1
Q
Quantitative Genetics
A
- The genetic analysis of complex characteristics
- vary along a continuous scale
2
Q
Quantitative Trait Loci
A
chromosome regions containing genes that influence a quantitative trait
3
Q
polygenic
A
encoded by multiple genes
4
Q
Quantitative characteristics
A
- many are influenced by genes at many loci (polygenic) and influenced by environmental factors.
- vary along a scale of measurement with many overlapping phenotypes.
5
Q
Quantitative Traits (continuous traits)
A
- the relationship between genotype and phenotype is more complex
- if the characteristics is polygenic, many different genotypes are possible, several of which may produce the same phenotype.
6
Q
Discontinuous Traits
A
- the relationship between genotype and phenotype is straightforward
- each genotype produces a single phenotypes, and most phenotypes are encoded by a single genotype
- dominance and epistasis may allow two or three genotypes to produce the same phenotype but the relationship is simple
7
Q
Multiple factor hypothesis
A
- expression depends on the additive effects of a number of genes
- the effect of each gene is small
- environment plays an important role in expression of the trait
8
Q
Meristic characteristics
A
- measured in whole numbers
- limited number of phenotypes but the underlying determination of the characteristics may still be quantitative
- ex. litter size
9
Q
Threshold characteristic
A
- simply absent or present
- the presence of some diseases can be considered a threshold characteristic
- when susceptibility is larger than a threshold value, a specific trait is expressed
10
Q
frequency distribution
A
- a graph of the frequencies (numbers or proportions) of the different phenotypes
- connecting the points of a frequency distribution with a line creates a curve that is characteristic of the distribution
11
Q
normal distribution
A
symmetrical bell shaped curve exhibited by many quantitative characteristics
12
Q
mean
A
- average
- provides information about the center of the distribution
- adding up all the individual measurements and diving by the total number of measurements in the sample
13
Q
variance
A
- the variability of a group of measurements, or how spread out the distribution is (width of the curve)
- the larger the variance, the greater the spread of measurements in a distribution about its mean
- s^2 or o^2 - the average squared deviation from the mean
14
Q
standard deviation
A
- s or o
- the square root of the variance
15
Q
Mean +/- standard deviation
A
+/- 1 SD - 66% of measurements in a normal distribution
+/- 2 SD - 95%
+/- 3 SD - 99%
16
Q
General Statistical Model
A
Phenotype = Genotype + Environment + Genotype*Environment Interaction
17
Q
The quantity that drives selection
A
- variance NOT mean
- if there is no genetic variation in the strain we are studying, there can be no genetic improvement
18
Q
Phenotypic variance
A
a measure of the degree of phenotypic differences among a group of individuals
19
Q
Components of phenotypic variance
A
- genetic variance
- environmental variance
- genie-environmental interaction variance
20
Q
Genetic variance
A
- VarG
- phenotypic variance due to differences in genotypes among different individuals of the population
21
Q
Environmental variance
A
- VarE
- phenotypic variance due to differences in environment among different individuals of the population
22
Q
Genia-Environmental interaction variance
A
- VarGE
- phenotypic variance due to interaction between genotype and environment.
- genotypes are expressed differently in different environments
23
Q
Simplified statistical model
A
- if we constrain the environment to a narrow range, then we may be able to ignore the interaction terms
- now becomes VarP=VarG+VarE
24
Q
heritability
A
- the fraction of phenotypic variation that is directly attributable to variation in genotypes
- can be used to predict the rate and amount of selection response in a breeding program
25
high heritability means
high percentage of phenotypic variance due to genotypic variance
26
Broad sense heritability
- the proportion of phenotypic variance due to some type of genetic effect (all sources)
H^2=VG/(VG+VE)
H^2=VG/VP
27
If heritability is high
VG is high and VE is small
28
If heritability is low
VG is small and VE is high
29
Partitioning Genetic Variance
- can be additive
- can be dominant
- can be epistatic
30
Additive variance
- VarA
- determines the resemblance between parents and offspring
- the phenotypes of the offspring will be intermediate of those of the parents
31
Dominant
- VarD
| - the effect of the allele depends on the identity of the other allele at the locus
32
Epistatic
- VarI
- epistatic interactions may occur between some genes and that is part of genetic variance.
- We wil ignore this in our calculations
V(G)=Var(A)+Var(D)
33
Narrow-sense heritability
- the proportion of phenotypic variance due to additive genetic effects.
- specialized heritability used in selection experiments
- the higher the heritability, the more progress we can expect from a selection/breeding program.
h^2/VA/VP
h^2=VA/VG + VE
h^2=VA/VA + VD + VE
34
Heritability and traits
heritability says how much genes affect the variation in the trait
35
the value of heritability
specific for a particular population in a particular environment and is not expected to be the same in a different population and/or different environment
36
A plant population with a heritability of 0.9 for height
90% of the phenotypic variation in the population is due to genetic effects and 10% is due to environmental effects
37
natural selection
may bring about genetic change in a quantitative characteristic
38
Artificial selection
promoting the reproduction of organisms with traits perceived as desirable
39
Selection response
the change in mean trait value over one generation
40
Selection response may change as a result of
selecting for a particular characteristic in a population for a long period of time
41
truncation (mass selection)
- we are interested in increasing some quantitative trait through selective breeding
- select individuals who are above/below a certain value as parents for the next generation
42
additive genetic variance
determines resemblance between parents and offspring
43
XT
truncation selection point
| - line in front of the colored in region
44
SD
selection differential
| - b/w mean of original pop and mean of selected individuals
45
SR
selection response
| - b/w mean of original pop and mean of offspring pop
46
X bar (u)
mean of original population
47
Xs bar
mean of selected parents
| - in the colored region
48
X' bar
mean of the offspring (next generation)
49
p
proportion selected
50
o
standard deviation
51
I
- selection intensity
| - get from chart
52
Z
- standardized selection point
| - get from chart
53
Values for I and Z are negative when
selection is left of the population mean
54
Truncation Point equation
XT=u+standardized selection point * o
55
selection differential equation
SD=I*o
56
mean of selected parents equation
Xs bar = u +SD
57
selection response equation
SR=h^2SD
58
mean of next generation equation
X' bar = u + SR
59
What is heritability not?
- Does not measure the amount that genes are involved in a trait
- Does not measure the relative important of genes and environments for a trait
- Does not involve just the genes
- Not the same for all traits for a population
- Not a statement about individuals
60
individuals to the right of the truncation point are chosen as
the next parents
GN 311
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