Meiosis, Gametes & Genetic Variation · Grade 7 Science
Chapter: Genes, Cell Division & HeredityMeiosis, Gametes & Genetic Variation
Learning goals
- I can explain meiosis, gametes & genetic variation.
- 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.
Here is an arithmetic problem the last unit leaves behind. Your cells carry 46 chromosomes. If a sperm with 46 fused with an egg with 46, the result would have 92, and the next generation 184, and so on. That does not happen, so something must halve the number before fusion restores it. That something is meiosis, the division that produces gametes — sex cells carrying half a set.
The vocabulary for the two states is worth fixing. A body cell with chromosomes in pairs is diploid; a gamete with one chromosome from each pair is haploid. In humans, diploid is 46 in 23 pairs and haploid is 23. Meiosis takes a diploid cell and produces four haploid cells, and fertilisation puts two haploid gametes together to restore the diploid number in the zygote. Halve, then combine: the cycle is stable because the two steps are matched.
Meiosis does something beyond halving, and it is the more important half of the story. Which member of each pair ends up in a given gamete is a matter of chance, and the pairs assort independently, so with 23 pairs the number of possible combinations is enormous before anything else happens. On top of that, recombination occurs: paired chromosomes exchange corresponding segments with each other, producing chromosomes that are themselves mixtures of the two originals. Between them, independent assortment and recombination mean that every gamete a parent produces is very likely unique — which is why siblings differ, and why sexual reproduction generates variation reliably rather than occasionally.
Build your understanding
Where this fits
Mitosis keeps the chromosome number constant, which is what growth and repair need. Sexual reproduction has the opposite problem: if two cells with full sets fused, the number would double every generation. This unit is the division that solves it, and it is also where the variation the next two units depend on is actually produced.
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 · Paired sets, and what halving means
Start by picturing the pairs rather than a list. A diploid cell with eight chromosomes has four pairs, and the two members of each pair carry the same genes in the same order, one having come originally from each parent. They may carry different alleles of those genes, which is what makes the choice between them consequential.
Halving means taking one member of each pair, not any four chromosomes at random. A gamete with four chromosomes that happened to include both members of one pair and neither of another would be missing genes entirely and carrying others twice — which would not work. The requirement is one complete set, and the machinery of meiosis is built to deliver exactly that.
So haploid is not simply half the number; it is one full set rather than two. That distinction is why the restored diploid number after fertilisation is a working genome rather than an arbitrary collection: each parent contributed one complete set, and the pairs reform.
Pause and explain
A species has eight chromosomes in a diploid body cell. How many are in a gamete, and why is it not just any four?
Check your reasoning after answering
Four — one from each of the four pairs. It cannot be any four, because a gamete containing both members of one pair and neither of another would carry some genes twice and others not at all. Haploid means one complete set, not merely half the number, which is why fertilisation can restore a working diploid genome.
Part 2 · Two divisions, doing two different jobs
Meiosis is two divisions in succession, and they are not repetitions of each other. DNA is replicated once, at the start, exactly as in mitosis. Then the first division separates the members of each pair, sending one of each pair to each side — this is the division that halves the number. The second division separates sister chromatids, as mitosis does, producing four cells in total.
Getting the jobs the right way round matters. The first division reduces the count from diploid to haploid by separating pairs. The second does not reduce it further; it simply separates the joined copies, so each of the four products has one complete haploid set.
Compare the outcomes side by side. Mitosis: one replication, one division, two diploid cells identical to the parent. Meiosis: one replication, two divisions, four haploid cells that differ from the parent and from one another. Same starting material, different machinery, different purposes — one for growth and repair, one for making gametes.
Pause and explain
What does the first division of meiosis do that the second does not?
Check your reasoning after answering
The first division separates the members of each chromosome pair, sending one of each to each side, which is what reduces the count from diploid to haploid. The second division separates sister chromatids, as mitosis does, and does not reduce the number further — it takes two haploid cells and makes four.
Part 3 · Where the variation actually comes from
Two mechanisms generate it, and both act during meiosis. The first is independent assortment. When pairs line up before the first division, which member of each pair goes to which side is independent of what the other pairs do. With four pairs that gives two to the power of four, which is sixteen possible combinations; with the human 23 pairs it is over eight million, before anything else is considered.
The second is recombination, and it adds far more. Before the first division the members of a pair come together and exchange corresponding segments, so a chromosome in a gamete can be part one parental chromosome and part the other. That means the combinations are not limited to the whole chromosomes a parent received; new mixtures are created within chromosomes as well.
Together these make it effectively certain that no two gametes from the same individual are identical, and that no two offspring of the same parents are either, barring identical twins who come from a single zygote. This is the source of the variation that natural selection acts on in the last chapter, and it is produced reliably every generation rather than waiting for a mutation.
Pause and explain
Why are two gametes from the same parent almost never identical?
Check your reasoning after answering
Because of independent assortment and recombination. Which member of each pair goes into a gamete is independent for each pair, giving over eight million combinations with 23 pairs. Recombination then exchanges segments between paired chromosomes, so the chromosomes themselves are new mixtures. Together these make identical gametes vanishingly unlikely.
Part 4 · Fertilisation restores the set
Fertilisation does the other half of the arithmetic. Two haploid gametes fuse, their chromosome sets combine, and the zygote is diploid again with the pairs reformed — one member of each pair from each parent. For a species with eight chromosomes in a body cell, gametes carry four and the zygote has eight. The number is restored exactly because halving and combining are matched operations.
