This question was not a popu1ar one and many candidates avoided it. Many candidates who
attempted it did not score high marks.
In stating inheritable characters many candidates stated skin, colour, hair colour, eye colour
which earned them just one mark for colour. Some stated blood instead of blood group,
anaemia instead of sickle cell anaemia and so lost marks. Many candidates could correctly
name the site of meiosis in plants but in animals they named gonads rather than testes and
ovary and so lost some marks.
A number of candidates cou1d correctly state the differences between mitosis and meiosis but
many candidates answer did not correspond and so they lost marks. Expected answers include:
DIFFERENCES BETWEEN MITOSIS AND MEIOSIS
MITOSIS |
MEIOSIS |
Nucleus divides only once |
Nucleus divides twice |
Number of chromosomes remains the same |
Number of chromosomes is halved after |
after division/diploid. |
division/diploid. |
Homologous chromosomes do not associate |
Homologous chromosomes associate to form |
|
bivalents |
Chiasmata are never formed |
Chiasmata are formed |
Crossing - over never occurs/no exchange of |
Crossing - over occurs/exchange of genetic |
genetic materials |
materials occurs |
Daughter cells identical to parent cell |
Daughter cells generally different from parent |
|
cell; |
Two daughter cells are formed |
Four daughter cells are formed |
Chromosomes form single row at the equator |
Chromosomes form double row at the equator |
Occurs in somatic cell |
Occurs in reproductive cell |
Occurs in a sexual reproduction |
Occurs in a sexual reproduction |
Many candidates performed poorly in question 4(c) on the paternity of woman's child. Most
candidates cou1d not write the genotype of the parents namely AA and AO. They wrote Aa
instead of AO. They would not do the crossing. Many candidates stated that Mr. James was
not the father of the child but could not give reasons for their conclusion .
Expected answers were as follows: |
|
|
|
|
|
|
|
WOMAN |
x |
MAN |
|
Blood group |
|
A |
|
|
A |
|
|
|
|
|
|
|
Phenotypes |
|
|
|
|
|
|
Genotype |
|
AO |
|
x |
AA |
|
|
|
Gametes: |
|
A |
0 |
A |
A |
|
|
|
|
|
. |
FI GENOTYPES AO AO AA AA
The Woman's claim is wrong
Reasons
Half their children are heterozygous blood group A and
Halfhomozygous blood group A
No child is group 0