Lec 18

• Selection (Artificial)


  Allowing some individuals more opportunities than others to reproduce<|>Choice of individuals to become parents for the next generation of offspring
• Selection remains the only means of directing genetic improvement in closed populations
• Response to selection

  Result or progress of a selection programme
• Response to selection is of interest because of genetic change expected in the population
• Selection of desirable genes

  Increases their frequency - more homozygous individuals will be selected
• Two interests of selection

  • Increase frequency of desirable genes
  • Change mean value of herd/population
• Traits

  • Reproductive: h^2 < 0.2 (low)
  • Growth: h^2 0.2 - 0.4 (moderate)
  • Carcass quality: h^2 0.4 - 0.6 (high)
  • Few traits have h^2 > 0.6
• Heritability (h^2)


  Proportion of phenotypic variation that is due to additive gene effects
• Generally moderate to highly heritable traits respond positively to selection
• Response to selection

  1. R = h^2 x S

  2. Where R is the response, h^2 is the heritability, S is the selection differential
• Selection differential (S)

  Deviation of mean of selected parents from population mean
• Standardized selection differential

  1. R = i x h^2 x δP

  2. Where R is the response, i is the intensity of selection, h^2 is the heritability, δP is the phenotypic standard deviation
• Standardized selection differentials for different proportions saved


  • 90% saved: i = 0.20
  • 80% saved: i = 0.35
  • 70% saved: i = 0.50
  • etc...
• Standardized selection differentials


  • 70% saved: i = 0.50, S = 1.6
  • 20% saved: i = 1.40, S = 2.8
  • etc...
• Within generation change


  Difference between mean of selected parents (μS) and population mean before selection (μO)
• Between generation change


  Difference between population mean after selection (μP) and population mean before selection (μO)
• Response to selection (R) is the average deviation of offspring mean from population mean = breeding value of parents
• Realized heritability


  h^2 = Response/Selection Differential
• Response to selection for average daily gain in beef cattle calves

  • h^2 = 0.4, Herd mean = 1.0 kg/day, Mean of selected parents = 1.2 kg/day, Response = 0.08 kg/day per generation
• Response to selection for ovulation rate in sheep

  • h^2 = 0.1, Herd mean = 6 ovulating follicles per ewe, Mean of selected parents = 9, Response = 0.3 ovulating follicles per generation
• Response to selection will eventually diminish when all favourable alleles at most loci become fixed in the homozygous type
• In livestock breeding, new genetic materials are constantly being introduced from outside the herd and this will maintain genetic variability in a herd
• Ways to increase response to selection

  1. More intense selection (keeping smaller proportion of animals as parents)

  2. Higher heritability estimates (making environment more uniform to all animals)

  3. Increase phenotypic variation (introducing new genetic materials)
• Response to selection for back-fat thickness in pigs

  • h^2 = 0.5, Herd mean = 28 mm, Mean of male parents = 20 mm, Mean of female parents = 28 mm, Response = -1.75 mm per generation
• Response to selection for weaning weight in beef cattle

  • h^2 = 0.30, % saved (males) = 5%, % saved (females) = 30%, Response = 9.66 kg per generation
• Correlated response


  Response on another trait (Trait Y) when selecting directly for a particular trait (Trait X), exists only when there is genetic correlation between the two traits
• Correlated response


  CR_Y = i h_X r_A δ_AY = i h_X h_Y r_A δ_PY
• Correlated response in average daily gain when selecting for weaning weight in beef cattle

  • h^2_X = 0.30, h^2_Y = 0.40, r_A = 0.8, Response = 0.13 kg/day
• Indirect selection


  Improve Trait X by selecting indirectly for Trait Y (secondary trait)
• Efficiency of indirect selection


  CR_X / R_X = r_A * (i_Y * h_Y) / (i_X * h_X)
• Indirect selection can be more efficient than direct selection if the secondary trait has higher heritability, higher selection intensity, and high genetic correlation with the primary trait
• Usefulness of correlated response

  Select indirectly for a secondary trait to improve the primary trait when the primary trait is difficult to measure, measurable only in one sex, or very costly to measure
• Examples of primary and secondary traits

  • Primary trait: Ovulation rate, Secondary trait: Litter size

  Primary trait: Milk yield in beef cattle, Secondary trait: Weaning weight of calves

  Primary trait: Carcass fat %, Secondary trait: Back-fat thickness
• Correlated response for feed efficiency = -0.5974 kg feed/kg gain/generation (when direct selection is for average daily gain)
• Direct selection for feed efficiency = 0.7513 kg feed/kg gain/generation (has to be interpreted as a negative value since lower feed efficiency is better)
• Response to selection


  R = h^2 x S

  Where R is the response, h^2 is the heritability, S is the selection differential
• Traits that are difficult to measure

  • Measurable only in one sex
  • Very costly to measure
• Primary trait


  • Ovulation rate
  • Litter size
• Secondary trait


  • Milk yield in beef cattle
  • Weaning weight of calves
• Primary trait's attributes


  • Difficult to measure
  • Only in females
  • Costly to measure