Sexual & Asexual Reproduction — Print · Grade 7 Science
Chapter: Coordination, Defense & ReproductionSexual & Asexual Reproduction
Learning goals
- I can explain sexual & asexual reproduction.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
A strawberry plant does both. It sends out runners that root and grow into new plants genetically identical to the parent, and it also produces flowers that make seeds. Same plant, two strategies, and the reason it keeps both tells you most of what this unit is about. Reproduction is the production of new individuals, and the two mechanisms differ in one respect that changes everything downstream: whether sex cells from two parents combine.
Asexual reproduction produces offspring from one parent without gametes fusing. A bacterium divides in two, a strawberry sends a runner, a potato grows from an eye, a hydra buds. The offspring carry the same genetic information as the parent, so they are copies, and the advantages follow directly: it is fast, it needs no partner, and a well-adapted parent produces equally well-adapted offspring. The cost is uniformity. A population of copies meets a new disease or a changed climate with the same resources, and what defeats one defeats all.
Sexual reproduction involves fertilisation — the fusion of two gametes, one from each parent — producing a zygote that develops into a new individual carrying a combination of genetic information from both. It is slower, usually requires finding a partner, and produces offspring that differ from their parents and from each other. That variation is the whole point: it is raw material, and a varied population is far more likely to contain individuals that survive a change. The strawberry hedges, using runners when conditions are good and seeds when a new generation may need to be different.
Build your understanding
Where this fits
Defence keeps an individual going. No individual lasts indefinitely, so a species continues by producing new individuals. This unit compares the two ways that happens and what each one does to the variation in a population — which is the idea the heredity chapter formalises and the evolution chapter depends on.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Comparing mechanisms, not just labels
The dividing line is gamete fusion, and nothing else. Not whether there are two parents in some social sense, not whether the offspring are attached to the parent, and not whether the process looks complicated. A bacterium dividing, a strawberry runner, a cutting rooted in soil and a potato sprouting from an eye are all asexual, because in each case the new individual's genetic information comes from one parent without fusion.
The consequence of fusion is a new combination. Each gamete carries half the parent's genetic information, and which half varies, so the combination in a zygote has not existed before. That is why siblings differ, and why sexual reproduction generates variation reliably rather than occasionally.
The practical comparison is a trade. Asexual: fast, no partner needed, offspring as well-adapted as the parent, and no variation. Sexual: slower, usually needs a partner, offspring unpredictable, and variation produced every generation. Neither is better in general. Which one pays depends on whether the environment is staying as it is or changing, which is why many organisms, like the strawberry, keep both.
Pause and explain
A gardener takes a cutting from a plant and roots it. Is that sexual or asexual reproduction?
Check your reasoning after answering
Asexual. No gametes fused: the new plant's genetic information comes entirely from the one parent, so it is a genetic copy. The dividing line is fusion, not whether the process involves a person, whether the offspring is separated from the parent, or how complicated it looks.
Part 2 · Fertilisation is one event; development is what follows
Fertilisation is the moment two gametes fuse to form a zygote — a single cell with a combination of genetic information from both parents. Everything after that is development: the zygote divides, the resulting cells specialise, and a multicellular organism is built. The two are separate stages and compressing them causes real confusion, because a zygote is not a small version of the organism; it is one cell from which the organism will be constructed.
Where fertilisation happens varies. Many fish and amphibians release gametes into water and fusion occurs outside the body, which allows enormous numbers and leaves the offspring unprotected. Other animals fertilise internally, which protects the process and limits the numbers. Flowering plants have their own arrangement, which the next unit covers.
Development varies just as much. Some offspring hatch or are born able to feed themselves; others need extended care. There is a consistent pattern in the trade: many offspring with little investment each, or few offspring with much investment each. A cod releases millions of eggs and cares for none; an albatross raises one chick over months. Both strategies persist because both can work.
Pause and explain
Why is a zygote not simply a very small organism?
Check your reasoning after answering
Because it is a single cell that has not yet developed. Fertilisation produces a zygote with a combination of genetic information from both parents; development is the separate process in which that cell divides repeatedly and the resulting cells specialise into the tissues and organs of a multicellular organism. The zygote carries the instructions, not the structures.
