What you need to know
- 3.9(i–ii)Loci and linkage of geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. on chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere..
- 3.10independent assortmentThe random way each homologous pair lines up in metaphase I, so gametes get different combinations of chromosomes. in metaphaseThe stage of mitosis when chromosomes line up at the equator, attached to spindle fibres by their centromeres. I and crossing overExchange of sections between non-sister chromatids of homologous chromosomes at chiasmata in prophase I of meiosis. in prophaseThe first stage of mitosis: chromosomes condense and the nuclear envelope breaks down. I.
The named substages of prophase I are not required. Recombination mapping calculations are enrichment, not additional core recall here.
By the end, you should be able to…
- Distinguish homologues from sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere..
- Explain reduction division and two meiotic divisions.
- Predict alleleA different version of the same gene, found at the same locus. combinations from assortment and crossing over.
Build your understanding
Loci, homologues and linkage
A locusThe position of a gene on a chromosome. is the position of a gene on a chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere.. homologous chromosomesA pair of chromosomes, one from each parent, with the same genes at the same loci but possibly different alleles. carry the same geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. at corresponding loci but may carry different alleleA different version of the same gene, found at the same locus.; one homologue in a pair is inherited from each parent. Sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere. are copies produced by replication of one chromosome.
Genes on the same chromosome are linked and tend to be inherited together. Linkage does not mean the alleles must always remain together: crossing overExchange of sections between non-sister chromatids of homologous chromosomes at chiasmata in prophase I of meiosis. between their loci can produce new combinations. Genes on different chromosome pairs can assort independently.
One replication, two divisions
DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. replicates before meiosisTwo nuclear divisions that produce four genetically different haploid nuclei, used to make gametes. I. In prophaseThe first stage of mitosis: chromosomes condense and the nuclear envelope breaks down. I homologous chromosomes pair as bivalents. Corresponding DNA segments can be exchanged between non-sister chromatids of homologous chromosomes at chiasmaThe point where non-sister chromatids of a homologous pair cross over and exchange sections.. This crossing over creates new combinations of existing alleles.
In metaphaseThe stage of mitosis when chromosomes line up at the equator, attached to spindle fibres by their centromeres. I each bivalent aligns with a random orientation relative to the poles, independently of the other bivalents. Homologues separate in anaphaseThe stage of mitosis when centromeres divide and sister chromatids are pulled to opposite poles. I; centromeres do not divide at this stage. Each resulting nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. has one homologue from each pair, so it is haploidHaving one set of chromosomes (n), as in gametes. even though its chromosomes still have two chromatids.
There is no further DNA replication before meiosis II. Chromosomes align individually; sister chromatids separate in anaphase II. A diploidHaving two sets of chromosomes (2n), one from each parent. starting cell typically produces four haploid nuclei with different combinations of alleles. Mammalian egg development partitions cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. unequally, so four equivalent functional eggs are not produced.
Two sources of variation
independent assortmentThe random way each homologous pair lines up in metaphase I, so gametes get different combinations of chromosomes. combines maternal and paternal homologues in different ways. With n chromosome pairs, 2ⁿ combinations of whole homologues are possible across many gameteA haploid sex cell, such as a sperm or an egg cell. before considering crossing over. A single meiosis does not necessarily produce every possible combination.
Two sources of variation (continued)
Crossing over exchanges corresponding segments, not whole unrelated genes at random. For linked alleles AB on one homologue and ab on the other, crossing over between the loci can produce Ab and aB chromatids alongside parental combinations. It recombines existing alleles; mutationA change in the sequence of bases in DNA. is the source of new alleles.
Random fusion of gametes adds variation at fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote.. It is distinct from the meiotic mechanisms named in specification 3.10.
Keep the quantities separate
For a 2n = 8 cell, there are eight replicated chromosomes before meiosis I. Each nucleus after meiosis I contains four chromosomes with eight chromatids. Each final nucleus after meiosis II contains four single-chromatid chromosomes. DNA per nucleus changes from 4C to 2C to 1C; ploidy changes from 2n to n to n.
NOT: meiosis is not two mitoses, and the products are not genetically identical.
- Chromosome number is halved
- Crossing over swaps alleles between homologues
- Pairs can line up in different orientations
- Needed for gametes so fertilisation restores diploidy
Two traps this diagram blocks
- Do not say chromatids separate in meiosis I as the main event. Homologues separate then.
