Form 4 · Common mistakes

Sexual Reproduction, Development and Growth in Humans and Animals, common mistakes

The mistakes SPM students make on Sexual Reproduction, Development and Growth in Humans and Animals, why each one loses marks, and the correct version.

Book a Trial Class

One-hour paid trial · Same-day reply

The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying ovulation happens during menstruation.	Menstruation is at the start of the cycle (about days 1 to 5), while ovulation is near the middle (about day 14).	State that ovulation occurs around day 14, roughly in the middle of the cycle, not during menstruation.
Confusing the roles of FSH and LH.	FSH matures the follicle and causes oestrogen secretion; LH causes the mature follicle to release its ovum at ovulation.	State that FSH matures the follicle and LH triggers ovulation.
Writing that the ovum is produced in the oviduct.	The ovum is produced and released by the ovary; the oviduct is only where fertilisation usually happens.	State that the ovary produces and releases the ovum and the oviduct is the site of fertilisation.
Saying the mother's and foetus's blood mix in the placenta.	The two blood supplies stay separate; substances cross a thin barrier by diffusion.	State that maternal and foetal blood do not mix; exchange is by diffusion across a thin barrier.
Writing that identical twins come from two ova.	Two ova give non-identical twins; identical twins come from one zygote that splits.	State that identical twins come from one fertilised ovum that splits into two embryos.
Saying the placenta makes its own food for the foetus.	The placenta does not make food; it allows glucose and other nutrients to diffuse from the mother's blood.	State that nutrients pass from the mother to the foetus across the placenta by diffusion.
Calling growth simply an increase in size.	Temporary swelling, for example after drinking water, is not growth because it is not permanent.	Define growth as a permanent and irreversible increase in size and dry mass.
Drawing the human growth curve as a straight line.	Human growth is not at a constant rate; it speeds up in infancy and adolescence and levels off in adulthood.	Draw the human growth curve as a sigmoid (S-shaped) curve with a plateau at the end.
Mixing up complete and incomplete metamorphosis.	Complete metamorphosis has a pupal stage and a larva unlike the adult; incomplete metamorphosis has a nymph and no pupa.	State that complete metamorphosis has egg, larva, pupa and adult, while incomplete has egg, nymph and adult.
Saying insects grow smoothly like humans.	The rigid exoskeleton cannot stretch, so insects grow in steps through moulting.	State that insects grow in steps because they must moult the exoskeleton to increase in size.
Confusing an embryo and a foetus.	The early dividing cells form an embryo; it is called a foetus only after the main organs have formed.	State that the embryo becomes a foetus once its main organs have developed.
Saying gametes are diploid.	Gametes are made by meiosis, which halves the chromosome number, so they are haploid.	State that sperm and ova are haploid, so fertilisation restores the diploid number in the zygote.

How to avoid these mistakes

Draw the menstrual cycle as a timeline with the key events and hormones.
Compare identical and non-identical twins with a simple diagram.
Sketch and interpret the human sigmoid growth curve.
Build a timeline linking FSH, oestrogen, LH and progesterone to the exact event each hormone triggers.
List the placenta's functions and state clearly that maternal and foetal blood stay separate.
Compare incomplete and complete metamorphosis in a table with one named example of each.

Frequently asked questions

What happens during the menstrual cycle?

In a roughly 28-day cycle, the uterus lining first thickens ready for a possible pregnancy. Around day 14, ovulation releases a mature egg from an ovary. If the egg is not fertilised, the thickened lining breaks down and is shed as menstruation, and the cycle begins again. Hormones from the ovaries and pituitary gland control these events. Being able to place ovulation, fertilisation and menstruation correctly on a single timeline is a frequently tested skill.

What is the difference between identical and non-identical twins?

Identical twins form when a single fertilised egg (zygote) splits into two, so both have the same genes and are the same sex. Non-identical (fraternal) twins form when two separate eggs are released and fertilised by two different sperm, so they are genetically as different as ordinary siblings and can be different sexes.

How is growth defined in biology?

Growth is a permanent and irreversible increase in the size and dry mass of an organism. In humans it follows a sigmoid (S-shaped) curve: slow at first, rapid during infancy and adolescence, then levelling off in adulthood. It results from cell division, cell enlargement and cell differentiation. A growth curve question often also asks for the phase in which growth rate is fastest, usually adolescence in humans.