Quantitative Genetics Flashcards
quantitative genetics
the investigation of continuously variable traits that are influenced by the combined effects of many loci as well as environmental factors
biometrical school
- initiated by Darwin’s cousin, Galton
- saw heritable variation as continuous
- maintained the idea of slight differences among individuals was the essence of evolution
mendelian school
- viewed variation as discrete and sudden, not continuous
- most laboratory geneticists did not believe in evolution by natural selection: too slow & required too subtle variation
what to capital letters (B, S) contribute
+1
what do lower case letters (b,s) contribute
0
many loci =
continuous variation
breeding experiment by East
- crossed short & long flowered plants
- produced F1s and F2s
- recovered parental phenotypes by artificial selection
logic of breeding experiment by East
if continuous variation is composed of discrete units, the original phenotype should be recoverable
variance equation
= sum of (Xi - X0)^2 / n - 1
= omega^2
standard deviation =
omega = square root of variance
3 kinds of genetic variance
Va + Vd + Vi
additive genetic variance, Va
when the phenotypic value for a heterozygote is exactly intermediate between both homozygotes
what causes Va?
independent effects of alleles on a phenotype
what is additive genetic variance responsible for?
heritable genetic variation
dominance variance, Vd
when phenotypic value deviates from additivity
what causes dominance variance?
interactions among alleles
are dominance effects transmitted directly from parent to offspring? why or why not?
no, because only one allele is contributed by each parent
epistatic variance
genetic variation due to interactions among loci
is epistatic variance heritable variance? why or why not?
no, because interactions among loci cannot be passed on to offspring due to independent assortment
epistasis
the effect of the interaction between two or more loci on the phenotype whereby their joint effects differ from the sum of the loci taken separately
is epistasis common?
yes
why do you need to know both loci in epistasis?
need to know both loci to predict the influence of an allele on the phenotype
environmental variance
differences among individuals in a population that are due to differences in the environments they have experienced
the causes of environmental variance, Ve
- environmental effects
- maternal effects
- developmental noise
environmental effects as a cause of environmental variance
variation in phenotypic values that is based on environmental factors
environmental variation: Clausen, Kreck, & Heisey
- planted cuttings at 3 different elevations in the Sierra Nevada
- very different phenotypes
what does environmental setting have an impact on?
phenotype
maternal effects as a cause of environmental variance
the effect of the mother on phenotypic values for non-genetic reasons
example of maternal effects
- egg nutrition levels
- egg hormones
developmental noise
variance due to random events in development, even under the same condition
example of developmental noise
we are not completely symmetrical
a phenotypic value is the consequence of the …
action of your genes within a specific environment
what does the variance in phenotypes with in a population often vary with?
the environment
what is heritability dependent on?
the environment and the genetic context
broad sense heritability
the fraction of phenotypic variation within a population that is due to genetic variation
broad sense heritability equation
h^2 = Vg / Vp
narrow sense heritability
the fraction of phenotypic variation within a population that is due to additive genetic variation
narrow sense heritability equation
h^2 = Va / Vp
what is heritability always between
0 - 1
is h^2 a symbol or do you square it?
symbol
how is narrow sense heritability estimated?
- parent-offspring regression
- sib analysis
- twin studies
- pedigree analysis
problems with parent-offspring regression
- misidentified parents
- environmental correlation between parents and offspring
- maternal effects
example of misidentified parents
- nest parasitism
- extra pair copulation
solution to misidentified parents
genetic testing of paternity
solution to environmental correlation between parents and offspring
cross fostering experiments
solution to maternal effects
father-offspring regression
example of heritability is an environmental-dependent measure
- fruit flies
- h^2 = 0.9 for wing width under normal conditions
- h^2 = 0.06 under stressful conditions
phenotypic plasticity
the capacity for a genotype to express more than one phenotype depending on the environment
norm of reaction
the set of phenotypic values of a genotype under different environmental conditions
genotype x environmental interaction
difference in the effect of the environment on the phenotype displayed by different genotypes
what is the variance of phenotypes in a population dependent on?
interactions between genotypes and the environment
can heritability differ between populations? why or why not?
yes, allele frequencies differ between populations even if the environment and everything else is the same
does a heritability of 0 mean the trait is not genetically determined?
no
