Observation

Some candidates answered this question so well while majority could not even explain chromosome. Some candidates could not make a diagram of chromosome and those that attempted it could not adhere to the rules of making diagrams in biology. In question 4(c), many candidates could not relate their knowledge of haploid number of chromosomes in meiosis and diploid number of chromosomes in mitosis to answer the question to correctly fill the number of chromosomes for Man, Mouse and Rhesus monkey. The knowledge that the number of chromosomes in sex cells is halved while those in somatic cells (muscle cells and skin cells) are not halved should have been used to answer the question. This made some candidates lose the marks. The candidates that applied the knowledge scored all the marks.

Some candidates could only state the X and Y chromosomes as sex chromosomes but could not relate them to male and female while explaining how chromosomes determine the sex of a child in humans.

The expected answers are:

(a) Chromosome
A thread-like structure of nucleic acids; and proteins; found in the nucleus of living cells; carrying genetic information/genes.

(b) Diagram of chromosome


Title (Tl)

Detail (D)
X - shaped structure with unequal lengths of the arms (XS)
Centromere (CS)
Quality (Q)
Clarity of lines (CL)
Neatness of labels (NL)
Size (SZ) (2 cm to 4 cm)

Labels (L)
Centromere; chromatid, long/q-arm short/ p-arm.

(c) Table


(d) Explanation of how Chromosomes determine the sex of a child in humans

The 23 pairs/XY chromosomes determine sex in humans; the female carries two X chromosomes while male carries one X chromosome and one Y chromosome; so the female genotype is XX while male genotype is XY; when fertilization occurs, between X–carrying sperm cell and X-carrying ovum, the child will be a girl; and if a Y–carrying sperm cell fertilizes an X carrying ovum the child will be a boy.