How to solve quantitative genetics problems

QUANTITATIVE GENETICS Quantitative genetics deals with the genetics of continuously varying characters. Rather than considering changes in the frequencies of specific alleles of genotypes, quantitative genetics seeks to “quantify“ changes in the frequency distribution of traits that cannot easily be placed in discrete phenotypic classes. The reason for the continuous variation is usually that the traits are polygenic (controlled by many genes) and there are environmental effects that alter the phenotypic state of each individual. Consider two inbreed strains that represent “extremes“ of a phenotypic distribution: high and low oil content in corn for example or long and short carrots. We will assume that the plants of each type are homozygous at all loci. Under this assumption the variation we see within each group is entirely environmental variation and the variation we see between the two groups is mostly (but not entirely) genetic variation. If we then cross an individual from the high group (ABCD) with an individual from the low group (abcd) we would get F1 hybrids (ABCD/abcd) that are intermediate in phenotype. We would notice that each individual is not identical in phenotype even though each is identical in genotype (all F1’s). We would then attribute all the variation in phenotype to an environmental component, VE. If we than crossed all the F1’s with each other, we would get an F2 distribution that would have a wider distribution. Because of independent assortment of chromosomes and recombination in the F1’s each F2 is likely to have a unique multilocus genotype. Thus the total phenotypic variance in the F2 distribution will have both a genetic component, VG and an environmental component (VE). In simple terms, these are related by the expression VP = VG VE. If you were given a bunch of plants with a smooth continuous distribution of phenotypes, how would you determine if there was a genetic basis to the variation? Simply select individuals from the distribution with distinct phenotypes, breed them (=parents) and compare the phenotypes of these parents to that of their offspring. If the mean phenotype of offspring was close to the mean of the parents this would be evidence for a genetic basis for the phenotype and the trait would be identified as heritable. If on the other hand, the offspring produced from two “high“ parents were extremely variable in phenotype and offspring produced from two “low“ parents were extremely variable there would be a weak genetic component to the trait. The heritability in a “broad“ sense can be expressed as the proportion of the total phenotypic variance that has a genetic component: h2B = VG/VP. This correlation between parent and offspring can serve as a simple means of quantifying the heritability of the trait: if there is a 1:1 correlation of phenotype between parents and offspring (e.g., a 45 degree slope of the regression of offspring phenotype vs. parent phenotype) then the trait has the maximal heritability. With no relation between parents and offspring (a slope of zero) the heritability would be zero. #QuantitativeGenetics #GeneticsFieldOfStudy #polygenic #phenotypic #inbreed #Homozygous #F2 #genetic #heritability #broadSenseHeritability #narrowSenseHeritability #GenotypicVariance #PhenotypicVariance #environmentalVariance #DeepBiology #CsirNerLifeScience #narrowHeritability #broadHeritability #Selection #NaturalSelection #selectionDifferential #QUANTITATIVETRAITS #partitioningTheVariance #Variance #normOfReaction