behavioural genetics 2 Flashcards
Mendel (1822-84) - mandelian inheritance
dichotomous traits
one form or other - never in combination
cross with green and yellow peas - found dominant and recessive alleles from different percentages of green and yellow produced
Mendel green and yellow peas
crossed green with yellow (parents = P gen)
F1 gen = all green pods
F2 gen = 3:1 ratio of green : yellow pods
green = dominant - G
yellow = recessive - y
genetics terms (all gcse/a-level stuff)
- phenotype
- genotype
- allele
- heterozygous
- homozygous
- dominant
- recessive
punnett squares
Phenotype – trait e.g. yellow versus green
Genotype – genetic material (e.g. GG)
Allele – different forms of a gene that control the same trait (G versus y)
Heterozygous: G y
Homozygous: GG or y y
G is dominant to y
y is recessive to G
also Punnett squares to work out inheritance of dom/rec genes
inheritance of gene for serotonin transporter - 2 forms
gene alleles = long (l) and short (s)
short form = higher incidence of depression
l + s are both recessive - so the effect is additive
additive - the A in variation equation (Vp=A^2+C^2+E^2)
heterozygous = have a combination of long and short serotonin transporters
why have 2 alleles - 2 copies of each gene
inherit one chromosome of each pair from either parent - therefore inherit 2 alleles
A-LEVEL - DNA, genes, mitosis, meiosis, replication, protein synthesis
quick recap if needed - slides 18-25
DNA replication - unwind and new strands form from free bases into new double helices
protein synthesis - using mRNA to form a new double helix, in sections called codons (3)
DNA vs RNA - ATGC or AUGC
mRNA = transcription from DNA
tRNA = translation from mRNA to form amino acid chains in a ribosome - AA is joined to a specific codon
formation of serotonin
tryptophan –> 5-hydroxytryptophan (5-HTP) –> 5-HT (serotonin)
tryptophan hydroxylase = rate-limiting enzyme which controls the formation of 5-HTP from tryptophan
tryptophan hydroxylase gene mutation
has a mutant form
in vitro cells with this gene make 80% less serotonin than cells with common gene
could relate to depression
tryptophan hydroxylase mutant gene study
Zhang et al (2005)
compared 87 depressed with 219 control participants
found 9 depressed with mutant gene
found 3 control with mutant gene
3 controls with mutant = anxiety, alcoholism, + family history of mental health problems
limitation of tryptophan hydroxylase mutant gene theory of depression
results in mental health problems
BUT many clinically depressed don’t have mutant form
could be that psychological measures are incorrect - use brain scanning to check it
long vs short gene for serotonin transporter
transporter = 5-HTT
serotonin = 5-HT
short form = sometimes linked to depression, sometimes not
physically shorter than the long form allele
check this with brain scanning
brain scanning study of amygdala in depressed patients (Weinberger)
fMRI scan
show participants different faces - angry and afraid expressions
observe responses in the amygdala - region for emotion and memory
make comparison between those with long (normal) or short form of 5-HTT gene
short group have higher brain activation to negative emotions than long form group (control)
fMRI long vs short form 5-HTT gene conclusions
brain over-responds to negative emotions with short form
could be risk factor for depression
but many with short form are not depressed
therefore environmental input
Brown (1993) - environment effect on depression
compared depressed with control participants
84% of depressed had severe stress in the previous year
32% of control had severe stress in past year
diathesis-stress model for depression
risk factor of genes
stress as a trigger
both risk and stress = depression
