The last page claimed that sexual reproduction creates variation. This page shows you the machinery that actually does it — and it turns out there are three separate shuffles, not one. It also explains why the two gametes ended up so absurdly different in size, which is a question examiners love and students rarely answer well.
📘 What you need to know
Meiosis is nuclear division that produces haploid cells from a diploid cell, in two successive divisions: meiosis I and meiosis II.
Three mechanisms generate variation: crossing over and independent assortment (both in meiosis I), plus random fertilisation afterwards.
Meiosis happens in the testes and ovaries of animals, and in the anthers and ovules of flowering plants.
Gametes are wildly unequal: the ovum is large, immobile and packed with cytoplasm; the sperm is tiny, motile and carries almost nothing but a nucleus.
You must be able to draw and annotate the male and female reproductive systems and state the function of each structure.
Fusion of gametes restores the diploid number and produces a new combination of alleles.
Halving, so that fertilisation can double
Meiosis takes one diploid cell and produces four haploid cells. It does this in two rounds of division. In meiosis I the homologous pairs separate, which is the step that actually halves the chromosome number. In meiosis II the sister chromatids separate, much like an ordinary mitotic division. Each division passes through prophase, metaphase, anaphase and telophase.
The point of meiosis
one diploid cell → meiosis I → meiosis II → four haploid gametes
A really common slip: meiosis does not “make sperm and eggs” directly in every case. It makes haploid nuclei. In plants those haploid nuclei then divide by mitosis to produce the actual gametes. If a question mentions pollen or ovules, expect mitosis to appear after meiosis.
The three shuffles
Here is the part worth understanding properly. Variation does not come from one clever trick — it comes from three independent random events stacked on top of each other.
The first two happen inside meiosis I. The third happens after meiosis is over, which is why “random fertilisation” is often forgotten in exam answers.
🧩 Naming the three sources under pressure
Crossing over — non-sister chromatids of a homologous pair exchange sections, so a single chromosome ends up carrying alleles from both grandparents.
Independent assortment — homologous pairs line up at random in meiosis I, so it is chance which member of each pair ends up in which daughter cell.
Random fertilisation — which particular sperm fuses with which particular egg is unpredictable, multiplying every combination already generated.
Why “independent” is the key word. Each homologous pair lines up without reference to any other pair. With 23 pairs in humans, that alone gives over 8 million possible gamete combinations before crossing over has added anything. If you can state that reasoning, you have the mark.
Sperm and egg: two answers to the same problem
Both gametes carry exactly the same amount of genetic information — one haploid set. Everything else about them is different, and the differences all follow from one split in strategy: the egg invests in resources, the sperm invests in mobility.
The dot on the left really is a sperm at the same magnification as the egg. Nearly all of the zygote’s cytoplasm, and all of its mitochondria, come from the mother.
Feature
Sperm
Egg
Size
Very small — a few tens of micrometres including the tail
Large — around 0.1 to 0.15 mm across, visible to the naked eye
Structure
Head, midpiece and tail; highly specialised for one journey
Rounded cell with a large cytoplasm and a protective jelly coat
Movement
Capable of swimming under its own power
Cannot move itself; carried along the oviduct by cilia
Food store
Almost none — carries only enough to reach the egg
Substantial, to support the embryo before implantation
Numbers
Produced continuously in enormous numbers
Normally one released per cycle from a store present since before birth
If a question asks you to explain the difference in numbers, do not just say “because most sperm die”. Say why that matters: only one sperm can fertilise the egg, the journey is long and hostile, so producing millions raises the probability that at least one arrives.
The reproductive systems
You need to be able to draw, label and explain both systems. The easiest way to remember them is not as a list of parts but as a journey: follow the gamete and ask what each structure does to it on the way.
The oviduct is the answer to a surprising number of questions: it is where fertilisation happens, and its cilia are what move the egg, since the egg cannot move itself.
