Lecture 12: Quantitive Genetics Flashcards
Name 3 types of phenotypic variation
discontinuous
continuous (quantitive)
Threshold
Discontinuous variation:
-Discrete classes of phenotype
- NO intermediates
- Follows Mendelian ratios
- Simple genetic basis, limited effect of the environment
e.g. red/white pea flowers
widow peak, dimple and blood groups
Continous variation:
Also known as: quantitive, complex or multifactorial.
- phenotypes do not fall into classes
- offspring of intermediate phenotype
- CANNOT observe Mendelian ratios
- Complex genetic basis plus effect of the environment
- E.g. Height, blood pressure, Beak size
Graph used to show continuous variation data:
‘Normal’ (Gaussian) Graph. Upside bell shape.
What does the graph used for continuous variation data show us?
Peak = MEAN.
68% oh width = standard deviation (measure of spread)
Variance =
sd^2
Threshold variation:
- appears like a discontinuous trait BUT is caused by a continuous distribution.
- distribution caused liability
- phenotype depends on critical threshold
- no simple segregation in families
E.g of threshold variation:
Type 2 diabetes.
As number of predisposing alleles in genotype increase past threshold zone then you’re considered to have DIABETES. Measure of liability.
How do genes cause continuous variation?
Think of coin tossing.
1 toss = 1H 1T
2 toss = 2H 1H1T 2T
3 toss = Even more inbetween values (More possible ways for middle values to be made) =NORMAL GRAPH SHAPE.
same thing if many genes contribute to variation in a trait
Lots of genes contributing to one trait that are close together ==
Quantitive Trait Locus (QTL)
Complications to continuous variation?
-Genes vary in the size of their effect
-Allele frequencies vary
-Some alleles are dominant/ co-dominant
AND ENVIRONMENT has a huge effect
You’re more similar to your siblings than a stranger? WHY?
you inherited some of the same alleles from your parents (NATURE) you grew up in the same environment (NURTURE)
Monozygotic twins:
IDENTICAL
Dizygotic twins:
NOT IDENTICAL
do genes or environment contribute to quantitive traits?
BOTH
The total variation within a population for a phenotype is measured by the..
PHENOTYPIC VARIANCE (Vp)
Phenotypic variance 2 components:
- Genetic variance (Vg)
- environmental variance (Ve)
Genetic variance (Vg)…
Variation due to differences in genotype
Environmental variance (Ve)…
Variation due to environmental effects
If genetic & environmental effects are independent you can write…
Vp = Vg + Ve
Heritability =
H^2 = Vg/Vp
H^2 is called the..
‘broad sense heritability’ of the trait
Broad sense heritability…
‘the proportion of the phenotypic variation in a population that is due to genetic differences among individuals’
Broad sense heritability is often expressed as a
%
H^2 = Vg/Vp =
Vg/(Vg+Ve)
What happens to heritability when we DECREASE genetic variance?
Heritability decreases
What happens to heritability when we INCREASE genetic variance?
Heritability increases
Do estimates of heritability for a particular trait change among populations?
Populations living in different environments can show different degrees of heritability for a particular trait.
Heritability estimates for a trait are…
relative to the genetic & environmental factors in the population
The effect of inbred individuals on heritability..??
= 0 heritability after long time. As Vg (genetic variance) = 0 over time
What does 0 heritability mean?
A zero heritability DOES NOT mean that a trait is not genetically determined.
Why is heritability an excellent predictor for short-term but NOT long-term response
Allele frequencies change over time due to selection and/or genetic drift. This change affects Vg & in turn, heritability.
Genotype-environment interaction is known as…
Nature vs nurture
Genotype-environment interaction is represented by th equation:
Vp = Vg + Ve + VgXe
One way to handle GxE is to consider….
And whats it called
the response to environmental variation as the quantitive trait
called ‘norm of reaction’
The norm of reaction:
a curve that relates, for a given genotype, the contribution of environmental variation to observed phenotypic variation.
Heritability is specific to…
the population and environment in which it is measured.
