Lec 25

• Inbreeding


  Mating of animals more closely related than the average of the population
• Identical by descent


  Having alleles originating from the same individual
• Many would not want to inbreed because inbreeding has been associated with reduced fitness and fertility (Inbreeding Depression)
• But inbred animals are prepotent … producing uniform offspring with a high degree of predictability
• Normally inbreeding level is high (> 0.1) in inbred lines
• Linebreeding


  A system of mating which maintains a substantial degree of relationship to a highly regarded ancestor while keeping inbreeding to a low level
• Linebreeding


  A milder form of inbreeding
• A single common ancestor appears several times in a linebreeding pedigree, usually > 3 generations back
• Linebreeding keeps inbreeding to less than 12.5%
• If we breed a sire back to his daughter or mating full sister to full brother, we can expect inbreeding level to reach 25%
• Coefficient of relationship (R)

  Degree of kinship
• In linebreeding, R > 0.25 to a single common ancestor
• Effects of linebreeding


  • Having a higher probability of acquiring common genes derived from a common ancestor
  • Individuals are related or identical by descent to the common ancestor
• Purposes of linebreeding

  • To develop consistency and uniformity within a herd or breed
  • To fix desirable alleles associated with superior phenotypes derived from an outstanding individual/ancestor
  • To uncover and eliminate harmful and undesirable recessive alleles within the population
  • To develop prepotent animals desirable in the development of distinct family lines
• Strain


  A sub-population within a breed that develops into a variant of the established breed
• Differences in qualitative traits, like coat colour in Brahman cattle, can lead to the development of a strain
• Differences in fecundity rate (ovulation rate) in Merino sheep led to the development of the Booroola Merino sheep strain
• Characteristics of a strain

  • Produces uniform offspring when carried on a long term basis with appropriate selection
  • Increased homogeneity leads to predictability of performance of offspring
  • Uniformity is a major attribute useful for breeding purposes and commercial production
• First degree relatives


  Offspring-parents, full brother-full sister
• First degree mating results in a high inbreeding level, often referred to as inbreeding and associated with inbreeding depression
• Potential problems with linebreeding


  • Loss of general vigour, especially reduced reproductive performance
  • Inbreeding will build up but needs to be maintained at a low level
  • May linebreed to an inferior son of an outstanding ancestor
  • Ideals may change
• With linebreeding we get a set of uniform offspring with both desirable and undesirable alleles in their genomes
• Their phenotypes show superior performance as well as inferior traits (congenital defects, reduced fertility)
• Need to do annual selection to keep the desirable phenotypes as parents and cull the undesirable offspring
• How to linebreed


  1. Sire A x Dam M, Dam N, Dam P, Dam Q

  2. Half-sibs interse mating with selection

  3. ¼ M ¼ N ½ A
• Linebreeding examples

  • Abdallah, Charles Kent, Mare, Mambino, Amazonia, One Eye, Bellfounder, Hambletonion10, Son of, Messinger, Bishop's, Hambletonion, Silver Tail, Imp. Messinger (F < .05, R > .25 to a single common ancestor)
  • 16 Tons, Leo, Tom Lady, Gray, Little Fanny, Joe Reed II, Lady Reed, Joe Reed (F too high, more than one common ancestor)
• Coefficient of inbreeding (F)

  Measure of inbreeding level
• In linebreeding, check coefficient of inbreeding F < 0.1
• In linebreeding, check coefficient of relationship to a common ancestor
• Who should linebreed?

  • Outstanding purebred herd - hard to find breeding stock to improve the herd
  • Someone who has clearly identified an outstanding ancestor
  • A breeder who has a good knowledge of pedigrees and inbreeding
• Effects of Inbreeding (to be aware when linebreeding)

  • Leads to increased homozygosity at most loci (both desirable and undesirable alleles)
  • Alleles tend to be fixed in the population
  • Frequency of heterozygotes decreases
  • Alleles that are fixed may have desirable and undesirable characteristics
• Results of Inbreeding (to be aware when linebreeding)

  • Loss of fertility (example as in reduction in litter size)
  • Some inbred lines failed to survive, especially in highly inbred lines of laboratory mice
  • Surviving lines developed into commercial inbred lines/strains
• Need to keep inbreeding at low level, F < 0.15
• Producers should not actively pursue inbreeding
• Avoid mating close relatives - nothing closer than cousins
• Keep ratio of males : females high
• Keep replacements from several families
• Avoid bottlenecks (period of reduced census number)
• Linebreeding aims

  • To increase homozygosity for all genes with desirable characteristics from a common ancestor
  • But at the same time keeping inbreeding level low
  • By identifying carrier ancestor, undesirable genes may be uncovered and reduced in frequency
  • Having undesirable genes in the population may affect health and fitness of the animals and shorten their life span