Meiosis is where the variation comes from. It does not just halve the chromosome number — it deliberately breaks up the combinations of alleles a person inherited from their own parents and deals them out again. Three separate mechanisms do this, and you need all three by name.
📚 What you need to know
Meiosis is nuclear division that produces haploid cells from diploid cells, making the gametes used in sexual reproduction.
It happens in two successive divisions, meiosis I and meiosis II, each with prophase, metaphase, anaphase and telophase.
Three mechanisms create variation: crossing over, independent assortment and random fertilisation.
Meiosis occurs in the testes and ovaries of animals, and in the anthers and ovaries of flowering plants.
It produces spermatozoa and ova in animals; plant male gametes travel in pollen grains and female gametes sit in ovules.
The fusion of gametes at fertilisation creates new combinations of alleles, adding still more variation.
Sperm and ova differ hugely in size, structure, motility and number, and those differences drive different reproductive strategies.
Meiosis and where the variation comes from
Meiosis takes one diploid cell and produces four haploid cells. But if that were all it did, every gamete from one person would be identical, and siblings would be clones. They are not, because of three separate shuffling mechanisms.
🧩 The three sources of variation
Crossing over — during meiosis I, non-sister chromatids exchange alleles, producing chromosomes that are a mixture of the two the person inherited.
Independent assortment — homologous pairs line up randomly during meiosis I, so which member of each pair ends up in which daughter cell is down to chance. This gives different combinations of alleles in the gametes.
Random fertilisation — any one of millions of sperm can fuse with any egg, so the combination that forms the zygote is itself random.
Different colours mean different allele combinations. No two of the four gametes are the same, which is the entire point of meiosis.
A neat check: if a question asks why siblings are not identical, you have three marks sitting there — crossing over, independent assortment and random fertilisation. Name all three.
Where gametes are made
In animals, meiosis takes place in the testes of males and the ovaries of females, producing spermatozoa and ova (singular ovum). In flowering plants it happens in the anthers and in the ovaries: male gametes are carried inside pollen grains, while female gametes are held in ovules within the plant ovary.
Sperm and ova are not the same kind of cell
Meiosis itself is identical in males and females, but the cells that come out of it could hardly be more different. Those differences are not random — each is built for a different job.
Feature
Sperm
Egg
Size
Very small, around 45 micrometres long
Large, around 0.15 mm across
Structure
Head region plus a flagellum, with many structural adaptations for swimming
Round cell with few structural adaptations, covered in a jelly coating
Motility
Capable of locomotion
Not capable of locomotion
Numbers
Produced every day in huge numbers, roughly 100 million per day
Thousands of immature eggs in each ovary, but only one released each month
Follow those numbers through and the strategies make sense. A human female releases a single egg per menstrual cycle, while a male releases many thousands of sperm cells per ejaculation — because the vast majority never reach the egg, and only one sperm can fertilise it.
The reproductive systems
You need to be able to draw and annotate diagrams of both systems, identify the structures and recall what each one does. The quickest way in is to follow the route a gamete takes.
The two routes mirror each other: a site of meiosis, a place to mature or store, a tube, and an exit.
Female structure
Function
Ovary
Contains ova, which mature and develop when hormones are released
Oviduct
Connects the ovary to the uterus and is lined with ciliated cells that push the ovum along; fertilisation happens here
Uterus
Muscular bag with a soft lining where the zygote implants and develops into a foetus
Cervix
Ring of muscle at the lower end of the uterus that holds the developing foetus in place during pregnancy
Vagina
Muscular tube leading to the inside of the body, where sperm are deposited
Vulva
The external structures, one function of which is to protect the internal parts of the system
Male structure
Function
Testis
Produces sperm and the hormone testosterone
Scrotum
Sac holding the testes outside the body, keeping sperm slightly below body temperature
Epididymis
Coiled tubes that store sperm until ejaculation
Sperm duct
Carries sperm to be mixed with fluids from the glands before passing into the urethra
Prostate gland and seminal vesicle
Produce the fluid that, with the sperm, forms semen and supplies the sperm cells with nutrients
Urethra
Tube down the centre of the penis carrying either urine or semen; a ring of muscle stops the two mixing
Worked examples
WORKED EXAMPLE
Explain how meiosis and fertilisation together produce genetic variation in offspring. [4]
Point 1 — crossing over
During meiosis I, non-sister chromatids exchange alleles, producing chromosomes with new allele combinations.
Point 2 — independent assortment
Homologous pairs align randomly in meiosis I, so daughter cells receive different combinations of maternal and paternal chromosomes.
Point 3 — random fertilisation
Any sperm can fuse with any egg, so which two gametes combine is a matter of chance.
Point 4 — the outcome
The zygote therefore carries a combination of alleles that has almost certainly never existed before.
4 marksName the stage (meiosis I) for the first two points — it is often a marking detail.
WORKED EXAMPLE
Suggest why males release many thousands of sperm cells while a female releases only one egg per cycle. [3]
Point 1 — sperm losses
The great majority of sperm never reach the egg cell, so releasing large numbers raises the chance that one arrives.
Point 2 — only one is needed
Only a single sperm can fertilise an egg, so any extra sperm that arrive are surplus.
Point 3 — cost of the egg
The egg is far larger and carries food reserves for the early embryo, so it is expensive to produce and only one is released at a time.
3 marksThis is a “suggest” question: link every point back to a cost or a chance of success.
💡 Exam tip
Learn the three sources of variation as a set. They come up again and again, worth a mark each.
Crossing over and independent assortment both happen in meiosis I. Do not attach them to meiosis II.
When labelling reproductive systems, give the function as well as the name if the question says “annotate”.
The oviduct is where fertilisation happens; the uterus is where implantation happens. Two different places.
Use the correct plurals: one ovum, many ova; one testis, two testes.
Adaptations of the sperm should link to swimming and delivering the nucleus; adaptations of the egg to nutrients and blocking extra sperm.
⚠ Common mix-up
Meiosis does not make the zygote. It makes gametes; fertilisation makes the zygote.
Random fertilisation is not part of meiosis. It is a separate, third source of variation.
The scrotum is not the testis. It is the sac holding them outside the body.
The urethra carries both urine and semen, but never at the same time.
Pollen is not the male gamete. The gamete is the nucleus carried inside the pollen grain.
Two divisions does not mean two halvings. The chromosome number halves once, at meiosis I.
Up next: The Menstrual Cycle — four hormones, one 28-day loop, and the feedback that makes ovulation happen on cue.
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