The combination is also a second source of variation layered on the first. Not only is each gamete unique, but which two gametes meet is itself a matter of chance, so the number of possible zygotes from one pair of parents is the product of two already enormous numbers.
After fertilisation, every subsequent division in the developing organism is mitosis, which is why all its body cells carry the same genome — the fact the first unit of this chapter started from. Meiosis happens only in making gametes, and the cycle repeats in the next generation.
Pause and explain
For a species with eight chromosomes in a body cell, how many are in a gamete and in a zygote?
Check your reasoning after answering
A gamete has four, one from each of the four pairs, because meiosis halves the diploid number. A zygote has eight, because fertilisation combines two haploid gametes and the pairs reform with one member from each parent. Halving and combining are matched, which is why the number stays constant across generations.
Apply the whole unit · Trace a fictional chromosome cycle
Illustrative species, not a real organism: a diploid body cell contains eight chromosomes, arranged in four pairs. Normal gametes are formed by meiosis, and two of them fuse.
Your task
Give the chromosome number in a gamete and in the resulting zygote, and explain how different gametes from the same parent can carry different combinations.
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
Apply meiosis: four pairs means a gamete receives one chromosome from each pair, so a gamete has four chromosomes.
Apply fertilisation: two haploid gametes fuse, so the zygote has 4 + 4 = 8 chromosomes, with the pairs reformed.
Explain independent assortment: which member of each pair enters a given gamete is independent for each pair, giving 2 to the power of 4 = 16 possible combinations of whole chromosomes.
Add recombination: paired chromosomes exchange corresponding segments beforehand, so the chromosomes in a gamete can be mixtures rather than whole parental ones, raising the number of possibilities far beyond sixteen.
A gamete has four chromosomes and the zygote has eight. Meiosis takes the diploid cell with its four pairs and produces haploid gametes carrying one chromosome from each pair — four in total, which is one complete set rather than any four chromosomes. Fertilisation then fuses two gametes, so the zygote receives 4 + 4 = 8, with the pairs reformed and one member of each pair coming from each parent. Halving and combining are matched operations, which is why the number is stable from generation to generation instead of doubling. Different gametes carry different combinations for two reasons acting together. Independent assortment: when the pairs line up before the first division, which member of each pair goes to which side is decided independently for each pair, so with four pairs there are 2 × 2 × 2 × 2 = 16 possible combinations of whole chromosomes. Recombination: before that division, the members of a pair come together and exchange corresponding segments, so a chromosome entering a gamete may be part one parental chromosome and part the other. That makes the possibilities far greater than sixteen, because the combinations are no longer limited to the whole chromosomes the parent inherited. The model simplifies in ways worth naming: four pairs rather than a real species' number, no account of how often recombination occurs or where, and the assumption that every gamete is equally likely to take part in fertilisation.
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
In the diploid sexual life cycle studied here, body cells have chromosome pairs and gametes carry one member of each pair. Meiosis follows DNA replication and includes two divisions that produce reduced chromosome sets. Fertilization combines two gamete sets, restoring the paired arrangement in the resulting zygote.
Worked examples
Example 1 · Count combinations from the number of pairs
- Identify the choice at each pair: one of two members goes into the gamete, independently of the other pairs.
- Count the independent choices: three pairs, each with two options.
- Multiply: 2 × 2 × 2 = 8 possible combinations.
Independent choices multiply. That is why the number grows so quickly with chromosome count.
Example 2 · Spot the consequence of the wrong division
- State what mitosis produces: cells with the same chromosome number as the parent, so diploid gametes.
- Apply fertilisation: two diploid gametes fusing would give a zygote with twice the diploid number.
- Follow it forward: the number would double every generation, and the variation from independent assortment and recombination would also be absent.
Halving and combining must be matched operations, or the arithmetic runs away.
Avoid these mistakes
- Mistake: haploid means any half of the chromosomes. It means one complete set, one from each pair; a gamete with both members of one pair and neither of another would not work.
- Mistake: both meiotic divisions halve the number. The first separates the pairs and halves it; the second separates sister chromatids and takes two cells to four.
- Mistake: variation comes from mutation alone. Independent assortment and recombination generate it reliably every generation, which is why siblings differ.
- Mistake: meiosis happens whenever cells divide. Every division in a developing organism is mitosis; meiosis occurs only in producing gametes.
Check your understanding
Why is “any random half of the chromosomes” inadequate?
Discuss the answer
A functional gamete in this simplified normal model needs one representative of each type, not an arbitrary subset missing some types.
Why is one mitotic division not a sufficient meiosis model?
Discuss the answer
It normally preserves the paired set rather than producing the reduced gamete set through the studied two-division process.
Words to know
- meiosis
- The division producing gametes: one replication followed by two divisions, giving four haploid cells that differ from the parent and from each other.
- gamete
- A sex cell carrying one complete set of chromosomes, so that fusion restores the diploid number rather than doubling it.
- diploid
- Having chromosomes in pairs — 46 in 23 pairs for humans — with one member of each pair originally from each parent.
- haploid
- Having one chromosome from each pair: one complete set, not merely half the number.
- recombination
- Exchange of corresponding segments between paired chromosomes during meiosis, so a gamete's chromosomes are mixtures rather than whole parental ones.
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
Why is reduction relevant before gamete fusion?
Check your explanation after answering
It permits the paired set to be restored rather than repeatedly doubling the set across generations in this model.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.