Every living thing has to make more of itself, and there are only two ways to do it. One is a photocopier. The other is a shuffle. Almost every mark in this topic comes from understanding why an organism would choose one over the other — not from reciting the definitions.
📘 What you need to know
Sexual reproduction needs two parents. Two haploid gametes fuse at fertilisation to make a diploid zygote, and the offspring are genetically different from each other and from both parents.
Asexual reproduction needs one parent. There are no gametes and no fertilisation, so the offspring are genetically identical clones.
The cell division is the giveaway: meiosis makes gametes for sexual reproduction, mitosis makes clones for asexual reproduction.
A gamete is haploid (n) — one copy of each chromosome. In humans that is 23. A zygote is diploid (2n) — 46, half from each parent.
Sexual reproduction generates variation; asexual reproduction generates speed and numbers. Every advantage and disadvantage in this topic flows from that one trade-off.
Variation matters because it lets a population survive an environmental change that no individual has met before.
The two routes, side by side
Start with what physically happens, because the exam answers all hang off it. In asexual reproduction a single parent divides by mitosis. Mitosis copies DNA exactly, so every daughter cell carries the same alleles as the parent. Nothing is mixed, so nothing new appears.
In sexual reproduction, two parents each make gametes by meiosis. Meiosis halves the chromosome number, so a gamete carries only one copy of each chromosome. When two gametes fuse, the full number is restored — but the combination of alleles in that new cell has never existed before.
Notice the ploidy. The only reason meiosis has to halve the chromosome number is so that fertilisation can double it again without the total creeping up each generation.
If you can only remember one thing: count the parents, then name the division. One parent means mitosis means clones. Two parents means meiosis means variation. Almost every short-answer question in this section is testing that chain.
Gametes, zygotes and the chromosome numbers
A gamete is a sex cell. In animals these are the sperm and the ovum; in plants they are the nucleus inside the pollen grain and the ovum in the ovule. What makes a gamete different from an ordinary body cell is that it has a haploid nucleus — it carries one copy of each chromosome rather than the usual two.
So a human body cell has 46 chromosomes, a human gamete has 23, and the zygote formed when they fuse is back to 46 — with 23 from the father and 23 from the mother. That “half and half” is exactly why an offspring resembles both parents but is identical to neither.
Why halving is not optional. If gametes were diploid, every generation would double the chromosome number: 46, then 92, then 184. Meiosis exists to stop that happening. Examiners like this reasoning because it shows you understand meiosis as a solution to a problem, not just a process to memorise.
Asexual reproduction: the photocopier
Asexual reproduction does not involve gametes or fertilisation at all. One parent divides by mitosis, and because mitosis produces genetically identical daughter cells, the offspring are clones — identical to the parent and to each other.
It is common in plants, which reproduce asexually through runners, bulbs, tubers and cuttings. Bacteria do it by binary fission, simply copying their DNA and splitting in two. Under good conditions a bacterial population can double every twenty minutes, which is exactly the point: when conditions are already favourable, copying a genotype that works is the fastest possible strategy.
💡 The trade-off in one line
Asexual wins when the environment is stable. The parent’s alleles are already proven to work there, so copying them exactly is the best bet, and it costs no time or energy finding a mate.
Sexual wins when the environment changes. Variation means that whatever the change turns out to be, some offspring may happen to cope with it.
Neither is “better”. Any question asking you to evaluate them wants this trade-off stated explicitly, usually with a named condition attached.
Why variation is worth the cost
Sexual reproduction is genuinely expensive. You have to find a partner, you only pass on half your alleles, and it is slow. So why does almost every complex organism do it?
Because a population of clones has a single shared weakness. If a new disease can infect one individual, it can infect all of them, since they carry identical alleles and therefore identical vulnerabilities. In a varied population, the same disease meets many different genotypes, and some of those individuals may carry alleles that give resistance. They survive, they reproduce, and the population continues.
The resistant individuals existed before the disease arrived. Selection does not create the useful allele, it just removes everything else.
The comparison table
Six features come up again and again. Learn the row headings and you can rebuild the whole table under pressure.
Feature
Asexual
Sexual
Number of parents
One
Two
Cell division involved
Mitosis only
Meiosis to make gametes, then mitosis after fertilisation
Genetic similarity of offspring
Identical to each other and to the parent
Different from each other and from both parents
Sources of variation
Mutation alone
Crossing over, independent assortment, random fertilisation — plus mutation
Number of offspring
Usually very large
Usually smaller
Time and energy cost
Fast and cheap; no mate needed
Slower; energy spent finding and attracting a mate
Worked examples
WORKED EXAMPLE
A strawberry plant reproduces both by seeds and by runners. Explain one advantage of each method to the plant. [4]
Spot what each method actually isseeds = sexual (two parents, meiosis); runners = asexual (one parent, mitosis)Advantage of the runnersoffspring are clones of a parent already suited to that exact spot, and they are produced quickly without needing a pollinatorAdvantage of the seedsoffspring are genetically varied, so if conditions change some may have alleles that let them cope1 mark for identifying each method + 1 mark for each linked advantagenote the marks are for the LINK, not the label — “runners are asexual” alone scores nothing
WORKED EXAMPLE
A fungus has 18 chromosomes in each of its body cells. State the number of chromosomes in a spore produced by mitosis and in a gamete produced by meiosis. [2]
Mitosis copies, it does not halvespore from mitosis = 18 chromosomes (still diploid, still a clone)Meiosis halvesgamete from meiosis = 18 ÷ 2 = 9 chromosomes (haploid)18 and 9the trap is assuming anything called a “spore” must be haploid — read which division made it
⚠️ Common mix-ups
Saying asexual offspring have “no variation” full stop. They have no variation from reproduction, but mutation can still occur. Write “no new variation from the reproductive process”.
Confusing haploid with “half a cell”. Haploid means one copy of each chromosome, not half a chromosome and not half a nucleus.
Writing that sexual reproduction “causes” adaptation. It produces variation; natural selection then acts on that variation. Keep the two steps separate.
Forgetting that fertilisation itself is a source of variation. Which particular sperm meets which particular egg is random, and that is worth a mark on its own.
Listing advantages without a condition. “Asexual is faster” is weak. “Asexual lets the population expand rapidly when conditions are already favourable” is the answer.
Calling binary fission mitosis. Bacteria have no nucleus, so they do not perform mitosis in the strict sense — binary fission is its own process, though the outcome is the same: clones.
Up next: Sexual Reproduction — how meiosis actually generates the variation we have just been claiming it does, and why sperm and eggs ended up so wildly different in size.
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