Flashcards in Lecture 12: Quantitive Genetics Deck (37):
Name 3 types of phenotypic 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
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)
-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)
do genes or environment contribute to quantitive traits?
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
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 =
What happens to heritability when we DECREASE genetic variance?
What happens to heritability when we INCREASE genetic variance?
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.