- Do not say meiosis produces two diploid cells.
From the diagram alone
Question
Explain how meiosis produces genetic variation and haploid cells. [4]
Show a model answer
Homologous pairs separate in meiosis I, which halves the chromosome number. Crossing over exchanges alleles between homologues, and the orientation of pairs is independent, so gametes receive different combinations. Meiosis II then separates sister chromatids, giving four haploid nuclei.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Nuclear divisions | One | Two after one DNA replication |
| Homologues pair | No | Yes, in prophase I |
| First separation | Sister chromatids | Homologous chromosomes |
| Final nuclei | Two, retaining ploidy | Usually four, haploid |
| Genetic outcome | Identical barring mutation | Different allele combinations |
Make your reasoning visible
Give the named stage
crossing overExchange of sections between non-sister chromatids of homologous chromosomes at chiasmata in prophase I of meiosis.: prophaseThe first stage of mitosis: chromosomes condense and the nuclear envelope breaks down. I. Independent orientation of homologous pairs: metaphaseThe stage of mitosis when chromosomes line up at the equator, attached to spindle fibres by their centromeres. I.
Describe the correct partners
Crossing over involves non-sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere. of homologous chromosomesA pair of chromosomes, one from each parent, with the same genes at the same loci but possibly different alleles..
Do not equate haploid with unreplicated
After meiosisTwo nuclear divisions that produce four genetically different haploid nuclei, used to make gametes. I a nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. is haploidHaving one set of chromosomes (n), as in gametes. even though each chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. still has two chromatids.
Key explanations
- In meiosis I, homologous chromosomes pair and then separate. Each daughter cell receives one chromosome from each pair.
Avoid this misconception
The claim: Crossing over creates new alleleA different version of the same gene, found at the same locus..
The correction: It normally creates new combinations of existing alleles by exchanging corresponding chromosome segments.
Why are you not identical to your brother or sister?
Your body cells are diploid: they have two sets of chromosomes, one set from your mother and one from your father. To make eggs or sperm, a cell has to halve that number, or every generation would have twice as many chromosomes. Meiosis does this with one copy of the DNA and two divisions. On the way, it shuffles the chromosomes, so almost every gamete is different.
What the model shows
A cell going through meiosis one stage at a time, with each chromosome coloured by the parent it came from. One pair carries two genes, A and B, so you can follow how alleles end up in the gametes.
What to look for
When chromosomes pair up, where they swap pieces, which way each pair faces before it separates, and how the number of chromosomes and chromatids changes.
What it tells us
Meiosis halves the chromosome number and makes four genetically different cells. Two things cause the variety: crossing over in prophase I and independent assortment in metaphase I.
Be careful what you claim
Chromosomes are simplified and the tallies are simulated. Only one pair is shown crossing over.
Try it in the model: press Next stage twice to reach prophase I, then drag the chiasma (×) so it sits between A and B.
Meiosis: halving and shuffling chromosomes
Step a diploid cell through one replication and two divisions. Place a chiasma, flip the pairs at metaphase I, and see why the four gametes differ.
What you see
- The top strip shows which stage you are on. Pink chromosomes came from the mother and blue from the father.
- The longest pair carries two genes: A and B on the mother's chromosome, a and b on the father's. The black dot is the centromere holding sister chromatids together.
- Below: two tallies from many simulated meioses. The left one counts the pair-1 alleles in each gamete; the right one counts which homologue of each pair a gamete received.
How to use it
- Next stage and Back step through meiosis; Play runs it. Choose 2n = 4, 6 or 8.
- In prophase I drag the chiasma (or use the slider) or turn crossing over off. In metaphase I tap a pair, or use Flip, to change which way it faces.
- Run 100 meioses adds random meioses to the tallies. Set A and B close together or far apart.
What it shows
- One DNA replication and two divisions: 4C → 2C → 1C per nucleus, and diploid → haploid after meiosis I.
- Crossing over between non-sister chromatids of homologues in prophase I, which makes recombinant chromatids (Ab, aB).
- Independent assortment of bivalents in metaphase I, giving 2ⁿ whole-chromosome combinations.
