Principles of Selecting and Mating Farm Animals (Chapter 9)

							   Principles of Selecting and Mating
       Farm Animals (Chapter 9)

• Genetic improvement of farm animals
  –Involves selection (choosing the best to be
   parents)
  –Involves mating systems (combining sires and
   dams to maximize efficiency)
        Quantitative Inheritance
• Quantitative traits – traits that can be
  measured
   –Have continuous variation – any two values could
    have an intermediate value
   –Generally controlled by many gene pairs
• Qualitative traits – traits that can be classified
   –Frequently controlled by few gene pairs
 Phenotypic Variation in Quantitative
               Traits
• Distribution of performance traits generally
  normal (bell curve)

• Majority of values near the mean

• Fewer values far away from the mean
Frequency of Genes in a Population

• Goal of genetic improvement

  –Increase frequency of desirable alleles
   (form of a gene)

  –Decrease frequency of undesirable alleles
Frequency of Genes in a Population
                                      Total
  Number       Genotype         Red      White
   49 red         RR            98            0
  42 roan        RW             42            42
  9 white        WW              0            18
   Total                        140           60
Freq R = 140/200 = .7     Freq W = 60/200 = .3
  Forces that Change Gene Frequency

• Mutation
• Migration
• Selection
• Genetic drift
                  Mutation
• Change in the base sequence
• Some mutations occur at regular frequency
• Mutation rate is low and regular change due to
 mutations is very small
• By chance, some mutations end up making a
 difference in livestock (dwarfism in beef cattle
 in the 1950s)
                 Migration
• Importing new genes into a population
  –Purchasing new sire
  –Opening up breed to new animals
  –Importing European breeds of cattle

• Very powerful force for changing gene
  frequency
                  Selection

• Choosing best young animals to be parents
• Eliminating inferior parents from population
• Progress is gradual but steady
• Should select on a balance of characteristics
               Genetic Drift
• Change in gene frequency due to chance
• Each sperm and egg contains random sample
 of genes from parent
• Sample may be above or below average
• Some offspring better than average of parents
• Some offspring worse than average of parents
          Phenotypic Variation

• Phenotype = Genotype + Environment
• Variance in phenotypes
  –Due to variance in genotypes and environments

• Environmental effects
  –Effects other than genetic effects
 Genotype x Environment Interaction

• Differences between genotypes may not be
  constant in all environments

• Example
  –Brahman crosses superior to British crosses in
   southern states
  –British crosses superior to Brahman crosses in
   northern states
                Heritability

• Proportion of phenotypic variation that is due
  to genetic variation

• Describes how easy to make progress through
  selection

• May be any value from 0 to 1

• Usually between 0 and .60
                 Heritability
• Generally:

• Reproductive traits – low heritability (0-.2)

• Growth traits – moderate heritability (.2-.4)

• Carcass traits – high heritability (.4-.6)

• There are some exceptions to these
  generalizations
             Selection with Different
              Types of Gene Action

• Effectiveness depends on whether gene action
  is additive or non-additive

• Additive
  –Easy to make selection improvement
  –Each gene has differential effect
         Selection with Different
          Types of Gene Action

• Non- additive (dominance or epistasis)
  –Some alleles may mask other alleles

  –Some gene pairs may affect other gene pairs

  –Reduces effectiveness of selection

  –Selection may move toward some intermediate
   gene frequencies instead of 0 or 1
 Progeny Testing for Recessive Alleles

• Important to identify carriers

• Mate suspected carrier to known carriers or to
  daughters

• If enough matings without affected offspring:
  –Can establish low probability that individual is a
   carrier
        Gene Action with Heritability,
          Inbreeding and Heterosis

• Additive effects large
  –Heritability high, effect of inbreeding and
   heterosis low

• Non-additive effects large
  –Heritability low, effect in inbreeding and heterosis
   high
 Selection of Superior Breeding Stock

• Selection on individual performance
  –If available – individual performance is single
   most important piece of information

  –Selection on individual performance most
   effective for traits with moderate to high
   heritability
 Selection of Superior Breeding Stock

• Selection on performance of relatives
  –Sibs, progeny, pedigree, other collateral relatives

  –Useful especially for traits with low heritability

  –Some traits not measured on potential parent
     •carcass traits
     •traits measured in only one sex (eg milk)
      Predicting Selection Response

• One generation of selection
  –Response = heritability x selection differential

  –Selection differential = difference between those
   selected to be parents and average of group

  –Selection differential larger for males
    • smaller proportion of young males need to be kept
      Predicting Selection Response

