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- Summary Twin research in psychology lectures (1 t/m 13) - English

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  • 15 december 2021
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  • 2021/2022
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Twin Research in psychology

Lecture 1 (1A)

Finish more than 80% of the assignment before the exam
- every lecture has one assignment  to keep you active
- Block assignment is an obligation: submit and give feedback (2 peers)
- 16 December  last lecture on zoom

Historical perspective
- where do all differences came from?

Parents pass traits on to offspring
- Pythagoras
- Aristotle  though children were made of blood testes + menstruation blood  because
menstruation stops by pregnancy

Nicolaas + Anatonie  microbiology
- theory of preformationism = we all come from miniature versions of ourselves  human have
sperm with an individual
- benefits with this theory: Adam  to all humans, original sin (zonde)

Francis Galton  psychometrics, behavior genetics
- family trees
- twins
- understand genetic influences

Charles Darwin  most important trip in all of science
- voyage
- father: waste of time
- Bird Galapagos: designed for the food what there was on the island
- Theory of evolution: all species arose through the natural selection of small, inherited variations
that increase the individuals’ ability to survive and reproduce
- also, emotions
- Every part of the body  egg  embryo
- Blending inherited: blending/mix of traits

Problem of this blending theories of inheritance for theory of evolution
- small inheritance  selected
- er komt na een tijdje geen middelgetal meer uit, omdat iedereen al average is, iedereen zal dan
uiteindelijk gemiddeld zijn/even groot zijn (dat is niet zo)

Mendel
- pedigree (= stamboom) of hemophilia (bloedziekte) in the royal families of Europe
- crossing to find any pattern (inheritance)  with seeds, plants, flowers (smooth one, wrinkle one 
all smooth = glad)  all smooth because one was dominant
- cross again  all smooth woman and all smooth man: children ¾ smooth and ¼ wrinkled
- How could that be? Trait of two generations before, come back now
- after mixing more colors  color is gone: all black = blending inheritance
- met draad door elkaar, je kan de kleur nog terugkrijgen = the case by the traits that coming back
 two elements of heredity for each trait in each individual  Mendel’s first insight = Mendel’s first
law: Two elements  give one of them  child get one of father and mother

,Punnet square  diagram to cross the genotypes  useful to determine the probability of an
offspring having a particular genotype
- Dominant Allele = expresses its phenotypic by hetero
- Recessive Allele = expresses not its phenotypic by hetero




Now it makes sense  the problem of blending inheritance = not true

What happened when Mendel looked at two or more traits at the same time?
- realized: one trait does not influence the other. Inheritance will be random (random change) = law
of independent assortment = Mendel’s second’s law
Gene = basic unit of inheritance
Allele = alternative variants of gene
Genotype = combination of alleles at a region/gene
Phenotype = observed trait of an organism (blue eye)
Homozygosity = 2 same alleles
Heterozygosity = 2 different alleles
Chromosome = physical units of inheritance
Genome = all DNA sequences of an organism

Mendelian disorders
- Huntington’s disease = brain disorder  one gene = dominant effect  there is no treatment
- PKU = metabolic disorder  recessive effect = you only get the disease if you have the two alleles
 there is a treatment: diet, newborn baby’s prik  makes sense to care about inheritance and
Mendel’s laws
- Scurvy = disorder caused by a single gene in humans. However, we don’t consider calling it a genetic
disorder, why? There is no genetic variation, everyone has that mutation = defective gene

Exception to Mendel’s second law = independent assortment  inheritance of one gene is not
affected by the inheritance of another gene
1. law of holds  for genes on different chromosomes, for genes that are far apart of the same
chromosome  The law does not hold for: genes in proximity on the same chromosome
2. X-linked inheritance = sex-dependent transmission (for example: color blindness)
3. Quantitative trains (complex traits)

,Lecture 2 (2A)

Nurture = experience, environment
Nature = genetics

Extremes of nurture
- John Watson  dozen healthy infants, random take one and train him to become any type of
specialist (correct nurture, I can turn you to anything)
- French revolution: born as a blank slate and experience shape you = the good

Bad consequences of extreme nurture view
- Freud’s psychoanalyses
- Noble savage

Extremes of nature
- Konrad Lorenz  natural selection determined the evolution of cultures in the same way as it did
that of species (culture that seems better, those are more fit)
- The good/positives about these extremes of nature: better treatment for many diseases and
disorders, critical because there are environmentalism movement, and animal welfare movement
- The bad consequences of extreme nature view: social Darwinism, racism, and eugenics (ras)

Extreme cases are very rare  nurture and nature overlap = interesting area

Exceptions to Mendel’s second law  quantitative (complex) traits
- two elements
- you get it by polygenic trait = a lot of elements = not caused by single genes
- 1 gene  3 genotypes  3 phenotypes
- increase number of genes  more geno and pheno

Qualitative traits  influences by a single gene
Quantitative traits  influence by multiple genes

Estimating genetic and environmental influences
- adoption study  experiment of nurture (biological fam + non-biological fam)
- twin study  experiment of nature (raised together + apart)
- family study

Correlation: 0 = no resemblance, 1 = perfect resemblance
Relative risk = change to develop a disorder when a family member has the disorder
Concordance/Discordance = presence/absent of a particular condition in two family members

Adoption studies  genetic-plus-environmental relatives
- adoptive parents  shared environment (relatives)
- biological parents  shared genes (relatives)

PKU = recessive (kid + biological parents by adoption share this because it is genetic)
PKU  adoptive parent + kid  0 correlation, because you can’t say anything, because they are not
genetic related

First adoption study
- 47 adopted children with biological mother with schizophrenia  5 kids with schizophrenia
- 47 controls (biological mothers no schizophrenia)  0 kids with schizophrenia

,  It has a genetic component

More recent study (adoption study)
- 361 families from Finland, biological mother had schizophrenia  kid: 4,9 % schizophrenia and 9,1%
schizophrenia spectrum disorder
- adopted children from mother without 1,1% schizophrenia
 genetics plays a role, interaction between genetic vulnerability and environment

What are some potential issues with the adoption design?
- genetic preposition, different environment  when you look at it, that’s a lot of nature, but it can
be that the experience can correlate with the genetic preposition
- adoption  they try to match the parents’ traits to the kid (same level of intelligence)
 they are biased

Issues in adoption studies
- reduction in number of adoptions  practical, good thing for society, but not for researchers
- not representative, can’t generalize
- prenatal environment (before they born, they have influence on the biological environment)
- selective placement

Twin studies:
Identical twins  single egg fertilized by single sperm, then splits in two = share all genetics
Fraternal twins  separate = share half their genes (50%)

How do we know they are twins?
- physical resemblance
- always same sex + same sex chromosomes
- DNA markers

Issues twin studies:
- twin method assumed: environmental is roughly the same for both types of twins reared in the
same family
- But the identical twins experience more similar environment than fraternal twins
- generalizability is an issue for the twin method: are twins’ representative of the general population?
Most evidence suggest a yes
- common environment can be experienced different between twins

Combining adoption and twin studies

Heritability = proportion of phenotypic variance that can be accounted for by genetic differences
among individuals  differences in group and related by genetics

A = genetic sharing
C = shared environment

r (correlation) (MZ twins) = A + C
r (DZ twins) = ½ A + C
 zie slides!

Heritability is not the same as genetic inheritance
Heritability = about differences between individuals and not about a particular individual

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