Lecture 4 - Quantitative variation 2 Flashcards
How does the number of loci affect the distribution of a quantitative phenotype?
More loci - more continuous the range of phenotypes
What is phenotypic variatiance?
How much of the phenotypic variation is explained by the genotype and how much by the environemnt
-Vp
What is the symbol for phenotypic variance?
Vp
What is phenotypic variation due to?
- Genetics
- Environment
- Interaction between genes and the environment (different genes may react differently to different environments)
What two exmaples demonstrate phenotypic variation in quantitative traits?
- Corolla length in longflower tobacco (genetics)
- Yarrow height, depnding on location (environment)
How do Yarrow show phenotypic variation?
Vary in height depending on their location
Clausen experiment
Measured heights at
-Stanford (30m above sea level) - tallest but most variable
-Mather (1400m above sea level) - middle height
-Timberline (3050m above sea level) - smallest
How can we disentangle genetic and environmental effects?
- transplant experiments
- common garden experiments
- twin studies
Describe the components of phenotypic variation (Vp) when plants are grown in two trays: one in a Normal uniform nutrient solution and one that is deficient (in terms of VG and VE)
- within each tray, phenotypic variation is mostly to do with genetic variation (VG)
- between trays it is mostly to do with environmental variation (nutrient availability) - (VE)
Therefore VP = VG + VE
What is the symbol for genetic variance?
VG
What is the symbol for environmental variance?
VE
What is the equation for VP (in terms of VG and VE)?
VP = VG + VE + V(GXE)
How does a shared environment affect phenotypes?
A shared environment increased phenotypic resemblance e.g. units of pigment shows a more continuous range of phenotypes (typical bell curve) when in a shared environment
What components affect genetic variation?
VA = additive genetic variance (effects of allele substitutions) VD = dominance genetic variance (effects of allelic interactions at the same locus) VI = epistasis/interaction genetic variance (effect of interactions between different loci)
What is dominance genetic variance?
VD
-the effects of alleleic interactions at the same locus
e.g. A1A1 - tall plant
A1A2 - tall plant
A2A2 - small plant
Tall is dominant
What is additive genetic variance?
VA
- additive effects of allele substitutions
- co dominance (midway phenotype)
e.g. A1A1 - tall plant
A1A2 - med height plant
A2A2 - small plant
Add up the effects of both phenotypes
What is epistasis/interaction genetic variance?
VI
- effects of interactions between different loci
- e.g. epistasis
e.g. A1A1B1B1 - tall plant
A2A2B2B2 - tall plant
A1A1B2B2, A2A2B1B1 - small plant
interactions
What is VG?
The components of genetic variation
What is the formula for VG (in terms of VA, VD and VI)?
VG = VA + VD + VI
What are the inheritance features of VA, VD and Vi?
VA - simple pattern of inheritance compared to VD and VI, caused by average of the phenotypic effects
VD and VI caused by phenotypic effects of genotypes
How are the inheritance effects of VD illustrated?
Box plot thing whatever they’re called
Typical mendelian first law of segregation box
How is the inheritance effects of VI illustrated?
Massive box plot thing
e.g. B = black, b = brown, E = pigment there, e = pigment not (Dog colour)
4 X 4 = 16 different traits
How did the Clausen (Timberland, Mather, Standford) experiment show that the interaction between genes and the environment also affects phenotypic variation?
At high altitude (timerland) - some plants died(San gregario), whilst others didn’t (big horn lake)
At intermediate altitude (Mather) - some plants grew tall (San gregario) whilst others were small (Big horn lake)
At low altitude - the phenotypes changed again, San gregario v tall, Big horn lake not the leafiness it was at intermediate, more like a small version of the flowring plant it was at high altitude
Showed phenotypuc plasticity
What is phenotypic plasticity?
When a genotype expresses different phenotypes depending on the environment
What are genotype-envirpnemtn interactions?
When a genotype expresses different phenotypes depending on the environemtn, and different genotypes have different kinds of phenotypic plasticity
What are the 5 types of GenotypeXEnvironment interactions?
- VG only
- VE only
- VG and VE
- V(EXG) - phenotypic plasticity
- V(EXG) - phenotypic plasticity - genoptype-environment interaction
How is correlation between twins different for monozygotic and dizygotic twins?
Higher phenotypic correlation with monozygotic twins
What is heritability?
-the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals within a population
What are some common misconceptions about heritability?
- does NOT measure whether a trait has a genetic basis or not
- a heritability when h2 = 0 does not mean that trait has no genetic basis
What is broad sense heritability?
- the proportion of phenotypic variation that is due to genetic variation (including addivitive variation, dominance and epistasis)
- relatively easy to measure, as long as there is no association between environment and genotype
What is the symbol and formula for broad sense heratibility?
H2
=VG/VP
=(VA+VD+VI)/(VP)
What is narrow sense heritability?
-The proportion of phenotypic variation that is due to additive variation
(mostly additive genetic variation VA contributes to parent-offsping resemblance)
-harder to measure
What type of variation mostly contributes to parent-offspring resemblance?
Additive genetic variation
What is the symbol and formula for narrow sense heritability?
h2
h2=VA/VP
How do heritability values vary and why?
Heritability values are a proportion
-vary from 0 to 1
How can we estimate narrow sense heritabilty in natural populations?
Resemblance between relatives
- parent offspring regression
- Sibling analysis
What is Midparent?
-the average phenotypic value of the mother and father
In a graph of Mid-parent offspring regression, what does the narrow sense heritability equal?
The slope of the regression line
h2 = b
How can narrow sense heritability in one parent offspring regression be calculated?
-the narrow sense heritability is twice the slope of regression
h2 = 2b
-based on the fact that one parent is contributing less to the offspring phenotype than both together
What are the features of mother/father one parent offspring regression?
-differences in heritability between mothers and fathers ffrom one parent-offsping regression = Maternal effects
What are maternal effects?
- offspring often resemble their mother more than their father
- due to a shared environment/experience e.g. in womb
- therefore father offspring regressions better indicator of narrow sense heritability
Why are father offspring regressions better indicators of narrow sense heritability?
- offspring often resemble their mother more than their father
- due to a shared environment/experience e.g. in womb