• For several years
  –Yearly selection response
   = heritability x selection differential
            generation interval

  –Generation interval
     •average length of time to replace parents
     •swine 2-3 years, cattle 4-6 years
            Genetic Correlation

• Selection for one trait causes genetic change in
  another trait

• Caused by pleiotropy (genes that affect more
  than one trait)
    National Performance Programs

• Was need for uniform performance
  information

• Dairy programs organized first

• Beef programs followed

• Swine and sheep programs came later
 Dairy Cattle Performance Programs

• Dairy Herd Improvement Association

• Cooperative with United States Department of
  Agriculture

• Standardized lactation length for measuring
  milk production at 305 days

• Huge genetic increase in milk production in
  last 50 years
  Beef Cattle Performance Programs

• Beef Improvement Federation

• “Guidelines for Uniform Beef Improvement
  Programs”

• Established standard recommendations for
  measuring growth, efficiency, reproduction,
  carcass traits
     Swine Performance Programs

• National Swine Improvement Federation

• “Guidelines for Uniform Swine Improvement
  Programs”

• Established standard recommendations for
  measuring growth, efficiency, reproduction,
  carcass traits

• Recommends indexes to use for selection
     Sheep Performance Programs

• National Sheep Improvement Program

• Established standard recommendations for
  measuring growth, efficiency, reproduction,
  carcass traits

• Although slower to develop than other classes
  of livestock, programs are well organized
      National Genetic Evaluation

• Problem – how to make fair comparisons
  between potential breeding stock raised in
  different environments?

• Solution – use ties between herds that are
  established because many sires are used across
  several herds due to artificial insemination
      National Genetic Evaluation

• Breed associations maintain large databases of
  performance records for their herd
  improvement programs

• Data used to compare genetic merit of animals
  across entire breeds
      National Genetic Evaluation

• Expected Progeny Difference (EPD)
  –Measure of predicted genetic merit

  –Used for comparison between animals
     Bull Weaning Weight EPD
      A              +40
      B              +10

  –Means that Bull A is expected to sire calves that
   weigh 30 pounds more than the calves from Sire B
      National Genetic Evaluation
• Expected Progeny Difference (EPD)
  –EPD is called the PTA for dairy cattle (Predicted
   Transmitting Ability)

• Dairy – conducted by USDA

• Beef – conducted by breed associations

• Swine – organized within STAGES program
  (Swine Testing and Genetic Evaluation
  System) directed by Purdue University
               Mating Systems
• Inbreeding
• Linebreeding
• Linecrossing
• Crossbreeding
               Mating Systems
• Inbreeding
  –Mating of related individuals

  –Increases homozygocity

  –Does not cause mutations

  –Does increase homozygous recessive frequency so
   increases frequency that mutant genes are
   expressed
                 Mating Systems

• Inbreeding
  –Inbreeding depression
   • recessive alleles tend to be inferior

   • causes decline in performance due to increase in
     frequency of recessive homozygotes

   • most decline in reproduction and livability
               Mating Systems

• Linebreeding
 –Mating system that causes large relationship to one
  outstanding ancestor while keeping inbreeding low

 –Useful to retain genes of outstanding individual
  who is not longer available for breeding purposes

 –Outstanding individual must appear in pedigree
  several times at least 3-4 generations back
               Mating Systems

• Linecrossing
  –Mating unrelated individuals within a breed

  –Causes some increase in performance (less than
   what is seen with crossbreeding)
                Mating Systems

• Crossbreeding
 –Mating of individuals from different breeds

 –Benefits
   • heterosis – advantage of crossbred individual compared
     to the average of the component purebreds

   • breed complementarity – using benefits from breeds
     while hiding the flaws
                Mating Systems

• Heterosis
  –Individual heterosis – advantage of crossbred
   offspring

  –Maternal heterosis – advantage of crossbred mother

  –Paternal heterosis – advantage of crossbred sire
               Mating Systems
• Heterosis
  –Opposite of inbreeding depression

  –Results from increase in heterozygocity

  –Reproduction – large advantage from heterosis

  –Growth – moderate advantage from heterosis

  –Carcass – little advantage from heterosis
          Crossbreeding Systems

• Terminal
  –Specific breed(s) of sire mated to specific breed(s)
   of dam

• Rotational
  –Breeds used in a regular cycle, daughters of one
   breed of sire mated to next breed of sire
          Crossbreeding Systems
• Terminal
  –Uses maximum breed complementarity

  –Uses maximum heterosis

  –Must bring in replacement breeding stock

• Rotational
  –Replacement females retained by system

  –No breed complementarity

  –Some loss of heterosis