Biology · 16 Reproduction

16.2 Sexual reproduction

Before you start

A cell's genetic material is carried on chromosomes inside the nucleus.
Asexual reproduction produces genetically identical offspring from one parent (16.1) — this lesson is the contrast to it.
Meiosis is a reduction division that halves the chromosome number and produces genetically different cells (Topic 17).
Variation means differences between individuals of the same species.
In flowering plants, pollen is made in the anther and the ovule is inside the ovary.

Syllabus objectives covered (4)

Describe sexual reproduction as a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other
Describe fertilisation as the fusion of the nuclei of gametes
State that nuclei of gametes are haploid and that the nucleus of a zygote is diploid
Discuss the advantages and disadvantages of sexual reproduction: (a) to a population of a species in the wild (b) to crop production

Part 1 of 6

Three words, and you cannot swap any of them

Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote, and the offspring produced are genetically different from each other. Everything in this lesson hangs off that sentence.
A gamete is a sex cell — a sperm or an egg cell in animals, a pollen nucleus or an egg cell in plants. Write "sex cell" and you will usually be credited; write "reproductive cell" or just "cell" and you may not be. Gamete is the word the mark scheme is written in, so use it.
A zygote is the single cell formed when two gamete nuclei fuse. It is not an embryo yet, and it is not a gamete any more. If a question asks what is formed at fertilisation, the one-word answer is zygote — "a fertilised egg" is a description, not the term.
Now the word that costs the most marks: nuclei. It is the nuclei of the two gametes that fuse, not simply the two cells bumping into each other.
Why insist on it? Because the whole point of the process is combining genetic material, and the genetic material is in the nucleus. "The sperm joins the egg" describes the outside of the event; "the nucleus of the sperm fuses with the nucleus of the egg cell" describes what actually matters, and only the second one reliably scores.
So a full-marks description has three beats: two gametes, their nuclei fuse, a zygote is formed. Then add the outcome — the offspring are genetically different from each other.

Part 2 of 6

Fertilisation is the fusion itself — not the journey to it

Fertilisation is the fusion of the nuclei of gametes. That is the entire definition, and it is worth learning as a single short sentence because examiners mark it as one.
Notice how narrow it is. Fertilisation is one event: the moment two gamete nuclei become one nucleus. It is not the transport that got them together, and it is not what happens afterwards.
This is where pollination gets confused with fertilisation, and it is the single most expensive mix-up in the whole reproduction topic. Pollination is the transfer of pollen grains from an anther to a stigma. It is a delivery step. Nothing has fused, and no new cell exists yet.
Fertilisation in a plant happens later and somewhere else: the pollen nucleus travels down the pollen tube into the ovule, and there it fuses with the egg cell nucleus. Pollination can happen and fertilisation still fail — if the pollen tube does not grow, there is no fertilisation and no seed.
Test yourself with the wording: if a sentence uses the words "transfer", "carried", "anther" or "stigma", it is describing pollination. If it uses "fuse", "nuclei" or "zygote", it is describing fertilisation.
One more thing fertilisation is not: it is not mating, and in humans it is not implantation. Mating brings gametes close; implantation is the embryo settling into the uterus lining afterwards. Only the fusion of the nuclei is fertilisation.
Watch the spelling too. Fertilisation is the fusion of gamete nuclei. A fertiliser is a substance supplying mineral ions to a plant. They are unrelated, and writing one when you mean the other is not treated as a slip, because the two words are not confusable versions of the same term.

Part 3 of 6

Where the genetic difference comes from

The definition claims the offspring are genetically different from each other. You should be able to say why, because that reason is a mark in its own right.
First: the gametes themselves are not all alike. Gametes are made by meiosis, which produces genetically different cells, so every sperm and every egg cell carries a different combination of alleles.
Second: the two gametes usually come from two different parents, so the zygote receives genetic material from two different individuals rather than a copy of one.
Third: which gamete fuses with which is a matter of chance. Any one of a huge number of sperm could reach the egg cell, and each pairing would give a different combination.
Put those together and each zygote gets its own unique combination of alleles. That is why the offspring of sexual reproduction differ from each other — and why brothers and sisters are not identical.
Contrast this with 16.1 and the two definitions lock into place. One parent, mitosis, genetically identical offspring — asexual. Two gamete nuclei fusing, genetically different offspring — sexual. If you can say which of those two facts a stem describes, you can answer almost any identification question in this topic.
The awkward case, and it is examined: a plant that pollinates its own flowers has one parent, yet gametes are still produced and their nuclei still fuse. That is sexual reproduction. The number of parents does not decide it — the fusion of gamete nuclei does.

