Question 3 

((a)   Explain the following terms:

        (i)   test cross;
       (ii)   recessive allele;                                                                                                             [8 marks ]
      (iii)   homozygote.

(b)    (i)   Why is sickle-cell anaemia considered a deadly disease?                                     [5 marks ]
        (ii)   Explain briefly how sickle-cell anaemia can be reduced in a population           [6 marks ]
        (iii)  State one advantage which a carrier of the sickle-cell anaemia trait has          [ 1 mark]

 Not many candidates attempted this question and those who did performed averagely.
Most candidates could correctly define, test cross, recessive allele and homozygote.
But in defining homozygote many candidates omitted to say that the identical genes were on the same locus and so lost marks.
Most candidates could state the advantage of sickle-anaemia carriers i.e. resistance to malaria but could not state why sickle cell anaemia is a deadly disease or how its incidence could be reduced.

Expected answers include:

Why Sickle Cell Anaemia is a deadly disease

• the red blood cell is sickle/abnormal in shape;
• has less haemoglobin; which reduces its oxygen carrying capacity;
• hence transport of oxygen to the vital organs/heart/lungs/brain is reduced;
• leading to serious crisis/pneumonia/headaches/paralysis/joint pains/

weakness/fainting/being sickly;

• and eventually death.

                                                                                     
 Reduction of sickle-cell anaemia

•   Sickle-cell is an hereditary/inherited disease;
•   Caused by a recessive gene;
•   Two individuals who possess the recessive gene; two carriers/ carrier and sufferer/

    two sufferers; should be counselled not to marry;

•   To prevent having an offspring with the disease.