Nature vs Nurture notes
HC 1: Overview of the field of genetics
All traits are heritable to some extent, there is a difference between traits.
The field of human genetics is special because there are collaborations on a large scale (all
over the world), this is mandatory to achieve new disease insights. Main question in the field
of genetics: why aren’t we all the same?
Issues in human genetic are:
- What is the relative influence of Genes and Environment, nature vs nurture? Are
traits mostly influenced by one or the other, or both?
To give an actual answer, we need more reliable estimates and an overview across
age, sex and population.
- What is the nature of the effects of Genetics? Are they all additives, can we just sum
up the effects, or are there interactions between genes that we need to take into
account? Different for every trait? Additive vs nonadditive debate.
- How do we determine causal mechanisms? Once we know something is heritable,
does that immediately lead us to understand the biology of the brain? Is the
knowledge that it is heritable sufficient?
Challenge of polygenic traits: many traits are not influenced by just a single gene of
environmental trigger but by many different variations at a genetic level. All these
different variations have tiny effects, this is why it’s very difficult to build a hypothesis
and understand the underlying causal mechanism.
If you want to make an estimation of the relative estimation of genes versus environment,
you can look at heritability. The extent in which something is heritable is variable. To make
an estimate you can look at twin studies. Monozygotic and dizygotic twins: monozygotic
share all of their genes, share 100% environment. Dizygotic share on average 50% of their
genes. Raised in the same family they have 100% shared environment. If you look at the
difference, a trait of 100% heritability would have a perfect correlation in monozygotic twins.
You can predict the height of person A if you know the height of person B. For dizygotic
twins the correlation would be 0.5. It can be used as a perfect design to dissect the relative
influence of genes and environment.
N = sample size
r = twin correlations in trait studies
MZ = monozygotic twins
DZ = dizygotic twins
h2 = estimated amount of heritability, relative influence of genes
c2 = influence of the shared environment (=common environment)
Because it’s squared, it’s standardized. If you have a whole population with individual
differences in a trait like in height, the total of the variation is fixed at 100%. If h 2 is 80, it
means that 80% of the observed variation is due to differences in genes.
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,To try and standardize the traits the ICF and ICD10 are used. ICD10 = international
classification of disease, medical. ICF = international classification of …, the non medical side.
Height is listed here and not in the ICD10 because it is not a medical disease.
In a meta analysis to look into the relative influence of genes on any trait in MZ and DZ, a
correlation was seen in DZ exactly half the MZ correlation. This indicates that the genetic
influences can be summed and that there is hardly any evidence for interaction between
genes. Another finding: what is the relative influence? We looked at the estimated
influences of genes and environment for all the traits. The estimated heritability of all these
traits is 49%. The estimated influence of the environment is the rest, of which 17% is the
shared environment. It’s almost 50-50. The heritability of all the traits in twin studies is 49%,
environment 51. So is it one or the other? On average, it’s both. There is a lot of difference
between traits and their heritability. Traits with low heritability mostly fall in the social
domain, how many friends you have etc.
Conclusions of 50 years of twin studies:
- All traits are heritable to some extent, huge variability
- Influence of c2 (shared environment) is relatively small
- Majority of traits are consistent with a model where all genetic variance is additive
Heritability: proportional estimate of trait variance that is linked to genetic variance. It also
tells you something about the extent in 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, it’s not a predisposition. It doesn’t lead to a mechanic inside of why
someone gets sick and someone else doesn’t.
If something is heritable you look at the DNA for the causal variants that lead to
disease/personality.
One mutation in 1000 leads to the genetic variations between people. Genetics variations
(SNPs) can occur in a gene but also outside the genes. In this way they can influence the
regulatory function of the genes. Genetic variations can be harmless (change in phenotype),
harmful (cause diabetes, cancer, hemophilia etc), latent (dependent on other factors) or
silent.
Monogenic disorders: disorder influenced by only one gene, one mutation. The effect of
having this mutation is quite strong, it makes you sick directly. Sickle cell, huntington’s.
Polygenic disorders/traits: influenced by multiple genes, each of small effect. Very hard to
find out which genes are responsible.
The gene area meant that a couple of genes were researched and linked to a lot of disorders,
because they are to be used in animal studies, there was a similar human mutation, there
was a similar gene. Selected a lot by labs to be tested. Always look for replication in gene
studies in the 90s! Because of false positives.
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, Genomewide association studies: upscale and improved quality of the research using
microarrays. Majority of human traits are decided by many genes, to gather new information
we need to work together across the globe and put together our information and research.
HC 2: Historical overview NvsN debate
Nature: genes, hereditary factors
Nurture: the way you are raised, childhood experiences, neighborhood/country you’re
raised in, diet given by your parents.
Why do we differ? Intelligence, athletic ability, aggressive behavior, political preference.
Nature
- “Behavior is caused by innate characteristics”
- “All behaviors are present from conception”
- “Genes provide the blueprint for behavior: some expressed at birth, some
programmed to emerge with age”
- Deterministic (probabilistic)
Nurture
- “Behavior is determined by the environment – wat people learn, observe,
experience”
- “Behavior results from interaction with environment”
- “Tabula rasa: born as a blank slate, an empty vessel, waiting to be filled up with
experiences”
- “Everything is possible given the right environment”
- Deterministic (probabilistic)
Debate: to what extent play genes and environment a role in shaping behavior?
- Hippocrates: thought about human behavior, founder of modern medicine. Intrigued
in how human behavior was the product of biology. Four body fluid types, the
balance of the types determined what kind of person you became.
- John Locke: introduced the idea of tabula rasa. We are all born equal, typically
nurture. All knowledge comes from experience, all behavioral traits are formed by
nurture. This type of stance was controversial because the Middle Age was very
religious.
- Darwin: founder of genetics, homo universalis specialized as a naturalist, geologist,
biologist. The Origin of Species: animals adapt to their environment in ways that are
inherited by future generations. The Descent of Man: controversial book about how
people descend from apes. The church wasn’t happy with this.
- Sir Francis Galton: became interested in human abilities and whether they were
inherited. Is the offspring of ‘eminent men’ also more eminent than expected? He
collected data and saw that family of eminent men were more often eminent
themselves. Additionally, the numbers of eminent men dropped off as a function of
the degree of relatedness. Human (cognitive) abilities are heritable!
- Mendel: studied pea plants, father of genetics.
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