Part 4 of 6

Haploid and diploid: why the numbers have to work

A haploid nucleus contains a single set of chromosomes. A diploid nucleus contains two sets. That is the definition — sets, not a vague "half" and "double".
The statement you are examined on is short: the nuclei of gametes are haploid, and the nucleus of a zygote is diploid.
Attach each word to the right structure, because swapping them is an instant zero. Gamete nucleus → haploid. Zygote nucleus → diploid. Body cells of the organism are diploid too, since they all descend from that zygote.
Now see why it must be this way. Fertilisation adds two gamete nuclei together. One set of chromosomes plus one set of chromosomes gives two sets.
$n + n \to 2 n$
Read that as haploid plus haploid gives diploid. If you can write that line, you can answer the ploidy questions even when you cannot remember which word is which — the zygote is the one made of two, so it is the diploid one.
And here is the reason it matters. If gametes were diploid, fertilisation would give a zygote with four sets of chromosomes, the next generation eight, and so on — the chromosome number would double every generation.
So the halving that produces haploid gametes is what keeps the chromosome number of the species constant from one generation to the next. That sentence is the answer to "explain why gametes must be haploid", which is a standard Extended question.
The arithmetic questions are easy marks if you keep track of which cell you are being asked about. If a body cell of a species contains $24$ chromosomes, that is the diploid number, so a gamete nucleus contains $12$ and a zygote nucleus contains $24$ again.
$gamete = \frac{24}{2} = 12 zygote = 12 + 12 = 24$
You are never expected to recall the chromosome number of a particular species. The stem always gives you the number it wants you to work from, so read it from the question rather than from memory.

Part 5 of 6

In the wild: variation is the payment and the prize

This objective asks you to weigh both sides, so build the answer around one central fact and take it in both directions: sexual reproduction produces offspring that are genetically different.
Advantage — variation lets the population survive change. Because the offspring differ genetically, when the environment changes or a new disease arrives, some individuals may happen to have alleles that let them survive.
Push that one step further, because the step is the mark: those survivors reproduce and pass on the useful alleles, so the population is not wiped out and can become better adapted over time. That is natural selection, and it needs variation as its raw material.
Advantage — dispersal, in plants. Sexual reproduction in flowering plants produces seeds, and seeds can be dispersed away from the parent, so the offspring compete less with the parent for light, water and mineral ions, and the species can colonise new places.
Disadvantage — you need a mate. Two parents are normally required, so an individual must find one. That takes time and energy, and in a small or scattered population it may not happen at all.
Disadvantage — it is slow and expensive. Energy goes into producing gametes, attracting a mate, growing flowers and nectar. Fewer offspring are produced than by asexual reproduction, and the population grows more slowly.
Disadvantage — fertilisation can simply fail. Pollination may not occur, or the gametes may not meet, and then no offspring are produced at all.
Disadvantage — a good combination can be broken up. A parent that is well suited to the habitat does not pass on its exact genotype. Its offspring get a reshuffled combination, and some of them will be less well suited than the parent was.
That last point is the one almost nobody writes, and it is the honest counterweight to "variation is always good". In a habitat that is not changing, being genetically different from a successful parent is a risk, not a benefit.

Part 6 of 6

In a field: the same variation, now a nuisance and a resource

Part (b) points the same biology at a farmer's decision. Nothing new to learn — the same fact, judged against a different aim.
Advantage — new varieties become possible. Because the offspring vary, two different varieties can be crossed and their desirable features combined, for example high yield from one parent with disease resistance from the other. Asexual reproduction can never produce anything new.
Advantage — the crop is not all vulnerable at once. The plants are genetically different, so if a disease or pest arrives, some plants may be resistant and survive. The whole crop is far less likely to be destroyed.
Advantage — seeds are practical. Sexual reproduction gives seeds, which are easy to store, transport and plant in large numbers.
Disadvantage — the crop is not uniform. The plants differ genetically, so they differ in height, yield, size of fruit and ripening time. That makes harvesting harder and the produce less consistent to sell.
Disadvantage — you cannot guarantee the good parent's features. A grower with an excellent plant cannot be sure its offspring will inherit what made it excellent, because each seed gets a different combination of alleles.
Disadvantage — it is slower and less certain. The crop must be grown from seed, and it depends on pollination and fertilisation actually happening, either of which can fail.
How to write these answers: state the genetic fact, then the practical consequence for the farmer. "The plants are genetically different" is half a mark's worth; "the plants are genetically different, so they ripen at different times and cannot all be harvested at once" is the whole point.

