Lecture 2 - Behavioural Genetics 2

genotype

-particular form of specific gene that a person has

phenotype

-physical expression of a gene

-observable characteristics of an organism

allele

-two or more alternative versions of the same gene

heterozygous

-having two different alleles for the same gene within one individual

homozygous

-having two same alleles for the same gene within one individual

mendel

-studied traits without knowing structure of DNA

-inferred by looking at presentation of traits

• dichotomous traits → only show in one form or another and never in common

inheritance pattern - mendel

-pea pods can be green or yellow

-some green and yellow pea pods

-bred them together

-next generation were all green

-second generation green pea pods and bred them together again

-third generation had a ration of 3 green pea pods for every 1 yellow pea pod

explaining mendel’s inheritance pattern

-green pea pods had two GG and yellow pea pods had two yy

-pea pod inherited a letter from each pea pod parent

-big G for green pea pod and little y for yellow pea pod -> so second generation is Gy

-big G means will be green no matter what you have, small y is recessive and so doesn’t produce any colour if combined with big G

-second generation bred → pea pod can inherit big G and small y, one pea pod will get two small y from parents and end up yellow

-> ratio of 3:1

punit square

-two biological parents in headers

-put alleles in square

-fill it in and it calculates the ratio

	G	y
G	GG	Gy
y	Gy	yy

serotonin transporter gene (5-HTT)

-removes excess serotonin from synaptic cleft by transporting it back into presynaptic cleft

-is a protein so is made by a gene

-comes in long form (l) and short form (s)

-short form causes greater risk of depression

short and long forms (serotonin transporter gene)

-not dominant or recessive

-effects are additive → more long alleles mean more long 5-HTT and more short alleles mean more short 5-HTT

chromosomes

-24 pairs of chromosomes

-live in the nucleus of the cell

-made of DNA

-two forms of every gene on chromosomes

-so two alleles for additive effects and dominance effects

DNA

-deoxyribose nucleic acid

-helical structure

-made of nucleotides in pairs

nucleotides (DNA)

• adenine (A)

• cytosine (C)

• thymine (T)

• guanine (G)

-A always paired with T

-C always paired with G

DNA replication

-DNA can unwind

-spare bases in the cell join up their pairs

-allows it to make complimentary strands of DNA and replicate itself

-allows new cells to grow

mitosis (DNA replication)

-replicate DNA and grow new cell

-skin cells

meiosis (DNA replication)

-replication and recombination

-through a series of cell divisions end up with cells that have just one of each chromosome pair

-gametes, sperms and eggs

making proteins (DNA)

-DNA unwinds and as it does it replicates

-rather than making a complimentary strand of DNA it makes messenger RNA

-RNA similar to DNA but has one different base pair → has U (eurosil) instead of T

transcription

-DNA unwinds

-get complementary stand of mRNA

-on the chromosome on the DNA are particular genes which are particular segments of our DNA

-have the coding to make proteins

-promotor region is just before the gene -> when this is activated the gene transcribes into mRNA

-single stranded mRNA in nucleus floats out into cytoplasm of the cell

translation

-after transcription

-ribosome moves mRNA along

-transfer RNA is dragged as the ribosome goes along the mRNA

-tRNA has got three bases on its bump

-code for protein works in triplets, three bases on each piece of tRNA and the ribosome goes along the mRNA

-tRNA got triplet code and it has amino acid on its head, as it moves along it joins them together -> forming strands of amino acid

-amino acid is primary structure of protein

-mRNA picks out tRNA that has particular amino acid on them to form particular amino acid chain that gives you a specific protein

tryptophan hydroxylase gene

-enzyme that makes serotonin

-gene determines how much serotonin you have

-take tryptophan from diet and then rate limiting enzyme tryptophan hydroxylase turns that into 5-HTP

-5-HTP gets turned into 5HT (serotonin)

mutant form (tryptophan hydroxylase)

-in vitro or mice models

-cells with this gene make 80% less serotonin than cells with more common gene

Zhang (tryptophan hydroxylase)

-87 depressed patients → 9 patients had mutant form

-219 control patients → 3 people had mutant form → had history of anxiety, alcoholism, family history of mental health problems

-not many people with it in each group so doesn’t explain every case of depression, many depressed people don’t have mutant form

-3 times as many people in the depression group have it

endophenotype

-phenotype that’s underlying a complex trait

Weinberger - method (gene for serotonin transporter)

-fMRI

-no patients had diagnosis of depression

-people with different forms of 5-HTT gene → short (s) or long (l)

-asked to judge angry and afraid faces

-this activates amygdala → deals with emotions and memory

Weinberger - results (gene for serotonin transporter)

-those with two short forms have bigger response in amygdala compared to those with two longer forms of the gene

-may be endophenotype -> risk factor for depression as responding more to negative emotions

-but many people have short genes and not depression so need to look at role of the environment

Brown (role of environment in depression)

-80-90% women diagnosed with depression had experienced stressful life events in past year

-only 32% of controls experienced severe stress

diathesis-stress model

-risk factor genes for depression + experiencing environmental stress leads to depression

-risk factor genes + no environmental stress so don’t get depressed

-won’t get depression without risk factor genes

caspi - method (diathesis-stress model)

-measured stressful events

-split into groups based on forms of serotonin transporter gene

-measured depressive symptoms

caspi - result (diathesis-stress model)

-no difference in number of stressful life vents between genetic groups → shows those with short gene do not overreport experiences

-found number of stressful life events was non-significant

-genotype was non-significant

-but interaction between genes and environment was highly significant

caspi - interaction graphs (diathesis-stress model)

-number of stressful life events across x axis

-different genotypes depicted by lines

-s/s on average more depressive symptoms with more stressful life events compared to s/l in the middle and l/l the lowest

-s/s self-report of depressive symptoms largely increases as number of stressful life events increase compared to other groups

caspi - conclusions (diathesis-stress model)

-more short forms you have the more likely you are to be depressed

-but only when combined with stressful life events

-demonstrates gene-environment interaction