Quantitative genetics the genetics of phenotypic traits that are controlled by multiple genes o Many genes responsible for one phenotype ex height o Discrete phenotypes traits are different ex green vs yellow peas o Quantitative phenotypes controlled by multiple genes trait is continuous Ex height skin color eye color intelligence glaucoma schizophrenia controlled by 1 gene classic Mendelian ratios are predictable Skin color is controlled by 8 different genes o More copies of allele darker skin Caused by complex relationship between genotype and phenotype o Threshold characteristic controlled by multiple genes but only 2 or less phenotypes Appears like a discrete phenotype but is actually continuous cross threshold into state of diagnosis Ex diseased vs healthy state o Heritability rather than other factors such as environment how much of the observed variance in a phenotype that is due to genetics Benefits of understanding heredity predicting phenotypes Predicts risk of disease Predicts outcomes of breeding effects in agriculture Specific for given population and given environment not universal Mean average Sum of all individuals number of individuals Variance average of how much each individual differs from the mean Shows variability in group Observed variance can be caused by different factors hereditability vs environmental factors vs gene by environment interactions VP VG VE VGE o VP total variance in phenotype o VG genetics o VE environment o VGE GxE interactions some genotypes behave differently in different environments H2 VG VP Equation represents fraction of total variance that is due to genetics Control VG and VE make 0 through twin and sibling studies as well as controlled lab studies to measure H2 Ex Guinea Pig Experiment Phenotype spots Most of variance was due to environmental influences Inbreed genetically identical controlled so only VE was tested Twin studies H2 2 rMZ rDZ o rMZ correlation coefficient among monozygotic twins identical measure effect of genetics o rDZ correlation coefficient among dizygotic twins independent fertilization measure effect of environment o r as x increases y increases o r as x decreases y decreases o r 0 no relationship o x and y represent twin 1 and twin 2 Heritability does not Explain individuals just groups Explain other groups populations only current study population Tell degree to which genes play role in phenotype Totally discount environment other factors may have effect in non experimental conditions even if heritability is high o Identification of genes responsible for quantitative traits Mapping search for specific genes Mapping studies controlled crosses experimental Quantitative trait loci analysis Shuffling genomes together Cross two lines hybridize with extreme variance in phenotypes of interest o Generate heterozygote back cross with one of extreme phenotypes resulting chromosomes will have crossing over and be combination different genotypes determine phenotype and genotype of resulting progeny o Compare genotypes and phenotypes between progeny to see if there is an association between phenotypic trait and location of gene on chromosome o Find several genetic markers and decide if explained by linkage or independent assortment Probability of observed data due to linkage Probability of observed data due to random assortment o Identify regions of chromosome that may be responsible for not linked phenotypic trait Association studies observing populations correlational GWAS Divide population into two groups those with trait and those without trait genes Genotype and identify QTLs quantitative trait loci associated with Compare genetic markers between two groups sick vs not sick o Markers SNPs o Find loci in sick people that are associated with phenotype make conclusions based on whether this association is caused by linkage or independent assortment Population genetics study of frequency and interaction of alleles and genes in a population o Calculate genotypes Number of AA individuals N N population size f AA p2 f Aa 2pq f aa q2 o Calculate allele frequencies Number of copies of alleles number of copies of all alleles at the locus Number of alleles at locus 2N multiply genotypic numbers x2 p F A 2nAA nAa 2N o nAA number of individuals with genotype AA q F a 2naa nAa 2N p q 1 p2 2pq q2 1 f A p f a q If have allele frequency can calculate genotype frequency and vice versa q2 q p Variance is max when p q Hardy Weinberg Equilibrium no change in allele frequencies between o Change in frequencies generations Assumptions o Large population o Random mating o No mutation o No migration o No natural selection o Applications p2 2pq q2 Can predict fyrequency of people in a population carrying a recessive allele responsible for a disease Change in frequencies of alleles within a population evolution o How to test if population is in Hardy Weinberg Calculate allele frequencies with given observed numbers Calculate genotypic frequencies with p2 2pq q2 equation Multiply genotypic frequencies by totally population number to get expected Use chi squared to test significance degrees of freedom n 2 1 where n is number of expected genotypes o Effects on population size why Hardy Weinberg doesn t work random event that changes allele frequency variance reduces Genetic drift genetic variability Caused by sampling errors Stronger in small populations Population is represented by those who reproduce Usually occurs all of the time in small amounts but can have stronger influence in some scenarios Causes o Limitation of food space or other resources o Bottleneck effect drastic reduction in population size which also affects allele frequency individuals groups colonize in secluded locations ex Amish population originates from few founder o Founder effect Non random mating assortative mating Positive assortative mating like individuals mate increases Negative assortative mating unlike individuals mate increases homozygosity heterozygosity Inbreeding mating between related individuals Increases homezygosity Type of positive assortative mating Normal in plants self pollination Mutations change allele frequency Adds variation Forward and reverse mutations eventually reach equilibrium no change in allele frequency but mutations still occur in both directions Rates are low and effects are small in the short term Gene flow movement of alleles in and out of population via migration Diversifies populations increases genetic variation in population