Quantitative genetics Flashcards
discontinuous (qualitative) traits
can be categorised into few classes
exhibit only a few, easily distinguished phenotypes
continuous (quantitative) traits
vary along a scale of measurement
exhibit continuous range of phenotypes
bimodal distribution (polyphenism)
two peaks
meristic traits
determined by multiple genetic and environmental factors
can be measured in whole numbers
threshold traits
measured by presence or absence
eg susceptibility to disease
polygenic traits
phenotype reflects cumulative effects of many genes/loci
influenced by many environmental factors
lots of different genotypes can produce the same phenotype
phenotypic variance
total amount of variation among individuals in some trait
equation for phenotypic variation
Vp=Vg+Ve+Vge
where g=genetic variance
e=environmental variance
ge=genotype-environment interaction variance
equation for genetic variance
Vg=Va+Vd+Vi
where a is additive/heritable variance that contributes to parent-offspring inheritance
d= dominance variation (non heritable)
I= is genetic interaction/epistatic variance (non heritable)
inherited
dna sequences passed from parents to offspring (chromosomes, mitochondria etc)
heritable
phenotypic trait differences of individuals reflect genetic differences between individuals
contributes to parent offspring resemblance
Heritable traits refer to characteristics that have a genetic basis and
narrow sense heritability
the proportion of phenotypic variation that contributes to the resemblance between parents and offspring
(additive genetic)
broad sense heritability
proportion of phenotypic variation attributable to all types of genetic differences between individuals
equation for narrow sense heritability
h^2=additive genetic variance/phenotypic variance
equation for broad sense heritability
H^2= genetic variance/phenotypic variance
regression (to measure narrow sense heritability)
predicting the value of one variable if the other value of the other is given
regression coefficient
represents the slope of the regression line, indicating how much one value changes on average per increase in the value of another variable
gives you the narrow sense heritability
genotype-by-environment interaction
the phenotypic difference between genotypes depends on the environment they experience
what causes correlation between traits
environmental correlation
genetic correlation (due to pleiotropy or linkage disequilibrium)
phenotypic vs genetic correlations on graphs
point are different individuals-phenotypic
points are average values for different genotypes=genetic correlation
pleiotropy
variation at a single locus leads to variation in multiple traits
linkage equilibrium
combinations of alleles at the A and B loci should be randomly assorted
expect equal frequencies of coupling and
repulsion gametes
linkage disequilibrium
occurs when there is an association between alleles at different loci.
if you know the allele present at one locus you can predict the allele at the other locus because they are inherited/ co vary.
not same as physical linkage
there is an overrepresentation of coupling gametes or repulsion
gametes
epistasis
occurs when the effect of alleles at a
locus depends on alleles present at other loci
appearance of interaction depends on the variation at each locus
linkage equilibrium equation, eg for AB
F(AB)=px
where p=F(A) and x=F(B)
linkage disequilibrium equation
(product of frequency of coupling combinations)-(product of frequency of repulsion combinations)
effect of recombination on LD
breaks up associations between alleles-removes LD
rate of recombination determines how quickly LE is restored
what is a quantitative trait locus (QTL)
a location in the genome that causes
different values of the trait in question
(that position in the genome is associated with differences in the phenotype)
what is genome wide association study
identifies loci associated
with trait variation, usually in natural populations
equation for overall phenotypic variation, taking into account heritable and non heritable genetic and environmental origins of variation
Vp=Va+Vd+Vi+Ve+Vge
genotypes generally do not produce distinct phenotypes because of…
codominance
incomplete dominance
epistasis
polygenic inheritance
environmental influence
recessive traits
trade offs
can be shown by correlations
an organism can do one thing well but at the detriment of another thing
eg making lots of small seeds or fewer large seeds
positive linkage disequilibrium
more coupling gametes
how does linkage disequilibrium lead to correlation, using height and foot size as an example
Imagine the A locus affects height and the B locus affects foot size. Assume that
the large alleles (A and B) lead to larger size than the small alleles (a and b)
If there is an excess of coupling gametes, individuals will tend to inherit A with B
and a with b, leading to a correlation between height and foot size
locating loci that affect quantitative characteristics
mapping QTLs
genome wide association studies
a QTL can be caused by differences in
coding regions
regulatory elements
do not often reflect changes to protein produced as can be caused by non coding differences
more often associated with differences in gene expression
QTL analysis
uses genetic markers to test
whether different genotypes have significantly different trait values
(phenotypes)
genetic markers
any type of variable locus where alleles can be distinguished
identify casual variation linked to these markers (co inherited with casual alleles)
Heritability refers to the proportion of phenotypic variation in a population that is attributable to genetic variation. When narrow sense heritability equals 1, it means that all phenotypic variation is due to additive genetic factors, making the trait highly predictable based on the genotype.
Inherited traits are characteristics passed from parents to offspring through genes. All heritable traits are inherited, but not all inherited traits are perfectly heritable (i.e., heritability = 1).