Part 3 · Structures, behaviour, and what counts as success
Producing offspring is not the same as reproducing successfully. A cod releasing millions of eggs of which two reach adulthood has achieved the same reproductive outcome as a bird raising two chicks — the measure that matters is offspring that survive to reproduce themselves, not offspring produced.
That framing explains structures and behaviours that otherwise look wasteful. Parental care, elaborate courtship, nest building, timing a breeding season to a food supply: each is an investment that raises the chance any given offspring survives, and each is worth its cost only if it raises that chance enough. An animal producing few offspring can afford to invest heavily in each; one producing millions cannot.
It also sets up the evolution chapter. Features that increase the number of surviving offspring become more common in a population over generations, and features that do not, do not. Here the point is simply to measure the right thing: count surviving offspring over a life cycle, not eggs laid or seeds produced.
Pause and explain
A cod releases millions of eggs and a bird raises two chicks. Which reproduces more successfully?
Check your reasoning after answering
You cannot tell from those figures, because success is measured in offspring that survive to reproduce, not offspring produced. If two cod and two chicks reach adulthood, the outcome is the same. The difference is strategy: many offspring with no investment each, or few with heavy investment, and both persist because both can work.
Part 4 · Studying reproduction as biology
This topic is studied the way every other topic in this course is studied: through mechanisms, evidence and models. The questions are what happens, in what order, in which organisms, and what the evidence is. The organisms used as examples here — strawberries, bacteria, cod, hydra — are chosen because their mechanisms are clear and varied.
Two habits keep the work scientific. Use the biological terms precisely, since gamete, fertilisation, zygote and development each name a distinct thing and the precision is what makes an explanation checkable. And keep the discussion about organisms and mechanisms rather than about anyone present; no part of this course asks a learner to discuss their own circumstances or family.
If a question about human health or development comes up that goes beyond the biology here, the right answer is that it needs a qualified professional rather than a textbook. That boundary is the same one the defence unit drew, and it applies for the same reason: explaining a mechanism is not the same as advising a person.
Pause and explain
Why do gamete, zygote and development need to be kept as separate terms?
Check your reasoning after answering
Because each names a distinct thing, and an explanation is only checkable if they are distinguished. A gamete is a sex cell carrying half a parent's genetic information; fertilisation is the fusion of two gametes; a zygote is the single cell that results; development is the separate process by which that cell divides and specialises into a multicellular organism.
Apply the whole unit · Compare offspring records
Illustrative organisms, not real specimens: organism A produces new individuals from parent tissue with no gamete fusion. Organism B begins with the fusion of two gametes. One of A's offspring later develops a new mutation.
Your task
Classify each mechanism, explain how A's offspring can nevertheless come to vary, and distinguish producing offspring from reproducing successfully over a life cycle.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Classify A: new individuals from parent tissue with no fusion is asexual reproduction, so the offspring begin as genetic copies of the parent.
Classify B: fusion of two gametes is sexual reproduction, producing a zygote with a combination of information from both parents.
Explain A's variation: a mutation is a change in genetic information arising independently of reproduction, so copies can come to differ even with no fusion involved.
Separate production from success: both counts say how many individuals were produced, not how many survived to reproduce themselves.
A reproduces asexually: new individuals come from parent tissue with no gametes fusing, so each begins with the same genetic information as its parent. B reproduces sexually: two gametes fuse at fertilisation to form a zygote carrying a combination of information from both parents, which is why B's offspring differ from each other and from either parent. The mutation in A's offspring is the interesting part, because it shows that asexual reproduction producing copies does not mean a population of copies forever. A mutation is a change in the genetic information itself, arising independently of how reproduction happened, and once it occurs it is passed to that individual's own offspring. So asexual populations do vary — they simply acquire variation slowly, one change at a time, instead of generating new combinations every generation the way sexual reproduction does. On the second question, neither record says anything about success. Producing offspring and reproducing successfully are different measures: what counts over a life cycle is how many offspring survive to reproduce themselves. A cod releasing millions of eggs and a bird raising two chicks can achieve the same outcome, and the records here give no survival data at all. To compare A and B you would need the number of offspring reaching reproductive age, across a stated period and set of conditions — and the answer would likely differ between a stable environment, which favours copies of a well-adapted parent, and a changing one, which favours variation.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Reproduction produces new organisms through different biological processes. In many asexual processes, one parent contributes the inherited material without fusion of gametes. In sexual reproduction, gametes combine through fertilization. Offspring inherit material from contributing parents, but sexual reproduction does not mean offspring receive an equal visual blend of every characteristic.