Female reproductive system
Structure
Function
Ovary
Holds the immature egg cells and releases one at ovulation; also secretes oestrogen and progesterone
Oviduct
Carries the egg from the ovary towards the uterus using ciliated cells; fertilisation takes place here
Uterus
Muscular organ with a soft lining in which a fertilised egg implants and develops
Cervix
Ring of muscle at the base of the uterus that holds the developing fetus in place during pregnancy
Vagina
Muscular tube connecting the cervix to the outside; sperm are deposited here
Vulva
The external structures, which protect the internal parts of the system
Male reproductive system
Structure
Function
Testis
Produces sperm by meiosis and secretes the hormone testosterone
Scrotum
Holds the testes outside the body cavity, keeping them slightly cooler than core temperature
Epididymis
Coiled tube in which sperm mature and are stored until release
Sperm duct
Carries sperm from the epididymis towards the urethra
Prostate gland and seminal vesicle
Add fluid containing nutrients, forming semen and providing the sperm with energy
Urethra
Shared tube carrying either urine or semen; a ring of muscle prevents the two mixing
Penis
Transfers semen into the vagina; also carries urine out of the body
Why the testes hang outside. Sperm production works best a couple of degrees below core body temperature. That single fact explains the scrotum, and it is a favourite one-mark question. The equivalent question on the female side is why the oviduct is ciliated: because the egg has no means of moving itself.
Worked examples
WORKED EXAMPLE
Explain how meiosis and fertilisation together produce genetic variation in offspring. [4]
Split the question into its two halves“meiosis” wants two mechanisms; “fertilisation” wants oneFrom meiosiscrossing over in meiosis I exchanges alleles between non-sister chromatidsindependent assortment means each homologous pair lines up at random, so gametes get different combinationsFrom fertilisationany sperm may fuse with any egg, so the combination in the zygote is unpredictable4 marks: 2 for the meiosis mechanisms, 1 for random fertilisation, 1 for stating the outcome is new allele combinations
WORKED EXAMPLE
Suggest why the egg cell contains a large volume of cytoplasm whereas the sperm contains very little. [2]
Ask what each cell has to doegg: sit still and then support an embryo. sperm: travel, fastBuild the two linked pointsthe egg’s cytoplasm is a food store supplying the zygote until it implants and can obtain nutrients from the motherextra cytoplasm would add mass to the sperm and slow it down, so it carries only a nucleus, mitochondria and a tail1 mark for the egg’s food store, 1 mark for the sperm’s mass being minimised for movement“suggest” means apply the idea — you are not expected to have memorised this answer
💡 Exam tips
When asked for sources of variation, always give three unless the mark allocation says otherwise. Random fertilisation is the one candidates leave out.
Say meiosis I specifically for crossing over and independent assortment. “During meiosis” can lose you the precision mark.
Practise drawing both systems from memory with a labelled arrow showing the route of the gamete — it makes the function questions almost automatic.
Structure-and-function questions want the link word: “ciliated so that the egg is moved along”, not just “has cilia”.
The oviduct, not the uterus, is where fertilisation happens. The uterus is where implantation happens. Keep them apart.
⚠️ Common mix-ups
Saying meiosis produces two cells. Mitosis produces two; meiosis produces four, through two divisions.
Claiming crossing over happens between sister chromatids. It happens between non-sister chromatids — sister chromatids are identical, so swapping between them would change nothing.
Writing that the sperm gives the zygote its mitochondria. The sperm’s mitochondria are destroyed after fusion; the zygote’s mitochondria come from the egg.
Confusing the sperm duct with the urethra. The sperm duct carries sperm to the urethra; the urethra is the shared exit tube.
Saying eggs are “produced monthly by meiosis”. The immature cells are already present; one completes its development and is released each cycle.
Mixing up ovary and ovule. Ovary is the animal or plant organ; ovule is the plant structure inside it. That trips people up badly on the plant pages.
Up next: The Menstrual Cycle — four hormones, two feedback loops, and one graph that appears in Paper 2 more often than almost anything else in this topic.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
