36 Quantitative Genetics Flashcards
Quantitative traits…
- have continuous variation
- have no distinct F2 classes
- often assume normal distribution
Multifactor Hypothesis
- Edward East
- expression depends on the additive effects of a number of genes
- the effect of each gene is small
- environment plays an important role in the expression of traits
What is the effect of the environment on the curve of frequency of phenotype classes?
it smooths the curve
environmental effects allow…
one genotype to produce multiple phenotypes
variance
average squared deviation from the mean
related to the width of the curve - more narrow, less variance
wider curve equals
more variation
the units for standard deviation are the same as
those for the original measure of the trait
threshold traits
quantitative traits with only two phenotypes
an accumulation of factors pushes the phenotype over the limit to the second trait
Phenotype =
genetic + environment effect
P = G+ E
Genetic effect =
additive + dominance effect
G = A + D
additive effect
average effect of substituting A for a in genotype
dominance effect
due to the fact that sometimes the heterozygote is not halfway between the two homozygotes
when does D (for dominance effect) equal 0
if the value of Aa is exactly between the values of AA and aa
complete dominance
the heterozygote and BB have the same value
Vp
s^2
phenotypic variance
variance calculated from the measurements of the phenotypes of the individuals in the population
Vg
genetic variance
Ve
environmental variance
Va
additive genetic variance
Vd
dominance genetic variance
Vp =
Vg + Ve
Vg =
Va + Vd
Vi
epistatic interaction
will not be included in our calculations
Vge
interaction between the gene and its environment
will not be included in our calculations
heritability
proportion of the phenotypic variance due to genetic effects
used to predict rate and amount of selection response in a breeding program
types of heritability
broad-sense
narrow sense
broad sense heritability
H^2 = Vg/Vp
proportion of phenotypic variance due to genetic effect
narrow sense heritability
h^2 = Va/Vp
proportion of phenotypic variance due to additive genetic effects
heritability = 0
all Vp is due to environmental variance
heritability = 1
all Vp is due to genetic variance
Will higher or lower heritability lead to increase progress in a selection program?
higher
Ve
all variation in the parental and F1 generations
each generation has only one genotype - pure bred line
the best estimate of variance due to environmental effects
V(parent1) + V(parent2) + V(F1)/3
phenotypic variance is estimated by
the variance in F2 since if has all possible genotypes
V(F2) = Vp = Vg+ Ve
Assume environmental variance in F2 is…
the same as the previous generation
broad sense heritability is better for asexual or sexual plants
asexual
response to selection
extent to which the characteristics change over a generation
R = xbar1 - xbar0
selection differential
the difference between the average of the initial population and the average of those selected to be parents
the potential for change
S = Xbars - xbar0
limits to selection response
selection response may decline after selecting for a particular characteristic in a population for a longer period of time because…
- genetic variation is bein lost
- extreme types may not be healthy
- two desirable traits may be negatively correlated making it difficult to select for both
Why does progress from selection level off?
often because the population is approaching a biological limiting factor
Why did corn with varying degrees of oil level off?
the low oil content approach a limit as a certain amount of oil must be present for growth
the high oil content continued to grow
Why did drosophila with varying number of bristle level off
the number increased for many generations and then leveled off as it approached a biological limiting factor - likely room for the bristles
pleiotropy
a change in value for one trait is accompanied by a change in value for another trait due to a gene that affects on also affects the other
can be pos or neg correlation
how does pleiotropy impact breeding
milk cow example
milk yield and milk fat % are correlated - as milk yield increases, milk fat decreases and visa versa - if you select for one, you lose the other
pleiotropy may create a challenge to overcome
QTL
quantitative trait loci
identified by linkage analysis between the trait and molecular markers
if the inheritance of a genetic marker is associated with the inheritance of a quantitative trait, the marker must be…
linked to a QTL involved in that trait
SNPs
single nucleotide polymorphisms
common markers used to identify QTL
SNPs
GWAS
genome-wide association studies
associate a trait and gene markers in a biological population as opposed to controlled crosses
use molecular genetics to scan entire genome for regions that show statistical sig for a trait
DGRP
drosophila genetic reference panel
developed to help identify QTLs
mouse collaborative cross
used to identify QTLs
limitations to heritability
- does not say how much genes affects the trait, but how much they affect the variation in the trait
- not seen in an individual - only in population
- specific to a particular population in a particular environment only
- assumes that the environment for related individuals is not more similar than the environment for unrelated peoples
monozygotic
identical twins
dizygotic
ferternal twins
concordant
twins same for trait
discordant
twins differ for trait
% concordance indicates…
a percentage of that twin group that showed the same phenotype
concordance can be expressed as…
- % concordance
- the ratio of MZ:DZ for a particular trait where the numbers in the ratio are the % concordance for each twin group
can be used to estimate heritability
High MZ: Low DZ
indicates significant role of the variance of genetic effects in the phenotypic variance
similar MZ and DZ
indicates a lot of variation is due to variance in environmental effects
low MZ but still much higher than DZ
indicate genetic predisposition, but variance due to environmental factor is importance in the variance of the phenotypes for this trait
