Lec 14

• Quantitative Genetic Model


  • P = G + E
  • P = A + D + I + E
  • A = Additive genetic effects
  • D = Dominance effects
  • I = Epistasis or Interaction effects
  • E = permanent (Ep) + temporary (Et) environments
• Permanent environmental effects (Ep)


  Environmental effects which are permanent in nature and carry over throughout the lifetime of an individual (e.g. severe illness, failure to have a calf one year, injury to udder)
• Temporary environmental effects (Et)


  Specific environmental effects which are temporary in nature and affect the current record of the individual (e.g. heat wave, slight illness, off feed)
• Phenotypic variance (VP)


  VP = VG + VE
• Genetic variance (VG)


  VG = VA + VD + VI
• Environmental variance (VE)


  VE = VEp + VEt
• Many metric traits are influenced by environmental effects
• In traits that are measured repeatedly on the same individuals over time, the performance shown is most often affected by environmental factors, both permanent and temporary in nature
• Repeatability (r)


  Proportion of phenotypic variance due to genetic and permanent environmental effects
• Repeatability


  • Shows how much to be gained by repetition of measurements
  • Sets an upper limit to heritability
  • Predicts future performance based on past records
  • Tells extent of environmental variation
• Repeatability is only important for traits measured repeatedly on the same individuals over time
• Estimates of repeatability (r) are greater than heritability (h2)
• Heritability (h2)


  Association between parent and offspring, resemblance among relatives
• Repeatability (r)


  Association between adjacent records on an individual due to permanent environmental effects
• Repeatability estimates for several traits of economic importance
• Traits with repeatability estimates

  • Milk yield (0.50)
  • Weaning wt (as a dam trait) (0.40)
  • Egg production (0.25)
  • Litter size in pigs (0.27)
  • Calving interval (0.15)
  • Fleece weight in sheep (0.34)
  • ¼ mile time in horses (0.32)
• Genetic principle of repeatability

  Ratio of all permanent components (genetic & environment) to phenotypic variation
• Understanding repeatability of a repeated trait allows us to make informed decision in culling our animals
• Repeatability is important in sire proofs
• Time and place of measurement influence the repeatability estimate
• Interpreting repeatability estimate for milk yield (r=0.50)

  50% of superiority shown in 1 year is expected to appear in the next year
• Interpreting repeatability estimate for conception rate (r=0.1)

  Very low relationship on number of services taken to conceive for a cow to become pregnant each time, temporary environmental factors have more influence
• Sire Proofs


  Increasing accuracy of BV estimate by using avg. records of daughters compared with avg. records of herd-mates, including information on no. of daughters, no. of records per daughter, no. of herds represented, no. of herd-mates per daughter, pedigreed information, repeatability
• Best Linear Unbiased Prediction (BLUP)

  Preferred sire evaluation method, uses simultaneously all information on sires, cows and relatives to improve accuracy of BV estimate, all relatives included are weighted by genetic relationship, repeatability is often referred to as reliability
• Bulls evaluated in one farm and proven can be used in another farm without any sire x farm interaction effect
• Selecting superior females as dams


  An estimate of a cow's milk producing ability is her first milk record, with more records more reliable is her estimate of milk producing ability, but takes a longer time to have many records
• Most Probable Producing Ability (MPPA)


  MPPA = Avg. dev. X nr / (1 + nr - r)
• For milk production, if select solely for milk yield can attain maximum genetic change of 2% per year and 90 - 92% of the change is contributed by sire selection
• BLUP methodology can provide a reliable estimate of genetic change by simultaneously considering genetic components adjusted for environmental differences (herd, year of birth, season of birth, management)
• Four paths of selection

  • Select males to produce herd sire
  • Select females to produce herd sire
  • Select males to produce herd dams
  • Select females to produce herd dams
• First 3 paths are important to produce sires of high genetic merit
• Less selection pressure on dams because most female offspring are kept as replacement heifers to maintain population size, and it is difficult to obtain estimate of true genetic merit of dams
• 70% of genetic improvement is contributed by selection of males to produce herd sires