- -  AGRICULTURE CORE CURRICULUM  - -

(CLF3000)  Advanced Core Cluster:  ANIMAL SCIENCE

  (CLF3250)  Unit Title:  LIVESTOCK BREEDING AND GENETICS

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  (CLF3255)  Topic:  SELECTION AND               Time          Year(s) 
                     HERITABILITY               1 hour          3 / 4        
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          Topic Objectives:  Upon completion of this lesson, the student
                             will be able to:

          Learning Outcome #:

            (D-10)  -  Define heritability and explain why selection is 
                       important in the livestock industry.

          Special Materials and Equipment:  None

          References:  Baker and Greer. (1981). ANIMAL HEALTH (2nd. ed.).

          Evaluation:  Quiz by instructor.


TOPIC PRESENTATION:  SELECTION AND HERITABILITY

A.  Livestock improvement is dependent upon selecting genetically superior
    animals for breeding purposes.

B.  Selection secures or retains the best of those animals in the current 
    generation as parents of the next generation.

C.  Selection is based on:

    1.  type and individuality which often represents standards of 
        perfection for each breed within a species,

    2.  pedigree,

    3.  show-ring winnings, and

    4.  production testing (use of records based on economic importance).

        a.  Performance testing:

            1)  selection based on individual merit or performance,

            2)  type and individuality,

            3)  regular reproduction, 

            4)  high numbers in litters, and

            5)  efficiency of production (utilization of feed to produce
                meat, milk, wool, power, etc.).

        b.  Progeny testing:  Selection based on merit of their progeny.

D.  Systems of Selection:  

    1.  Tandem selection is selecting for one trait at a time until the 
        desired improvement is reached.

        a.  It is hard to select for only one trait.

        b.  Usually the traits are dependent on one another.

    2.  Establishment of minimum standards for each characteristic and 
        selection simultaneously but independently for each characteristic 
        is the most common system.

        a.  It sets minimum standards for each characteristic and culls out
            the others.

        b.  It allows several characteristics to be selected for 
            simultaneously, but independently of each other.

    3.  The selection index combines all important traits into one overall 
        value or index.  It,

        a.  keeps relative the importance of different traits.

        b.  balances the strong points against the weak points of an 
            animal.

        c.  gives an overall score and ranks animals.

        d.  achieves an objective degree of emphasis for the important 
            traits.

        e.  is a convenient way to correct for any environmental effects.

E.  Quantitative traits are commercially important characters such as meat,
    wool, and egg production, and are due to many genes.

    1.  The extremes tend to swing back to the average, and the progeny of 
        the two average parents will probably be superior.

    2.  It is difficult to determine the mode of inheritance of such 
        characters.

F.  Heritability of quantitative traits are influenced by training, feeding, 
    and other environmental factors.

G.  The breeder can not select for too many of the desired characteristics 
    at one time.

H.  Factors to determine the emphasis to place on selection of each desired
    characteristic:

    1.  economic importance of the characteristic to the producer; 

    2.  the heritability of characteristic - the more heritable, the greater 
        the priority;

    3.  the genetic correlation (the breeder would like a high correlation so
        that one trait automatically selects for another);

    4.  amount of variation in each character should be neither too low or 
        too high; and

    5.  if the level of performance is already attained, it would not be 
        selected as a desired characteristic.

I.  It is crucial to stress that the maximum development of characteristics 
    of economic importance CAN NOT be achieved unless there is optimum 
    nutrition and management.



4/16/90 bkm/sg/gv


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