Lec 13

• Traits


  Influenced by genetic and environmental factors
• Trait example


  • A bull having a high body weight at 12-month - Will it produce calves of high body weight at 12-month too?
• Phenotype


  Genotype + Environment
• Genotype


  Additive effects + non-additive effects
• Additive effects


  Average effects of genes (breeding value)
• Heritability (h2)


  Degree of resemblance between relatives for a trait in a population
• Phenotype


  Additive Gene Effect + Dominance + Interaction/Epistasis + Environment
• Heritability (h2)


  Proportion of phenotypic variation that is due to additive gene effects (heritable component)
• Heritability in the narrow sense (h2)

  Proportion of differences in performance for a trait that are attributable to differences in breeding values for the trait
• Heritability in the broad sense (H2)

  Extent to which genotypic value determines phenotypic value
• Heritability estimates for different traits

  • Reproductive (< 0.2 - low)
  • Growth (0.2 - 0.4 - moderate)
  • Carcase (0.4 - 0.6 - high)
  • Few traits have h2 > 0.6
• Heritability estimates for beef cattle traits

  • Birth weight (0.3)
  • Yearling weight (0.4)
  • Daily gain (0.4)
  • Reproductive rate (0.1)
  • Dressing % (0.4)
  • Loin eye area (0.4)
• Heritability estimates for dairy cattle traits

  • Body weight (0.55)
  • Milk yield (0.25)
  • Fat yield (0.25)
  • Solids-not-fat yield (0.25)
  • Protein yield (0.25)
  • Reproductive efficiency (0.15)
  • Mastitis resistance (0.10)
  • Body type score (0.30)
• Heritability estimates for swine traits

  • Ovulation rate (0.39)
  • Litter size (0.10)
  • Survival to weaning (0.05)
  • 21-day litter weight (0.15)
  • Average daily gain (0.30)
  • Days to 100 kg (0.25)
  • Back-fat thickness (0.41)
  • Feed efficiency (0.30)
  • Loin eye area (0.47)
  • Carcass length (0.56)
• Heritability estimates for goat traits


  • 90-day weight (0.25)
  • Post-weaning gain (0.40)
  • Doe fertility (0.05)
  • Prolificacy (0.10)
  • Kid survival (0.05)
  • Carcase weight (0.35)
  • Dressing % (0.10)
  • Loin eye area (0.50)
  • Fat thickness (0.30)
• Heritability estimates for horse traits

  • Pulling ability (0.25)
  • Cutting ability (0.04)
  • Thoroughbred earnings (0.09)
  • Handicap wt (0.33)
  • Trotter earnings (0.20)
  • Time (0.32)
  • Height at withers (0.43)
  • Heart girth (0.29)
  • Services /conception (0.12)
• Heritability estimates for layer chicken traits

  • Chick livability (0.05)
  • Adult livability (0.10)
  • Fertility (0.05)
  • Body depth (0.25)
  • Adult body weight (0.55)
  • Egg production (0.15)
  • Egg weight (0.55)
• Heritability estimates for broiler chicken traits

  • 7-week weight (0.45)
  • Feed consumption (0.70)
  • Feed conversion (0.35)
  • Breast fleshing (0.10)
  • Fat deposition (0.50)
• Heritability (h2)


  Extent in which heredity (genetics) influences a trait
• Broad sense heritability (H2)


  Variance of genotype / variance of phenotype
• Narrow sense heritability (h2)

  Variance of additive effects / variance of phenotype
• Higher h2


  Greater genetic control on the trait, and higher response to selection
• Breeding Value (BV) or A


  h2 x phenotypic deviation
• Phenotypic value (PV) is used to choose animals as parents as a guide to BV, but it is BV that is transferred from one generation to another</b>
• BV represents the average effects of genes (additive effects of genes)
• When all of the phenotype is heritable


  Regression slope = 1, h2 = 1
• When one half of the phenotype is heritable

  Regression slope = 0.5, h2 = 0.5
• Regression


  Change in dependent variable per unit change in independent variable
• Heritability can be estimated in 2 ways: Resemblance between relatives, and Realized heritability from selection response
• Resemblance between relatives


  Proportion of variation among individuals due to variation in additive genetic values (breeding values)
• Breeding value of an individual is not directly observable, but has to be inferred from mean value of its offspring or using phenotypic values of other known relatives
• There are 3 classes of relatives: Ancestral, Collateral, and Pedigree with both
• Phenotypic resemblance between relatives


  Provides an indication of the amount of genetic variation for a trait
• Covariance between relatives


  Extent the phenotypes of the pair of relatives vary together, due to sharing genes and environment
• Parent-offspring regression


  Cov(O,P) / Var(P)
• Collateral relationships (ANOVA)


  Intraclass covariance, Var(B) / Var(P) = t = Cov(FS) / Var(P)
• When Var(B) = 0, members of a family resemble each other no more than members of other families, and there are no significant differences in average phenotype between families
• When Var(B) = 2.5, Var(W) = 0.2, Var(P) = 2.7, members of a family resemble each other more closely than members of other families, and there are large differences in average phenotype between families
• Cov(O,P)


  Phenotypic resemblance between relatives
• Var(P)


  Collateral relationships: ANOVA