Complex traits are influenced by multiple genes, interactions, variation in environment, and interaction between alleles and environment
Analyzing quantitative trait variation
Factors causing continuous variation of quantitative traits- number of genes that determine the trait and genetic and environment factors that affect penetrance and expressivity of genes
Goal of quantitative effects from the environmental effects
Studies of dandelions can help sort out the effects of genes vs the environment
Goal is to compare influence of genes and environment on length of stem at flowering
Environmental Variance
most dandelion seeds arise from mitotic divisions (all seeds from single plant are genetically identical)
grow in variable environment, such as a hillside
Genetic Variance
to calculate genetic variance- keep environment constant by growing genetically different seeds in a greenhouse
Total Phenotypic Variance
grow genetically diverse seeds on a hillside
total phenotype variance= genetic variance + environmental variance
wider graph
Heritability- proportion of phenotypic variance due to genetic variance
Heritability of a trait is the porportion of total phenotypic variance to genetic variation (VG/VP)
broad sense variability is measured only when comparing identical twins to each other
Equations
VP = VE + VG
VG = VA + VD + VI
VA = Variance due to additive genetic effects
VD = Variance due to dominance effects
VI = Variance due to loci interaction
Narrow-sense
VA/VP
VA/(VA+VD+V1+VE)
narrow sense heritability looks at the additive variance
genetic relatedness- average fraction of common alleles at all genetic loci shared by relatives
only 1 allele at any given locus is shared
Relationship between heritability and mean phenotypic values of parents, offspring, and overall population
Measuring the heritability of bill depth in populations of Darwin’s finches
Geospiza fortis on Daphne Major in the Galapagos Islands
correlation between beak size of offspring and the average of parents’ beak size
Results in finch populations had no environmental or genetic effects
if environment had no effect- heritability would be 1
if no genetic contribution- heritability would be 0
Concordance of a trait in two children raised in the same family
heritability 0- no differences observed between monozygotic, dizygotic, or unrelated by adoption
if heritability is 1- differences would be observed in monozygotic, dizygotic, or unrelated by adoption
A trait’s heritability determines its potential for evolution
selection differential (S)- difference between value for this trait in the parents and value for this trait in the entire population (breeding and non-breeding)
Response to selection (R)- amount of change in the mean value of a trait that results from selection
R=h^2S
Two approaches for mapping QTLs
Quantitative trait loci- genes that contribute to complex traits
mapping QTLs depends on producing individuals with different genetic compositions
Direct QTL mapping- requires crosses between individuals that differ in the phenotype of interest
Association mapping- takes advantage of the hisotry of a random breeding population
Direct QTL mapping- looks for joint segregation of the phenotype and genetic markers
rough mapping:
cross individuals showing 2 extremes of a phenotype
identify DNA markers that segregate with phenotype
markers strongly correlated with phenotype are likely to be near genes that influence the trait
Rough mapping- involved in tomato fruit size
start with isogenic large (l) and small (p) tomato strains
BC1 generation has many sizes due to variation at many genes
at each gene, the backcross generation consists of p/l and l/l individuals
QTLs identified based on correlation with phenotype
genotyped and measured weight of hundreds of tomatoes
differences in mean weight of l/l and p/l for a given marker indicates the marker is linked to a QTL
Genome-Wide Association Studies have identified genes that control only a fraction of the total trait variation
most variation has occurred recently due to population growth
recent variation hasn’t spread widely around the globe
