BSCI222 Lecture 15 10 24 13 Chapter 24 Quantitative Genetics Mendellian genetics has discontinuous characteristics here we have continuous characteristics where the population has a bell curve for the trait with a wide range continuous distribution of phenotypes o Why would you have this Might have multiple genes affecting the trait or multiple alleles at a locus or interactions among genes epistasis or environmental effects on the phenotype o If you plant many cuttings from the identical genotype they will not all be the same in phenotype environmental variation such as water availability etc o One locus with 2 alleles in equal frequency get 3 genotypes AA Aa aa same bell curve as genetically identical but more spread out wider o Many more alleles much broader distribution of genotypes and have a continuous trait curve when including the environment Partition the variants partitioning the phenotypes o V G variation due to genetics and V E variation due to environment B is equal to V G V P what Heritability h2 in a broad sense h2 proportion of the variance is due to genes and what proportion is due to the environment Example 2 alleles 2 loci additive effects on the phenotype o 2 pathways to producing purple pigment in a wheat plant A wild type or B wild type adds pigmentation units 2 different genes with additive effects F1 hybrid interbreed F2 TEST know how to do this table with wheat colors Purple is 1 16 in F2 More loci more columns bell curve starts appearing Even with just 2 alleles with a lot of environmental interference it becomes impossible to know the genotype from their phenotype Example height in people tall dwarf or short normal Analysis of variance sum of squared deviations from the mean Greater variance means a broader curve Components of Genetic Variance o V G V A additive components V D dominant components o Perfectly additive adding one allele gives you an exact phenotype didn t hear bb a bB 0 and BB a Usually don t have the case of it being perfectly additive bB s distance from 0 is called d If the trait were perfectly additive then the heterozygotes should fall right on the line between the two homozygotes If the heterozygote is above the line this indicates some degree of dominance amount that it is above the line is d Why is there a distinction between additive and dominant genetic variance Maybe heterozygotes grow faster a farmer breeder will select for it Keep selecting heterozygotes will never get any improvement always 1 2 1 Cannot breed for dominance Can however select for the additive components which will not leave you stuck Actually selecting for BB will give you a higher mean for the population overall Narrow sense heritability is what breeders care about h2 N V A V P How do you know your breeding will give you genetic gains Must somehow estimate what improvements you will get o First must get estimates of heritability will the trait respond to selection o Also need estimate of how hard you must select in order to get that gain If you know what kind of response you expect can calculate the potential economic benefit o Heritability is typically estimated from offspring of parent regressions make a bunch of crosses among the adults in your population plot the average of the 2 parent phenotypes called midparent value i e one big one small Two juicy parents 1 against the phenotype of the resulting offspring Crazy points everywhere low heritability or none no relationship If there is a positive correlation must have additive genetic variance for which you can select The heritability in the narrow sense is equal to the slope of the line steep slope means strong heritability If heritability for one trait is higher than the heritability for another then you can change the trait with higher heritability faster can get bigger cows faster than cows with higher milk yields Traits linked to reproduction do not have much additive variation left or any genetic variation because evolution has selected for it for so long Heritability estimates are not absolute o Depend on the particular population and environment of study and sensitive to exact methods used to measure the trait i e how long are you measuring growth To 5 weeks To 10 weeks Did you use a ruler correctly o Different populations have different allele frequencies to start with thus having different heritabilities Extreme case if population is fixed for a locus will have zero heritability no room for improvement o Genotype that works well in a wet environment will not work as well in a dry environment o So why do it Indication of response to selection we can expect funding Typically good investment because it is a one time cost to change the population permanently gain will be in the population forever giving the benefits for a long time Like improving the software Margins are of small percentages an organism growing 2 3 more efficiently will take over the market Another measurement is to take a group of parents that have a slightly higher mean phenotype from the population Offspring then have a slightly higher mean phenotype than the average population o Called the response to selection is another measure of heritability o R h2 S the selection applied or h2 R S how far does the population move after applying some amount of selection Limits to selection o Continuously select get best locus across the genome Run out of genetic variation Is Clifford possible o Might be possible but not in every case Appears that there is a selection limit past a certain point Thoroughbred horses have hit a limit for increased speed Experiment on corn oil content began in 1896 still continuous improvement the low content has stopped because can t have less than 0 oil called a phenotypic limit high content has so far seen a difference of 32 SD s Limits in number of bristles on drosophila Why the difference The more genes you have the bigger target for mutations to happen and selecting on new variations that weren t present before o Starting with a smaller population makes it more likely to hit a selection limit starting with less variability exhaustion of genetic variability o Essentially selection limits are not a problem for most agricultural crops o Some traits are negatively correlated chickens that grow fast put all their energy into muscles and don t make as many eggs decide between meat and eggs broiler chickens or layer chickens and can either have fewer large eggs or more but smaller eggs opposing selection o