Final Review

• Polygenic Inheritance


  Many genes influence 1 trait, when >3 genes = polygenic
• Examples of polygenic inheritance

  • Head shape in horses
  • Litter size in pigs
  • Eggs laid/day
  • Retrieving ability in dogs
• Genotype


  (A)dditive, (D)ominance, (I)nteractive
• Environment (E)


  Feeding, housing, stress, disease
• Phenotype


  Genotype + Environment
• Additive Effects


  Each allele adds to the effect of the gene
• Additive Effects


  • LLLLLL = 2200L vs. llllll = 1400L
• Interactive Effects


  Two or more loci interact on one trait
• Interactive Effects


  • E locus = MC1R red or black

  D locus = dilute gene (PMEL17)
• Epistasis


  A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited
• Hypostasis


  Masked gene (in albinism it would be E)
• Broad Sense Heritability


  Genetic variance over phenotypic variance
• Narrow Sense Heritability


  Additive genetic variance over phenotypic variance
• Heritability estimate is based on how much relative resemble each other (compared to non-relatives)
• Heritability levels


  • High (>0.40): Growth traits (pig backfat, milk % butterfat, sheep wool length, ADG, feed efficiency)

  Medium (>.10<0.40): Function/behaviour traits (post-weaning gain, milk yield, time in thoroughbreds, weaning weight)

  Low (<0.10): Fertility/stamina/reproduction (egg hatchability, pig litter size, sheep conception rate)
• Heritability


  An assessment of how successfully or quickly a trait can be improved by selection
• Repeatability


  Measures environmental variation within a single genotype
• Correction Factors for Comparison

  • Sex

  Age of dam

  Age of calf

  Singleton vs. twin
• Individual Selection


  Mass Selection - based on animal's own performance, accurate if h2 high
• Individual selection is practical if trait is measurable and shows early expression
• Ancestor Selection


  Pedigree Selection - based on weighted avg of relatives, useful when selection has to be made before candidates performance can be measured
• Ancestor Selection


  • Milk yield
• Ancestor Selection


  Low accuracy, cheap cost
• Sib Selection


  Relatively accurate if h2 is low, expensive, can come close to accuracy of individual selection, need 15-20 sibs
• Progeny Selection


  Very accurate, time consuming and expensive, 100% possible if h2 is high
• Maximum accuracy for selections

  • Individual = 71%

  Progeny = 100%

  Sib = 0.71 (full) 0.50 (half)

  Ancestor = low
• EDP


  Expected Progeny Difference
• ETA


  Expected Transmitting Ability
• EBV


  Estimated Breeding Value
• BLUP


  Best Linear Unbiased Prediction
• Recurrent Selection


  Specialized progeny selection to improve crossbreeding and develop a company "line"
• Recurrent Selection


  • Pig companies
• Reciprocal Recurrent Selection


  Specialized progeny selection to establish a hybrid line
• Reciprocal Recurrent Selection


  • Poultry
• Positive Correlations


  • Milk Yield: Protein Yield

  Marbling: backfat

  Milk yield: fat yield

  Birth wt.: yearling wt

  Weaning wt: ADG
• Negative Correlations


  • Milk yield: fat percent

  Egg: body weight

  Eggs: egg size

  Calving ease: birth wt.

  Broiler size: fertility

  Marbling: lean yield
• Independent Culling


  Select for minimum value per characteristic, every animal > minimum for every trait
• When to rebreed cow in herd

  • Must produce calf

  Must mother calf well

  Must not be dangerous to owner
• Tandem Selection


  Selection for most important trait this season, sequential selection of traits, efficient if all traits are positively correlated
• Index Selection


  Use a formula to select for several traits, emphasizing some