Lec 26

• Outbreeding


  A system of mating involving individuals less closely related than the average of the population
• Inbreeding


  A system of mating of individuals more closely related than the average of the population
• Outbreeding systems

  • Linecrossing
  • Crossbreeding
  • Rotational breeding
  • Terminal breeding
• Linecrossing


  Mating of members of different lines within a breed
• Crossbreeding


  Mating of individuals or members of different breeds
• Systems of Mating Involving Inbreeding and Outbreeding

  • Inbreeding
  • Linebreeding
  • Outbreeding
• Inbreeding


  Mating of closely related individuals
• Linebreeding


  Mating of individuals with a special type of relationship such as an excellent ancestor
• Outbreeding


  Mating of individuals less closely related among themselves
• Outbreeding


  Increases heterozygosity
• Heterozygotes are observed to be more adaptive to harsh environments compared to inbreds
• Heterozygotes have higher fertility and fitness levels
• Inbreeding leads to increased homozygosity
• Crossing between lines within breeds

  Linecrossing
• Crossing between different breeds

  Crossbreeding
• The more divergent the two lines/breeds in the crossing, the higher the magnitude of the advantage of crossbreds over their parents (heterosis)
• Heterosis


  Superiority of crossbreds over their parents
• Heterosis is due to divergence or differentiation among lines within breeds or breeds of livestock species
• Different breeds originate in different environments and natural selection has selected for adaptation to specific environments
• Breeds have different gene frequencies of desirable alleles
• Outbreeding masks the effect of deleterious recessive alleles
• Most deleterious alleles present in the heterozygotes
• Natural and artificial selection are not effective in eliminating deleterious alleles present in the heterozygotes
• Outbreeding would restore fitness lost from inbreeding
• Without selection, inbreeding followed by crossing of inbred lines and of subsequent crossbreds in a large population, no permanent change in population mean is expected
• With continued crossing among crossbreds, no change in population mean and inbreeding coefficient is expected
• Parental


  Population of individuals as parents
• F1 generation


  Offspring from the crossing of 2 parental lines or groups
• F2 generation


  Crossing of F1 with F1 crossbreds
• Backcross


  Crossing of F1s to one of their parental lines/breeds
• Reciprocal cross


  Crossing of different sire and dam breeds, e.g. sire A x dam B, sire B x dam A
• Heterosis (H)


  Superiority of crossbreds over their parents, measured as a deviation of mean of crossbreds of F1 or F2 generation from the mean of their mid-parent (mean of the sire and dam breeds/lines)
• Heterosis is highest when one allele is fixed in one population and the other allele in the other population
• Heterosis is dependent on dominance and difference in gene frequency of the two breeds involved
• Heterosis must have directional dominance
• The magnitude of heterosis is specific to a particular cross, different pairs of lines have different values
• If the 2 parental lines are highly inbred and completely homozygous, heterosis is the sum of all dominance deviations of these loci that have different alleles in the two lines
• The more divergent the two breeds in the crossing, the higher the magnitude of Heterosis
• Directional dominance


  One allele is dominant over the other allele
• Heterosis is calculated as the deviation of mean of crossbreds from the mean of their mid-parent, usually expressed as a percentage