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Agricultural Science Paper 2, Nov/Dec 2008  
Questions: 1 2 3 4 5 6 7 8 9 10 Main
General Comments

Question 5

(Explain the following terms as used in crop improvement:

  •   mass selection;
  •   in-breeding;
  •   back crossing;
  •   hybridization.                                                                                             (16 marks)



This question was not popular with the candidates.  In 5(a-d), majority of the
candidates could not satisfactorily explain mass selection, in-breeding, back crossing and hybridization.

The expected answers include:

Mass selection

  • This is the selection of crops from a large population of the crops;
  • The group with desirable and undesirable traits is mixed initially and
    their produce evaluated;
  • Progressive selection is carried out from the group and those
    selected are further grouped;
  • The undesirable ones are progressively eliminated;
  • The groups selected are further subjected to further planting and
  •    screening until the desired plants are finally obtained;
  • It is based on natural selection.


  • This is the same thing as selfing;
  • It is the use of pollen from the male flower of a plant to pollinate the stigma of the
    female flower of the same plant or the female flower of another plant which is an offspring of the original plant;
  • It is effected by collecting the pollen from the flower and sprinkling it on the
    stigma of the same flower which must have been covered up and protected from being pollinated with pollen from different plants;
  • In both monoecious and dioecious plants, both pollens and stigmas must be well
       protected against pollen from unrelated plants;
  • Such selfing produces homozygous plants;
  • Offspring from inbreeding are less vigorous.

Back crossing

  • This is the crossing of the offspring of two unrelated parents with
       one of the original parents;
  • Carried out by collecting the pollen from the offspring and using it
       to pollinate the stigma of one of the original parents or vice versa;
  • When carried out progressively, the resultant offspring of such
    back-crossing tends to be very similar to one of the parents in certain traits;
  • It is used in transferring single identifiable and desirable traits from
       parents to offspring;
  • Genotype of the offspring tends to be homozygous.


  • Two homozygous lines must first be obtained from inbreeding or
       selfing over some time until the desirable ones are produced;
  • The two homozygous lines are then cross-pollinated to produce
       heterozygous offspring or hybrid seeds;
  • In the development of the homozygous lines through selfing or
    inbreeding, there must be selection against undesirable traits from the group so that the
    final homozygous plant is pure;
  • Crossing of these inbred homozygous lines produces an offspring
    with superior traits than either of the two original parents.  This is "hybrid vigour".
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