BIOL214 Exam 2 - Genes (Ch. 7+8)

Polygenic Trait

• influenced by many genetic loci

• tend to vary continuously among individuals (ex. human height)

• epistasis

• phenotypic plasticity

Epistasis

• interaction between alleles

Phenotypic Plasticity

• interaction with environment

Quantitative Genetics

• study of the genetic mechanisms and evolution of continuous phenotypic traits

Population Genetics vs. Quantitative Genetics

• Population Genetics

  • focused on understanding the subtle signals left in genetic data by selection over longer time scales

• Quantitative Genetics

  • focused either on mapping the genetic basis of relatively simple traits, or in the absence of any such knowledge

Complex Traits Vary Continuously

• traits that vary widely about the mean value have bigger variance

• those that are identical have smaller variance

Components of Phenotypic Variation

• V_P = V_G = V_E

  • V_P = total phenotypic variance in population

  • V_G = variance due to genetic differences

  • V_E = variance due to environmental differences

Notes on Phenotypic Variation

• in cases where environmental factors play a bigger role in phenotypic variations that do alleles, we have

  • V_E > V_G

• likewise, when environmental factors are neglibible we have:

  • V_P ~ V_G

Genetic Sources of Variation

• genetic sources of variation can themselves be divided into several subcategories

  • additive variance (V_A)

  • dominance variance (V_D)

  • epistatic variance (V_I)

• together, the values for each of these subcategories yield the total amount of genetic variation (V_G) responsible for a particular phenotypic trait

  • V_G = V_A + V_D + V_I

Additive Genetic Variance

• deviation from the mean phenotype due to the inheritance of a particular allele and this allele’s relative effect on phenotype

Dominance Genetic Variance

• deviation due to interactions between alternative alleles at a specific locus

Dominance Variance

• if there is an interaction between the alleles such that their expression is not equal, then the phenotype of the heterozygote would not be an approximate midpoint

• it would more closely resemble one of the homozygotes

Epistatic Variable

• involves an interaction between alleles

• alleles are associated with different loci

Hypostatic Genes

• they have their expression controlled by

Trait Heritability

• Heritability: a measure of the proportion of phenotypic variance attributable to genetic variance

  • important predictor of the degree to which a population can respond to artificial or natural selection

Waldmann (2001)

• Scabiosa canescens

  • found in small populations → low genetic diversity → limited ability to adapt

• in this study, genetic lines of S. canescens from both a small and large population were used to estimate additive and dominance genetic variation

• did not find a lower level of additive genetic variance in offspring from smaller population

• the smaller population still had the ability to evolve

Broad Sense Heritability (H²)

• proportion of the total phenotypic variation of a trait that is attributable to genetic differences among individuals

• H² = V_G/V_P = V_G/(V_G + V_E)

Environmental Sources of Variation

• subcategories:

  • specific environmental variance (V_Es)

  • general environmental variance (V_Eg)

  • genotype by environment interaction (V_GxE)

  • together, these values yield the total amount of environmental variance (V_E) responsible for a particular phenotypic trait

    • V_E = V_Eg + V_GxE + V_Es

Specific Environmental Variance (V_Es)

• deviation from the population mean due to the environmental conditions that are uniquely experienced by each individual

• called the error or residual variance

• random noise in the expression of the true phenotype, and it can interfere with the artificial selection of traits of commercial interest by weakening the correlation between genotype and phenotype

General Environmental Variance (V_Eg)

• nongenetic sources of variation between individuals that are experienced by multiple individuals that are experienced by multiple individuals in a population

  • largest component of variation in populations in natural conditions

• replicates of each genetic lines need to be assessed in each natural or experimental environment of interest

  • if one environmental treatment is used, then all nongenetic sources of variation are due to the specific environment

Estimating V_Eg

• estimates of general environmental variance can be determined by growing or raising the same set of genetic lines in all environments of interest

• for instance, water is required for all plants, but the amount that promotes the maximum growth differs among both species and genetic lines of a species

• changes in growth rate due to the different water treatments = general environmental variance

Genotype by Environment Interaction (V_GxE)

• involves the unique or different responses of genetic lines to general environmental variation

Variance with Components

• V_P = V_E + V_G

• V_Eg + V_GxE + V_Es + V_A + V_D + V_I

Narrow Sense Heritability

• proportion of phenotypic variance explained by the additive effects of alleles

Are selection and evolution the same thing?

• NO!

• populations can experience selection even if they cannot evolve in response to it

Quantitative Trait Locus (QTL) Analysis

• statistical method that links two types of information

  • phenotypic data (trait measurements)

  • genotypic data (usually molecular markers)

• allows researchers in fields as diverse as agriculture, evolution, and medicine to link certain complex phenotypes to specific regions of chromosomes

• goal is to identify the action, interaction, number, and precise location of these regions

Evolutionary Response to Selection

• the degree to which populations change in response to selection depends on

  • selection differential

  • heritability of a trait

Selection Differential (S)

• the difference in the mean of a trait in reproducing individuals and the mean of the trait for the general population

Heritability of a Trait (h²)

• the proportion of phenotypic variance due to genetic differences (additive effects) among individuals

Calculating the Evolutionary Response to Selection

• R = h² x S

• R = evolutionary response

• h² = heritability (narrow sense)

• S = selection differential