Worked examples

Example 1. A student writes: "Fertilisation is when pollen lands on the stigma of a flower." Explain what is wrong with this statement and give the correct definition of fertilisation.

Show the working

Name what the student has actually described, rather than just saying they are wrong. Pollen being transferred to a stigma is pollination.
Say why that cannot be fertilisation: at that moment nothing has fused. Two nuclei are still in separate cells, and no new cell has been formed.
Give the definition in its exact, short form: fertilisation is the fusion of the nuclei of gametes.
Then place it in the flower to show you understand the sequence — the pollen nucleus travels down the pollen tube to the ovule, and fuses with the egg cell nucleus there.
Finish with the evidence that they are genuinely different events: pollination can happen and fertilisation still fail, if the pollen tube does not reach the ovule.

Answer: The student has described pollination, which is the transfer of pollen grains from the anther to the stigma. No nuclei have fused at that point. Fertilisation is the fusion of the nuclei of gametes — in a plant, the pollen nucleus fuses with the egg cell nucleus inside the ovule, forming a zygote.

Example 2. A body cell taken from the leaf of a plant contains $18$ chromosomes. Determine the number of chromosomes in the nucleus of a gamete of this plant and in the nucleus of a zygote of this plant, and state the term describing each.

Show the working

Work out which number you have been given. It came from a body cell, and body cells are diploid, so $18$ is the diploid number — two sets of chromosomes.
A gamete nucleus is haploid: one set. Halve the diploid number.
$gamete = \frac{18}{2} = 9$
A zygote is formed when two gamete nuclei fuse, so add the two haploid numbers back together.
$zygote = 9 + 9 = 18$
Check the result makes sense: the zygote has the same number as the parent's body cells, which is exactly what must happen if the chromosome number of the species is to stay constant.
Attach the terms carefully — haploid to the gamete nucleus, diploid to the zygote nucleus. Swapping them loses both marks even though the numbers are right.

Answer: A gamete nucleus contains $9$ chromosomes and is haploid. A zygote nucleus contains $18$ chromosomes and is diploid.

Example 3. Two farmers grow tomatoes. Farmer P grows plants from seeds produced by sexual reproduction. Farmer Q grows plants from cuttings of one parent plant. Compare the risk to each farmer if a new tomato disease reaches the area, and suggest one disadvantage of farmer P's method.

Show the working

Start from the genetics of each crop, because every consequence follows from it. Farmer P's plants came from seeds, so they are genetically different from each other. Farmer Q's plants came from one parent asexually, so they are genetically identical.
Take farmer Q first, since the outcome is starker. All the plants are genetically identical, so all are equally susceptible; if the disease can infect one, it can infect every plant, and the whole crop could be lost.
Now farmer P. The plants vary genetically, so some may carry alleles that make them resistant. Those plants survive, so the whole crop is unlikely to be destroyed.
Now find the cost of that same variation, because the question asks for a disadvantage of the varied crop.
Because the plants differ genetically they differ in yield, size and ripening time, so the crop is not uniform, cannot all be harvested at once, and its quality is less predictable to sell.

Answer: Farmer Q's plants are genetically identical, so all are equally susceptible and the entire crop could be destroyed by the disease. Farmer P's plants are genetically different, so some may be resistant and survive, and the crop is unlikely to be lost completely. A disadvantage for farmer P is that the varied plants do not grow to the same size or ripen at the same time, so the crop is not uniform and cannot be harvested in one operation.

Where people go wrong

Mistake: Using pollination and fertilisation as if they meant the same thing.

Instead: Pollination is the transfer of pollen grains from an anther to a stigma. Fertilisation is the fusion of the nuclei of gametes, which happens later, inside the ovule.

They are separate events in separate places, and pollination can happen without fertilisation following. A question that asks for fertilisation and receives a description of pollination scores nothing, however well written.

Mistake: Writing that "the two gametes fuse" or "the sperm joins the egg" instead of naming the nuclei.

Instead: Say that the nucleus of one gamete fuses with the nucleus of the other.

The genetic material is in the nucleus, and combining genetic material is the entire purpose of the process. Both objectives in this subtopic contain the word 'nuclei', which tells you the examiner is looking for it.

Mistake: Attaching haploid and diploid to the wrong cells — or defining haploid as 'half the chromosomes' rather than 'one set'.

