HMB265H1 Lecture 13: Quantitative Trait Loci (Lecture 13)

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Published on 23 Mar 2016
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Quantitative Trait Loci (Lecture 2)
How can we determine if there are genetic effects ‘buried’ within a quantitative trait?
Classic nature vs. nurture debate
Are certain aspects due to nature in genetics or nurture in the environment?
Most traits almost allis due to a complex mixture in both genetics and environment
How much of the phenotypic variation is attributable to genetic variation vs. environmental variation?
ℎ= +
VP= Vg+ VE
VP = variation of phenotype
VG = variation due to genetics
VE = variation due to the environment
The interest is looking at the proportion of variation due to genetics
Broad Sense Heritability
The extent of phenotypic variation that is attributable to genetic variation is the broad sense heritability of the
trait
This includes all the variation that could be attributed due to genes
H2=
VG
VP
If all phenotypic variation is attributable to genotypic variation
H2 = 1 (maximum)
If all phenotypic variation is attributable to environmental effects
H2 = 0 (minimum)
What does H2 tell us?
If a lot of phenotypic variation is attributable to genotypic variation H2 is close to 1
o The larger the H2 value the more the variance in the phenotype can be attributable to the
variance associated due to variance in the genotype
How much of this variation is attributable to genetic
variation?
If there is certain susceptibility to a disease (like
blood disease) and it is certain to be attributed
to nurture change in life style
If the disease is attributed to just genetics/nature
go straight into drugs for treatment
Trait measure = phenotypic variation
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o High values suggest that genotype is important in determining whether quantitative trait arises
due to segregation of particular alleles
BUT – H2 does not predict how progeny will perform on the basis of the phenotype of their parents
Definition of broad sense heritability determined by the variations in the genotypes over the variance in
phenotypes
Both of these are based on context
It depends on the phenotype being measured and people in the population
It is ideal to use twin studies to study H2 for traits in humans those separated at birth
Easy to determine the ‘genetic’ component with monozygotic twins
The degree of correlation between the twins in two different environments will give an idea to the
genetic contribution of the phenotype
Covariance of twinshelps to estimate
What H2 does and does not tell us
H2 tells us
If H2 is highthe phenotype of an individual is likely to be attributable to its genotype in that family
Only in the population in which the broad sense heritability has been calculated
What H2 does not tell us
It does not tell us what phenotype an individual will have on the basis of its parents’ phenotype
(nothing about transmission at phenotypic level)
Even if H2 is highan individual’s precise phenotype cannot be predicted on the basis of its parent’s
phenotypes (all context dependent)
H2 is family specific
H2 is only applicable to which the broad sense heritability calculation has been performed
Useful for single families, but not in determining differences of different populations
o Environment could be vastly different and also basis genetics can be different
What H2 tells us
Tells us something about the proportion of the
phenotypic variation that is attributable to
genetics
But! Broad sense heritability encompasses all the
different ways by which genes can contribute to
phenotypic variation NOT just additive effects
epistatic, dominant, etc
What H2 does NOT tell us
There is no direct relationship between the
genotypes of these individuals and the
offspring
This is because of all the different ways in
which context of the gene matters
The only component of genetic variation
that will be transmissible from the parents
to offspring is additive effects
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Document Summary

Classic nature vs. nurture debate: are certain aspects due to nature in genetics or nurture in the environment, most traits almost all is due to a complex mixture in both genetics and environment. If there is certain susceptibility to a disease (like blood disease) and it is certain to be attributed to nurture change in life style. If the disease is attributed to just genetics/nature. The interest is looking at the proportion of variation due to genetics. The extent of phenotypic variation that is attributable to genetic variation is the broad sense heritability of the trait: this includes all the variation that could be attributed due to genes. If all phenotypic variation is attributable to genotypic variation: h2 = 1 (maximum) If all phenotypic variation is attributable to environmental effects: h2 = 0 (minimum) It depends on the phenotype being measured and people in the population. What h2 does and does not tell us.

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