‘eminent men’
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