CHAPTER 5

• Quantitative genetics


  The study of traits that can be described numerically
• Quantitative traits are usually controlled by more than one gene
• Quantitative traits


  • Height
  • Metabolism rate
• Quantitative traits


  • They vary measurably in a given species
  • They are easily measured and described numerically
• Measurement of quantitative traits is essential when comparing individuals or evaluating groups of individuals
• Biometric field of genetics

  The field developed by Francis Galton and Karl Pearson to study quantitative traits
• Discontinuous traits


  Traits that fall into two or more discrete categories
• Quantitative traits


  Traits that show a continuum of variation within a group of individuals
• The distribution of measurements of many biological characteristics is approximated by a symmetrical bell curve
• Quantitative traits do not naturally fall into a small number of discrete categories
• Frequency distribution


  An alternative way to describe quantitative traits by dividing the trait into a number of discrete phenotypic categories
• Mean


  A statistical measure of central tendency calculated using the formula: X = ∑X/N
• Variance


  A statistical measure of the amount of phenotypic variation in a group, calculated using the formula: VX = ∑(Xi - X)^2/(N-1)
• Standard deviation


  The square root of the variance, provides an intuitive grasp of the variation in a population
• In a normal distribution, approximately 68.3% of individuals are within 1 standard deviation of the mean, 95.4% within 2 standard deviations, and 99.7% within 3 standard deviations
• Correlation coefficient (r)


  A statistic that evaluates the strength of association between two variables, ranging from -1 to 1
• Covariance


  A measure of the degree of variation between two variables within a group, calculated using the formula: CoV(X,Y) = ∑[(Xi - X)(Yi - Y)]/(N-1)
• A statistically significant correlation coefficient does not necessarily imply a cause-and-effect relationship
• Polygenic inheritance


  The transmission of traits that are governed by two or more genes
• Quantitative trait loci (QTLs)


  Locations on chromosomes that affect the outcome of quantitative traits, may contain multiple genes
• Molecular genetic tools have greatly enhanced the ability to find regions in the genome containing QTLs
• Polygenic inheritance and environmental factors both contribute to the continuum of phenotypic variation in quantitative traits
• First demonstration that continuous variation is related to polygenic inheritance occurred

  1909
• Nilsson-Ehle's cross


  1. P: True-breeding red x true-breeding white

  2. F1: Intermediate red

  3. F2: Great variation in redness (White, light red, intermediate red, medium red, dark red)
• Nilsson-Ehle discovered the colors fell into a 1:4:6:4:1 ratio
• Wheat is diploid for two different genes that control hull color


  • Each gene exists in two alleles: red or white
  • The contribution of each red allele to the color of the hull is additive
• A third gene was later discovered to affect hull color
• Many polygenic traits are difficult or impossible to categorize into several discrete genotypic categories
• Reasons why polygenic traits are difficult to categorize

  • The number of genes controlling the trait increases
  • The influence of the environment increases
• A Punnett square cannot be used to analyze most quantitative traits
• Statistical methods must be employed to analyze quantitative traits
• Overlap of genotypes and phenotypes for polygenic traits

  • One gene: Separate, well-defined categories
  • Three genes: Nearly all individuals fall into a single phenotypic category that corresponds to their genotype, Different genotypes have regions of overlapping phenotypes
• QTLs


  Quantitative Trait Loci - Regions of the genome that contain or are linked to the genes that underlie a quantitative trait
• QTL mapping


  1. Identify molecular markers close to QTLs

  2. Associate genetically derived phenotypes (quantitative traits) with molecular markers
• Detailed genomic maps have been obtained from model organisms and organisms of agricultural importance
• Heritability is the amount of phenotypic variation within a group of individuals that is due to genetic variation
• Heritability


  • If all the phenotypic variation in a group was due to genetic variation, heritability would have a value of 1
  • If all the phenotypic variation was due to environmental factors, heritability would have a value of 0
• Phenotypic Variance


  V_T = V_G + V_E

  V_T is the total variance

  V_G is the relative amount of variance due to genetic variation

  V_E is the relative amount of variance due to environmental factors
• Genetically homogeneous populations have V_G = 0, so all phenotypic variation is due to V_E
• Genetically heterogeneous populations have phenotypic variance due to both V_G and V_E