Worked examples
Example 1 · Explain a hedged strategy
- Identify what runners give: fast, partner-free production of offspring genetically identical to a parent already doing well in this spot.
- Identify what seeds give: offspring with new combinations of genetic information, which can differ from the parent and from each other.
- Match each to a condition: copies pay when the environment stays as it is; variation pays when it changes or when offspring must colonise somewhere different.
Ask which mechanism pays under which conditions rather than which is better.
Example 2 · Trace variation in an asexual population
- Check the mechanism: division without fusion copies the genetic information, so offspring start as copies.
- Identify the other source of change: a mutation alters the genetic information itself and is independent of how reproduction occurs.
- Follow the consequence: once a mutation arises, it is inherited by that cell's descendants, so lineages within the population diverge.
Asexual means no new combinations, not no change.
Avoid these mistakes
- Mistake: asexual reproduction means one parent in a social sense. The dividing line is gamete fusion — a rooted cutting and a potato eye are asexual because no fusion occurred.
- Mistake: asexual populations never vary. Reproduction generates no variation, but mutations arise independently and are inherited, so lineages diverge over time.
- Mistake: a zygote is a tiny organism. It is one cell carrying the combined instructions; development is the separate process that builds the organism from it.
- Mistake: more offspring means more successful. Success is offspring surviving to reproduce; a cod with millions of eggs and a bird with two chicks can end up equal.
Check your understanding
Does asexual reproduction forbid all variation?
Discuss the answer
No. Mutations and environmental effects can produce differences, even when offspring share much inherited material.
Is fertilization the same event as every later cell division?
Discuss the answer
No. Fertilization combines gametes; later divisions and development produce and organize cells.
Words to know
- reproduction
- The production of new individuals; success is measured in offspring that survive to reproduce, not offspring produced.
- asexual reproduction
- Production of offspring from one parent with no gamete fusion, giving genetic copies; fast and partner-free, and it generates no variation.
- sexual reproduction
- Production of offspring by the fusion of gametes from two parents, giving new combinations and therefore variation every generation.
- fertilization
- The fusion of two gametes, one event at one moment, which is separate from the development that follows.
- zygote
- The single cell formed at fertilisation, carrying a combination of both parents' genetic information; not a miniature organism but the cell one is built from.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Can a reproductive diagram establish personal family information?
Check your explanation after answering
No. It describes a general process and should not be used to infer or demand a learner’s private history.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
Answer and explanation
No. Mutations and environmental effects can produce differences, even when offspring share much inherited material.
Problem 2
Answer and explanation
No. Fertilization combines gametes; later divisions and development produce and organize cells.
Problem 3
Answer and explanation
Some eggs may not be fertilized or survive development, and later survival and reproduction depend on conditions.
Problem 4
Answer and explanation
No. It describes a general process and should not be used to infer or demand a learner’s private history.
Problem 5
Answer and explanation
A represents asexual reproduction and B sexual reproduction. A’s new mutation shows that asexual offspring are not guaranteed identical in every detail. The records describe production mechanisms, but later survival and reproductive success depend on development and conditions. More information is needed to compare outcomes, not merely count starting cells.
Identify no fusion in A and fusion in B.
Connect A’s new mutation with variation.
Consider later survival and reproductive stages.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 6
Answer and explanation
No. They can receive different combinations of inherited material and experience different environments. Sexual inheritance is not an exact visual blend.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 7
- Gamete fusion
- Every offspring is identical
- No inherited material
- Only one possible environment
Answer and explanation
Gamete fusion
Sexual reproduction involves gamete fusion; later development and conditions also affect outcomes.
Problem 8
Answer and explanation
60 %
30/50×100=60%; this does not give later survival.