behavioural genetics 2 Flashcards
Mendel (1822-1884)
- studied dichotomous traits
–> one form or another
–> NEVER in combination - quite different to what psychologists tend to focus on (tend to focus on traits on a spectrum/scale)
Mendel’s pea pods summary
- Green pods (GG) bred with yellow pods (yy)
–> GG x yy - first gen of offspring were green
–> Gy - second gen of offspring had ratio of 3:1
–> 3 green (Gy)
–> 1 yellow (yy)
phenotype
- physical manifestation of the genotype
- trait/characteristic
–> e.g. blue eyes
genotype
the genetic material
allele
different forms of a gene that control the same trait
–> e.g. G vs y
heterozygous
- one dominant allele and one recessive allele
–> e.g. Gg
homozygous
- same type of allele
- two dominant or two recessive
- e.g. GG or yy
Dominant and recessive
- dominant = capital letter (G)
- recessive = lower case letter (g)
–> recessive need two alleles to present the characteristic
gene for serotonin transporter
- gene for serotonin transporter (reuptake of serotonin into the presynaptic neuron - ready for rebuilding in vesicles) has 2 forms
1. long (l)
2. short (s) - both are recessive
- l isn’t recessive to s
forms of serotonin transporter
- short form (s) associated with higher incident rate of depression
additive genes?
- both l and s are recessive
- they are additive
- heterozygous genotypes lead to about 50% of each
–> e.g. ls will lead to about 50% long and 50% short - homozygous will produce all of one type
–> e.g. ss will produce all short proteins - additive genotypes do not produce a definitive phenotype due to dominance, rather the effects of genotypes sort of add up
additive genes and the variance equation
- Vp = A squared + C squared + E squared
- A is the ADDIDITVE genetic component
why two alleles in a genotype?
- alleles refer to chromosomes
- chromosomes come in pairs
- autosomes = all other traits, practically identical (22 pairs)
- allosomes = sex chromosome (XX or XY)
chromosomes and DNA
- made of DNA
- DNA is made up of 4 bases:
1. adenine
2. Thymine
3. Cytosine
4. Guanine - in 2 pairs
- AT, GC (@ gemma collins)
what does DNA do?
- replicates
- double helix (2 strands) unwind
- exposed pairs line up with each strand
- 2 new complementary strands form - makes proteins
- DNA unwinds, complementary strand of mRNA lines up
–> old DNA and ONE NEW mRNA
–> uses tRNA, have codes which bind to complementary codes on the mRNA, amino acids on tRNA bind together
RNA recap
- ribose
- uracil not thymine
- base pairs = AU, GC
the Tryptophan Hydroxylase gene (+ mutations)
- enzyme that makes serotonin
- mutant forms of the gene can result in less serotonin cells
–> in vitro (grown in a dish) cell with this gene made 80% less serotonin than cells with the more common gene
Zhang et al. (2005)
- 87 depressed Ps
–> 9 patients with mutant form - 219 control group Ps
–> 3 people with mutant form - suggests that the mutant form led to less serotonin and thus higher chances of depression
- history of mental health, anxiety and alcoholism were found in the 3 mutant subjects in the control group
issues with Zhang et al. (2005)
many clinically depressed Ps didn’t have the mutant form of the gene
does short forms of the serotonin transporter protein always result in higher depression levels?
- no
- sometimes linked to depression and sometimes it isn’t
–> could be that psychological measures of depression are inaccurate
–> could be that the link doesn’t exist - solution: use brain scanners
brain scanners and negativity
- use fMRI scanner
- ask people to judge angry and afraid faces
- look at response of amygdala
–> brain region that deals with emotions and memory - compare 2 groups
1. those with normal gene
2. those with 2 forms of the short gene - look at size of response (who responds more to negative emotions?)
–> likely to find higher brain activation to negative emotions in short form of the gene
effects of the short form of the serotonin transporter gene
- makes the brain “over-respond” to negative emotions
- risk factor for depression?
–> but many people have short gene(s) and not depression? - NEED THE ENVIRONMENT
Brown (1993) environment and depression
- 84% of depressed Ps had severed stress in the previous year
- only 32% in the control group has severe stress in the past year
diathesis-stress model
- can have risk factor genes or NO risk factor genes
can have stress or NO stress - having risk factor genes AND stress is the only combination linked to depression
–> risk factor + stress = depressed patient
Caspi et al. (2003)
- subjects from Dunedin multidisciplinary Health and development study
–> 1037 subjects
–> age 26 - measured stressful events
- divided subjects into groups based on forms of serotonin transporter gene
- measured depressive symptoms
results of Caspi et al. (2003)
- no difference in number of stressful events between genetic groups
- depressive symptoms:
–> number of stress full life events = non-significant
–> genotype = non-significant
–> interaction of genes AND environment = highly significant
Caspi study summary
- 5HT-T gene short (s) and long (l)
- more short forms you have more likely to be depressed
–> ss versus ls versus ll
–> BUT only when combined with stressful life events - gene x environment interaction
Nicotine Acetylcholine Receptor α4 Subunit Gene
- ** smoking **
- addiction in general
- ADHD
- internet gaming in male Koreans
GABRA2 – a GABA receptor (inhibitory)
- ** alcohol dependence**
- autism
- epilepsy
DAT1 Gene – dopamine reuptake transporter
- cocaine addiction
- ADHD
- schizophrenia
- alcohol use
comorbidity
- as gene variants are associated with lots of traits/diagnosis this could suggest comorbidity
- could be that a gene underlies lots of different disorders (common theme?)