Instead: A gamete nucleus is haploid, meaning it contains a single set of chromosomes. A zygote nucleus is diploid, meaning it contains two sets.

'Half' only makes sense if you already know what it is half of, and it invites the error of describing a zygote as haploid because it is a single cell. Count sets, not cells: two haploid nuclei fuse to give one diploid nucleus.

Mistake: Deciding that reproduction must be asexual whenever only one parent organism is involved, so calling self-pollination asexual.

Instead: Self-pollination is sexual reproduction. Gametes are produced and the nuclei of two gametes fuse, so the offspring are genetically different.

The number of parents is not the test. The fusion of gamete nuclei is what defines sexual reproduction, and it happens whether the pollen came from the same plant or a different one.

Mistake: Assuming sexual reproduction is simply the better option, and writing only its advantages when asked to discuss it.

Instead: It is a trade-off. It needs a mate, it is slower, it produces fewer offspring, fertilisation may fail, and a well-adapted parent's combination of alleles is broken up in its offspring.

The objective asks for advantages and disadvantages of sexual reproduction, and the marks are split between them. In an unchanging habitat, or in a field where a farmer wants a uniform crop, the variation it produces is a genuine drawback.

Check yourself

Describe sexual reproduction.
Answer
A process involving the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other.
Define fertilisation, in one sentence.
Answer
The fusion of the nuclei of gametes.
State two differences between pollination and fertilisation.
Answer
Pollination is the transfer of pollen grains from an anther to a stigma, whereas fertilisation is the fusion of gamete nuclei. Pollination happens on the outside of the flower and comes first; fertilisation happens inside the ovule and produces a zygote.
Explain why the offspring produced by sexual reproduction are genetically different from each other.
Answer
The gametes are produced by meiosis and are genetically different from one another, they come from two different parents, and which gamete fuses with which is random, so every zygote receives a different combination of alleles.
State whether each of these is haploid or diploid: a sperm nucleus, an egg cell nucleus, a zygote nucleus.
Answer
The sperm nucleus and the egg cell nucleus are haploid; the zygote nucleus is diploid.
A body cell of an insect contains $12$ chromosomes. How many chromosomes are in the nucleus of one of its gametes, and in a zygote of this species?
Answer
A gamete nucleus contains $6$ chromosomes; a zygote nucleus contains $12$ .
Explain why the nuclei of gametes must be haploid.
Answer
At fertilisation two gamete nuclei fuse, so their chromosome numbers add together. If the gametes were diploid, the chromosome number would double in every generation. Haploid gametes keep the chromosome number of the species constant from generation to generation.
Give one advantage and one disadvantage of sexual reproduction to a wild population of a species.
Answer
Advantage: the offspring are genetically different, so if the environment changes or a new disease appears, some individuals may survive and reproduce, allowing the population to adapt. Disadvantage: two parents are normally needed, so a mate has to be found, which takes time and energy and may fail in a small population.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A biologist is studying reproduction in a species of flowering plant.

(a) Define fertilisation. 1 mark No calculator

Mark scheme

B1 the fusion of the nuclei of (two) gametes (the word nuclei is required)

(b) Describe sexual reproduction in this plant. 3 marks No calculator

Mark scheme

B1 the nuclei of two gametes fuse
B1 a zygote is formed
B1 the offspring produced are genetically different from each other

(c) One flower of this plant is pollinated using pollen from its own anthers, and seeds are produced. Explain why this is still described as sexual reproduction, even though only one plant is involved. 2 marks No calculator

Mark scheme

B1 gametes are still produced and the nuclei of two gametes fuse to form a zygote
B1 so the offspring are genetically different from each other and from the parent; the number of parents does not determine the type of reproduction, owtte

Question 2. A body cell taken from a species of fish contains 44 chromosomes.

(a) Determine the number of chromosomes in the nucleus of a gamete of this fish, and in the nucleus of a zygote of this fish. 2 marks

Mark scheme

B1 gamete nucleus 22
B1 zygote nucleus 44

(b) State the term used to describe the number of sets of chromosomes in the nucleus of a gamete, and the term used for the nucleus of a zygote. 2 marks No calculator

Mark scheme

B1 gamete nucleus is haploid / contains a single set of chromosomes
B1 zygote nucleus is diploid / contains two sets of chromosomes

(c) Explain why the nuclei of gametes must be haploid. 2 marks No calculator

Mark scheme

B1 at fertilisation the nuclei of two gametes fuse, so the chromosome numbers add together
B1 if gametes were diploid the chromosome number would double each generation; haploid gametes keep the chromosome number of the species constant, ORA

Question 3. A wild grass species reproduces sexually, producing seeds that are dispersed by the wind. A farmer grows a related grass as a cereal crop, also from seed.

