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Biology Paper 2, Nov/Dec 2009  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 4

  1. )   Explain the following terms giving one example in each cases:
            (i)     complete dominance;
            (ii)    co-dominance;
           (iii)    mutation.                                                                                                                                 (9 marks)

    (b)   (i)   What kind of variation is exhibited by human blood groups?                                          (1 mark)
           (ii)   With the aid of genetic diagrams explain how parents with blood groups
                  ‘A’ and ‘B’ respectively can give birth to an offspring with blood group ‘O’.                  (10 marks)



This was the least popular question in this section and the few candidates who attempted it performed poorly.   Some of the candidates were able to properly define mutation, but most of them could not define co-dominance and complete dominance or give appropriate examples.  A good number of candidates could state that the blood group is an example of discontinuous variation but most candidates who attempted (b)(ii) performed woefully and did not give any logical explanation as to how parents with blood group ‘A’ & ‘B’ could have an offspring with blood group ’O’.  They could not state the genotypes of the parents or the gametes.  Some candidates were crossing AB x Bo and TT x TE.  Expected answers include.

Complete dominance:   this occurs when the effect of the gene is seen in the phenotype; as long as one dominant  gene is present; e.g. Tall dominant over short.

Co-dominance:  This is when both alleles exert their influence; so the hybrid has a phenotype that is intermediate between that found in both parents; e.g. red and white flower produce pink flowers.

Mutation:  This is an abrupt/sudden inheritable change; in the gene/chromo some/chromosome number; brought about by a fault in the replication of the genes; e.g Albinism.

Genetic cross to explain Blood group ‘O’ offspring

(ii)   Parents                        AO                  X                                                    BO
        Gametes        (A)         (O)             X                        (B)                (O)

F1 generation      AB             BO                     AO                           OO
                             NB:  Punnet square can be used as alternative to the above.


Blood group A and B could be AA or AO and BB or BO; for the offspring to be O the parents have to be AO and BO

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