lectures 11-20 Flashcards
Quantitative traits
influenced by many loci
often interact with the environment
quantitative genetics
study of the genetic mechanisms of continuous phenotypic traits
Heritability
the proportion of a populations phenotypic variance that is attributes to genetic differences
measured in a particular population at a particular place and time
population parameter not a feature of individuals
H2 =
Broad sense heritability
genetic variance (Vg)/phenotypic variance (Vp)
SNPs
Single-nucleotide polymorphisms (SNPs) are variations in a single nucleotide that occur at a specific position in the genome.
How many SNP’s contribute to human height
10,000
Vp =
phenotypic variance
VG+VE
VG
variance due to genetic differences
= VA + VD + VI
VE
variance due to environmental differences
“Common Garden” experiment
put replicates of genotypes in same and different environments and see how and if they differ
Narrow sense heritability
h^2
the proportion of genetic variation that parents can reliably pass on
(cannot pass on dominance, epistatic events or environmental effects)
parent - offspring regression slope
= VA / VA+VD+VI+VE
sources of genetic variation
if the genes are additive vs non-additive
and sex (M orF)
accounts for 18% of variation
sources of Environmental variation
maternal condition
maternal environment
age of mother
parity (birth order)
intangible
accounts for 82% of variation
VA
additive variance
VD
variation in dominance
VI
Variance in Gene interaction
additive
the addition of each allele changes the mean phenotype by the same amount
Dominance
depends on the combination of alleles
represents modification of the heterzygous individual
Epistasis
two or more genes interact to affect a trait
within generation
phenotypic variation and fitness differences (not all parental generations contribute equally)
part of natural selection
Between generation
phenotypic selection is coupled with heritability to produce a genetic response
part of natural selection
long term selection
may eventually exhaust additive genetic variation (Heritability decreases)
continued response depends on new mutational input and/or gene flow
types of selection on quantitative traits
none
directional
stabilizing and
disruptive
directional selection
one extreme is favored and the population mean moves in that direction
reduces variation (not substantially)
Stabilizing selection
the median has the highest fitness
reduces variation
mean value stays the same
disruptive selection
the two extremes are favored
increases variance
population mean does not change
the rate at which a population can respond to selection depends on
strength of selection (S)
and
heritability (h^2), or the proportion of variation that is due to additive genetic variation
R (response to selection)
= h^2 x S
breeders equation
between generation
= X’-Xp (mean phenotypes of offspring generation - mean phenotype of the population
S
strength of selection/selection differential
the difference between the means (mean of selected parents (Xs) - mean of the whole population (Xp))
S = Xs - Xp
within a generation
VGxE
Genotype by environments interaction
variance in the phenotypic trait that is due to different genotypes responding differently to environmental variation
phenotypic plasticity
a single genotype produces different phenotypes depending on the environment
VGxE
allows organisms to respond to unpredictable changes and regularly occurring ones
can be acted upon by evolution (could be favorable if individuals frequently experience different environments )
seen in a wide range of traits
not always adaptive
reaction norm
the phenotypes produced by a single genotype exposed to different environmental conditions
polyphenisms
simple genetic polymorphisms that respond to their environment and produce multiple discrete phenotypes
often due to a theshold sensitivity to the environment
norm of reaction
a plot of carefully measured phenotype in a large pool of genetically identical individuals grown under a range of environments
slopes and intercepts of lines are different
genotypes respond to the environments differently
genotypes show same plasticity if
direction and amount of plasticity is about the same (lines are roughly equal)