(a) A new fungal disease spreads through the area. Explain why the wild grass population is likely to survive. 3 marks No calculator

Mark scheme

B1 the plants are genetically different from each other / there is genetic variation in the population
B1 some plants may have alleles that make them resistant to the disease, so they survive
B1 the survivors reproduce and pass on the alleles for resistance, so the population is not wiped out and becomes better adapted

(b) Suggest two disadvantages of sexual reproduction to the wild grass population. 2 marks No calculator

Mark scheme

B1 two parents are normally needed, so pollen must reach another plant, which may fail / fertilisation may not occur
B1 fewer offspring are produced and reproduction is slower, or energy is used in producing gametes and flowers (accept: a well-adapted parent's combination of alleles is broken up)

(c) The farmer could instead grow a crop produced asexually from one parent plant. Compare the two methods in terms of the uniformity of the crop. 3 marks No calculator

Mark scheme

B1 the crop grown from seed is genetically different / varied, so plants differ in height, yield or ripening time
B1 the asexually produced crop is genetically identical, so the plants are uniform and ripen at the same time
B1 so the asexually produced crop can be harvested in one operation and its quality is more predictable, ORA

How this is examined

Weighing up variation — why it saves a wild population and why it annoys a farmer: a four-mark Suggest or Explain, usually split into two halves: advantages to a population in the wild, then advantages or disadvantages to crop production Every point must pair the genetic fact with its consequence. In the wild: 'the offspring are genetically different, so if a new disease arrives some may be resistant and survive, so the population is not wiped out'. For crops: 'the plants are genetically different, so they do not all ripen at the same time and the crop is not uniform'. A bare list of the words 'variation' and 'not uniform' with no consequence attached takes about half the marks.
Describing sexual reproduction in full: a three-mark Describe, often the opening part of a longer question that then moves into plant or human reproduction Three separate beats: the nuclei of two gametes fuse; a zygote is formed; the offspring are genetically different from each other. Losing the word 'nuclei' costs the first mark, and stopping at 'a zygote is formed' without the genetic outcome costs the last one.
Haploid and diploid, and the chromosome arithmetic that goes with them: a two- or three-mark State or Determine, where the stem gives you the chromosome number of a body cell and asks for the number in a gamete and in a zygote The number given is the diploid number, so halve it for the gamete and add the two halves back for the zygote; then attach the terms the right way round — gamete nucleus haploid, zygote nucleus diploid. If asked why gametes must be haploid: 'because two gamete nuclei fuse at fertilisation, so if they were diploid the chromosome number would double each generation; haploid gametes keep the chromosome number constant'.
Fertilisation defined precisely, and kept separate from pollination: a one- or two-mark Define or State, or a distractor built into a longer plant-reproduction question where a description of pollination is the tempting wrong answer 'The fusion of the nuclei of gametes' — the word 'nuclei' is the mark. If the question contrasts the two, add that pollination is the transfer of pollen grains from the anther to the stigma and happens first, whereas fertilisation happens inside the ovule.
Why sexually produced offspring differ from each other: a two-mark Explain, frequently placed straight after a question about asexual reproduction so that the contrast is being tested Two of these three: the gametes are genetically different from one another (produced by meiosis); the genetic material comes from two different parents; which gametes fuse is random. 'Because they are made sexually' restates the question and scores nothing.
Self-pollination is still sexual reproduction: a one- or two-mark Explain, in a stem built to look asexual because only one plant is involved Gametes are produced and the nuclei of two gametes fuse to form a zygote, so it is sexual reproduction and the offspring are genetically different. Saying 'one parent, so asexual' is the trap the question is set for.

Words to know

Sexual reproduction: A process involving the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other.
Gamete: A sex cell, such as a sperm or egg cell in animals or a pollen nucleus or egg cell in plants. Its nucleus is haploid.
Fertilisation: The fusion of the nuclei of gametes.
Zygote: The single cell formed when the nuclei of two gametes fuse. Its nucleus is diploid.
Haploid: Describes a nucleus containing a single set of chromosomes. The nuclei of gametes are haploid.
Diploid: Describes a nucleus containing two sets of chromosomes. The nucleus of a zygote is diploid, as are the nuclei of body cells.
Pollination: The transfer of pollen grains from the anther to the stigma. It is not fertilisation — no nuclei have fused at that stage.

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