Lec 16

• Inbreeding


  Mating of individuals more closely related than the average of the population/breed
• Outbreeding


  Breeding between unrelated animals
• Systems of Breeding


  • Inbreeding
  • Outbreeding
• Types of Inbreeding


  • Line breeding
  • Purebred breeding
• Types of Outbreeding


  • Crossbreeding
  • Terminal crossbreeding
  • Rotational crossbreeding
• Inbred animals


  Identical by descent - having more genes in common because parents are related or related to a common ancestor/ancestors
• Inbreeding Depression


  Loss of fitness and fertility arising from the mating of closely related individuals
• Prepotent


  More like their parents (true to type)
• Inbreds often more sensitive to environment than crossbreds
• Inbreds are often the optimum breed for high input or intensive system
• Hybrid vigour


  Resulting from the crossing of inbred lines
• General genetic result from inbreeding

  Aa x Aa -> 1/4 AA: 1/2 Aa: 1/4 aa -> 1/2 AA : 1/2 aa
• General genetic effect of inbreeding

  • Leads to a dispersion of gene frequencies among lines
  • Lines become differentiated in gene frequency as inbreeding proceeds
• Coefficient of inbreeding (F)


  Degree of homozygosity, % increase in homozygosity above the average of the population/breed
• Minimum value of F = 0, Maximum value of F = 1
• For livestock, F is rarely > 0.6, for laboratory animals F can approach 0.9, for plants with self-fertilization F can approach 1.0
• Inbreeding depression


  Decline in performance from mating of related individuals, especially those traits associated with reproductive rate/fertility, general vigour/fitness
• Inbreeding depression affects traits with low heritability
• Inbreeding


  Increases homozygosity, leads to more uniform phenotypes, recessive homozygotes tend to be inferior, large loss in reproduction and livability, some loss in growth and efficiency, little loss in carcass merit
• Phenotypic effects of inbreeding on reproduction traits

  • Litter size
  • Libido
  • Sperm count
  • Conception rate
  • Survival rate
  • Ovulation rate
  • Rebreeding interval
• Need to keep inbreeding at low level (< 0.1)
• Ways to avoid inbreeding

  • Avoid mating close relatives - nothing closer than cousins
  • Keep ratio of males : females high
  • Keep replacements from several families
  • Avoid bottlenecks (periods of reduced number of animals in the herd)
• Producers should not actively pursue inbreeding
• Homozygotes


  Highly inbred extremes
• Recessive homozygotes


  • Tend to be inferior
  • Lead to large loss in reproduction and livability
  • Some loss in growth and efficiency
  • Little loss in carcass merit
• Phenotypic effects of inbreeding on reproduction traits

  • Litter size
  • Libido
  • Sperm count
  • Conception rate
  • Survival rate
  • Ovulation rate
  • Rebreeding interval
• Phenotypic effects of inbreeding on growth

  • Growth
• Inbreeding


  • Need to keep at low level (< 0.1)
  • Producers should not actively pursue
• Ways to avoid inbreeding

  • Avoid mating close relatives - nothing closer than cousins
  • Keep ratio of males : females high
  • Keep replacements from several families
  • Avoid bottlenecks (periods of reduced number of animals in the herd)
• Inbreeding coefficient (F)


  • Degree of homozygocity
  • % increase in homozygocity above the average of the population/breed
  • If F = 0.25 then the animal is 25% more homozygous than the average of the breed
• Inbreeding coefficient (F)


  • Minimum value = 0
  • Maximum value = 1
  • For livestock rare for F > 0.6
  • Laboratory animals F can approach 0.9
  • Plants with self-fertilization F can approach 1.0 e.g., inbred lines of maize and rice
• Calculation of Inbreeding Coefficient (F)


  1. Generate an arrow pedigree

  2. List all paths that connect the sire and the dam

  3. Identify the common ancestor in each path

  4. Calculate the inbreeding coefficient of the common ancestor

  5. Count the number of arrows in each path to determine "n"

  6. Calculate the value of each path connecting the sire and dam

  7. Add together the value for each path connecting the sire and dam
• Measurement of relationship


  1. Generate an arrow pedigree

  2. List all paths that connect the X with D

  3. Identify the common ancestor in each path

  4. Calculate the inbreeding coefficient of the common ancestors

  5. Count the number of arrows in each path to determine "n"

  6. Calculate the value of each path connecting the sire and dam

  7. Add together the value for each path connecting the sire and dam

  8. Divide by value in denominator
• Relationship coefficient (R_XY) is the proportion of genes in common between relatives
• If R_XY = 0.25, the two individuals have 25% more genes in common than two random members of the breed