Lecture 10 Flashcards
(60 cards)
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
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high heritability means
high percentage of phenotypic variance due to genotypic variance
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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
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If heritability is high
VG is high and VE is small
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If heritability is low
VG is small and VE is high
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Partitioning Genetic Variance
- can be additive
- can be dominant
- can be epistatic
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Additive variance
- VarA
- determines the resemblance between parents and offspring
- the phenotypes of the offspring will be intermediate of those of the parents
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Dominant
- VarD
| - the effect of the allele depends on the identity of the other allele at the locus
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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)
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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
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Heritability and traits
heritability says how much genes affect the variation in the trait
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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
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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
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natural selection
may bring about genetic change in a quantitative characteristic
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Artificial selection
promoting the reproduction of organisms with traits perceived as desirable
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Selection response
the change in mean trait value over one generation
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Selection response may change as a result of
selecting for a particular characteristic in a population for a long period of time
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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
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additive genetic variance
determines resemblance between parents and offspring
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XT
truncation selection point
| - line in front of the colored in region
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SD
selection differential
| - b/w mean of original pop and mean of selected individuals
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SR
selection response
| - b/w mean of original pop and mean of offspring pop
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X bar (u)
mean of original population
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Xs bar
mean of selected parents
| - in the colored region
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X' bar
mean of the offspring (next generation)
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p
proportion selected
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o
standard deviation
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I
- selection intensity
| - get from chart
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Z
- standardized selection point
| - get from chart
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Values for I and Z are negative when
selection is left of the population mean
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Truncation Point equation
XT=u+standardized selection point * o
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selection differential equation
SD=I*o
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mean of selected parents equation
Xs bar = u +SD
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selection response equation
SR=h^2SD
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mean of next generation equation
X' bar = u + SR
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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
