using twin studies to estimate heritability Flashcards
what are complex diseases due to
alleles at a large number of different loci as well as environmental effects
there is not much dispute that physical disorders have a genetic component. what is there much more dispute over
whether psychiatric disorders have a genetic component
what is meant by additive variance
if you have 2 alleles at a locus the phenotypic outcome is a arithmetic sum of the alleles that are influencing it
what is meant by dominant variance
influence of one allele affecte the outcome of the other (depends on the actual configuration of the alleles)
what cannot be transmitted from parents to children
dominance effects
what can twin studies do
disentangle the effects of gene and environment
what is special about identical twins
have exactly the same genotype
how much of the genotype do identical twins share
50%
differences in identical twins are said to be from
experiences that one twin has but not the other twin
If identical twins are considerably more similar than fraternal twins (which is found for most traits) what does this indicate
genes play an important role in these traits
what does the acronym in the ACE model mean
A = additive effects C = common environment E = non shared environment
what does the additive variance A of identical twins =
1
what does the additive variance A of dizygotic twins =
0.5
we make the assumption that the common environment (C) = what for both monozygotic and dizygotic
1
what does the phenotypic correlation of monozygotic twins =
A + C
he difference between the identical and fraternal correlations is due entirely to a halving of the genetic similarity, the additive genetic effect ‘A’ is simply twice the difference between the identical and fraternal correlations:. this can be written as
A = 2(rMZ-rDZ)
how do we work out the effect of common environment using correlation of monoxygotic twins
C = rMZ - A
how do we work out effect of non shared environment using correlation of monozygotic twins
E = 1 - rMZ
what is additive variance ususally denoted as
h^2
what are discontinuous traits measured by
concordance
what does concordance mean
the presence of the same trait in both members of a pair of twins
when can you say twins are “concordant”
twins are concordant when both have or both lack a given trait.
A twin study compares the concordance rate of identical twins to that of fraternal twins. what can this help suggest
whether a disease or a certain trait has a genetic cause
Because identical twins are genetically virtually identical, it follows that a genetic pattern carried by one would very likely also be carried by the other. If a characteristic identified in one twin is caused by a certain gene, then it would also very likely be present in the other twin. Thus, the concordance rate of a given characteristic helps suggest whether or to what extent a characteristic is related to genetics T/F
T
DZT - what % have identical genotypes
25
25% of any dominance effects in DZT will be shared
T
how do dominance effects compare in monozygotic and dizygotic twins
any dominance effects will be shared by MZT but dominance effects will only be shared with 25% DZT
what is meant by epistasis
phenotype of an allele at one locus is affected by the genotype of another locus
what is meant by broad sense heritability
the degree to which a trait is genetically determined, expressed as the ratio of the total genetic variance to the phenotypic variance (VG/VP).
what is narrow sense heritability
the degree to which a trait is passed from parent to offspring expressed as the ratio of the additive genetic variance to the total phenotypic variance (VA/VP).
twin studies are nearer to estimating … sense heritability
broad
what are the limitations to twin studies
- not representatie
- in utero environment is different for twins than it is for singular births (may be expoed to different levels of hormones)
- assumes MZT and DZT share environment
across wide range of phenotypes about 50% of population variance is due to what
genetic variance
