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behavioural genetics-2

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what is a candidate gene?
A candidate gene is one whose function suggests that it may be associated with a given trait or condition.
what does a CGAS do?
Investigates whether a particular allele (a version of a particular gene) is more common in patients than controls. The gene chosen for examination is normally selected on the basis of an existing theory of the physiological basis for the condition or the variation. E.g. studying DRD4- the gene for the D4 receptor for dopamine. Garcia et al (2010)- link between D4 and number of one night stands
what did Johnson et al find regarding candidate genes for schizophrenia?
Johnson et al- compared 25 candidate genes typically associated with schizophrenia to 25 random genes- no more strongly associated with the condition than were a non-candidate set of genes
what is a GWAS?
DNA is obtained from >1000 ppts whose behaviour or score on some trait has been measured. Their genomes are probed at 1 million SNPs. genome wide association study
what are the stages of investigating a whole genome set during GWAS?
Whole genome amplification and fragmentation
Denaturation and hybridisation on BeadChip
Single base extension and staining
Array scanning and genotype scoring
is there a single gene that accounts for intelligence or personality or schizophrenia?
There is no single gene that accounts for even 1% of the variance in intelligence or in personality. There are however rare genes that have tiny effects that are significant when huge cohorts are used.
•There is no common allele that is present in most cases of schizophrenia and autism, although a number of significant susceptibility loci have been identified- each has a v small effect
•Schizophrenia may be polygenic- it is the cumulative results of many small effects
what is a GCTA?
Genome wide complex trait analysis
•the data comes from whole-genome association studies
•asks if all the measured SNPs are considered concurrently, how much of the variance of a behavioural trait can we account for.
•Thus, it helps to obtain a direct estimate for the heritability of the trait
what is the heritability of intelligence?
Heritability of intelligence is around 40% for crystallised intelligence and 51% for fluid intelligence
•Intelligence is less heritable when you are a child than when you get older (paradoxical)- cardiovascular disease
what is the heritability of neuroticism?
The heritability of neuroticism is only 0.06% and extraversion 0.12%. Maybe twin studies are wrong or maybe other variations in the genome cause it
how many chromosomes is the human genome made up of?
•The human genome is shared out amongst 46 chromosomes.
how can chromosomes be viewed?
Chromosomes can be viewed by light microscopy and cytological staining (which produces characteristic banding along the length of the chromosome
what is the long arm called and what is the short arm called? what separates them?
Long arm- q arm
Short arm- p arm
Separated by the centromere- contains no genetic information and isn't necessarily in the centre of the chromosome but has an important role in cell division
how is chromosomal location described? what does each part of 7q31.2 mean?
7q31.2- chromosome 7, q- long arm, region 3, band 1, sub-band 2
describe down syndrome as a chromosomal disorder? what causes it? how many per 1000 suffer from it? which chromosome is duplicated?
Extra copy of smallest chromosome- number 21
First described by Hogn Langton Down in 1866
It is the most common form of intellectual disability- 1 in 1000 birth
Normally the presence of an extra chromosome is fatal to the foetus- with the except of down syndrome
Characteristic anatomical features- short stature, speckled iris etc
what is William's syndrome and what causes it?
William's syndrome- deletion of part of chromosome 7
Wide mouth (elfin face), aortic stenosis (narrowing of the aortic valve in the heart).
The deleted region includes 25 or more genes
Have intellectual disability and spatial skills are impaired
what is balanced translocation? what can it cause? (Scottish family)
Translocation- a bit of a chromosome gets transferred with another chromosome-
E.g. tip of 1 chromosome with tip of chromosome 11. This was found in a large Scottish family. The breakpoint disrupts a gene that was christened DISC1
DISC1- disordered in schizophrenia 1
Can conclude that gross translocation was the source of psychiatric illness in the original family but it is not the sole cause of schizophrenia
what is meiosis?
Meiosis is a process that leads to the formation of gametes- sperm and egg cells. Gametes contain 23 pairs- so that when they are combined it restores the full 46 chromosomes. his process therefore ensures that half the genetic material comes from the mother and half from the father.During meiosis a process of recombination takes place- when homologus chromosomes come together and part of one chromosome is exchanged for the corresponding part of its sister. This allows shuffling of genetic inheritance but errors can occur if the sister chromosomes are misaligned before recombination occurs.
what is linkage?
Linkage- the closer genes are on a particular chromosome- this creates a greater probability that specific alleles will travel together through successive generations of a family without being separated by recombination
what is linkage equilibrium?
Linkage equilibrium- when the alleles of 2 genes are inherited independently as they are on genes of different chromosomes
describe the X chromosome... what does it mean that conditions are X linked? name some
• The X chromosome carries a disproportionate number of genes that have a role in the eye or brain
• some conditions are 'X linked', occurring only, or more severely, in men.
• X linked conditions are necessarily inherited from the mother
• E.g. Fragile X syndrome
• E.g. colour-blindness
how does colour blindness occur?
•Normal colour vision is dependent on the presence of three types of cone cell in the retina
•Each type of cone cell has peak sensitivities in different parts of the spectrum
•The light absorbing proteins in the long wave and middle wave cones are encoded by adjacent genes and this means they can misalign at meiosis. This leads to the lack of one gene on the x chromosome which causes dichromatic vision
what are the 3 types of colour-blindness?
•There are 3 types of colour-blindness- dichromatic, anomalous trichromacy and normal trichromacy
what is X Chromosome inactivation? is it an advantage or disadvantage? is it ordered or random?
X chromosome inactivation
• Although every woman has two X chromosomes, only one of them is expressed - makes protein - in any cell of her body.
• Discovered by the geneticist Mary Lyon (Girton)
• The inactivation is random and occurs early in the embryo, when there are a small number of cells
• The official explanation of X-chromosome inactivation is 'dosage compensation'- if both chromosomes were expressed (making protein) in women while only 1 was in men then many adjustments would be needed in those systems that don't differ between men and women
• There is another advantage of X chromosome inactivation- this is that the two proteins in different cells are segregated
• E.g. suppose a woman has one gene for a normal long wavelength photopigment but on her other X chromosome she has a gene for an anomalous pigment. X activation ensures that the different proteins are segregated in different cone cells in her retina meaning that she has 4 types of light absorbing cone cell while everyone else has three. This gives her an extra dimension of colour experience.
Do twin studies accurately estimate heritability?
• Non-additive genetic variations- the assumption of simple additivity underestimates the difference between MZ and DZ twins in terms of their genetic similarity
• Random X inactivation in female twins- although MZ twins are nominally identical the level of expression, owing to the random nature of X chromosome inactivation, may differ. This explains discordance for X linked conditions such as fragile X syndrome
• Assortative mating
• Differences in the similarity of enviornments- MZ have a more similar environment as well as sharing more genes
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