Limits of the model
- Chromosomes are drawn condensed in interphase so you can count them; really they are spread out as chromatin. Prophase I substages are not shown (not required).
- Only pair 1 crosses over, always at exactly one chiasma (or none). Real bivalents usually form at least one chiasma each, and sometimes more.
- The tallies are simulated, not real data. Meiosis II is drawn on the same axis as meiosis I to keep the picture simple, and all four products are shown as equal cells (in egg production they are not).
Guided tour · step 1 of 11
- In Stages, just below these steps, press Next stage.
- Watch each chromosome in the simulator become two sister chromatids joined at a centromere.
DNA replicates once, before meiosis I. The number of chromosomes stays the same, but the DNA doubles from 2C to 4C.
Stages
Interphase: before replication.
Flipping works in metaphase I.
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Define a locus and explain genetic linkage.
Show suggested answer
A locus is a gene’s position on a chromosome. Genes on the same chromosome are linked and tend to be transmitted together, although crossing over can separate alleles at different loci.
Practice question
Retrieval and explanation · suggested answer
Ignoring crossing over, how many whole-chromosome combinations are possible with five homologous pairs?
Show suggested answer
2⁵ = 32 across many gametes.
Practice question
Retrieval and explanation · suggested answer
A cell with 12 chromosomes starts meiosis. How many chromosomes are in each nucleus after division I and after division II?
Show suggested answer
Six after each division: division I separates homologues; division II separates sister chromatids.
Practice question
Retrieval and explanation · suggested answer
Explain how AB/ab can yield an Ab gamete.
Show suggested answer
A crossover between the A and B loci in non-sister chromatids exchanges corresponding segments, creating a recombinant Ab chromatid that can enter a gamete.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two loci are very close together on one chromosome. Why might their parental allele combinations be more common than recombinant combinations?
Show a hint
Consider the chance of an exchange occurring between the two positions.
Explore the reasoning
There is less opportunity for a crossover between nearby loci. They tend to remain linked. This is a qualitative prediction, not a requirement to calculate map distance.
Recall this lesson
Lesson recall questions
Question 1 of 5: Explain, in causal order, why linked genes do not show independent assortment.
Locus and linkage
Practice question
Retrieval and explanation · suggested answer
Explain, in causal order, why linked genes do not show independent assortment.
Lesson: 3B 3: Sexual Reproduction and Meiosis
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Breeding begins with chromosome behaviour
3B · Sexual Reproduction and MeiosisThe John Innes Centre studies meiotic recombination in Arabidopsis and wheat. Understanding where chromosome exchanges occur can inform plant breeding. Crossing over creates new combinations between homologous chromatids;…Specification 3.10Read & use the scienceClose article
Why this matters
The John Innes Centre studies meiotic recombination in Arabidopsis and wheat. Understanding where chromosome exchanges occur can inform plant breeding. Crossing over creates new combinations between homologous chromatids; independent assortment changes which whole homologues enter each gamete. Plant breeders cannot choose the exact crossover in every meiosis, but mapping recombination frequencies can show whether useful alleles are strongly linked. The John Innes work is therefore a research connection, not an exam requirement to know a crop variety. In an explanation, identify the chromosome event and the resulting allele combination. Saying that plants simply mix their traits skips the mechanism the specification assesses.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.10 understand the role of meiosis in ensuring genetic variation through the production of non-identical gametes as a consequence of independent assortment of chromosomes in metaphase I and crossing over of alleles between chromatids in prophase I
Use the science
Why might tightly linked alleles make combining desirable traits harder?
Explore a possible response
If desired alleles occur on opposite homologues, a crossover between their loci may be needed to bring them together; close linkage makes this less likely.
Explore the source
- John Innes Centre: meiotic recombination researchJohn Innes Centre
When chromosomes do not separate
3B · Sexual Reproduction and MeiosisMeiosis normally reduces chromosome number and produces genetically varied gametes. The National Human Genome Research Institute explains that nondisjunction can leave a reproductive cell with an abnormal chromosome…Specification 3.10Read & use the scienceClose article
Why this matters
Meiosis normally reduces chromosome number and produces genetically varied gametes. The National Human Genome Research Institute explains that nondisjunction can leave a reproductive cell with an abnormal chromosome number, as in many cases of trisomy 21. This is different from crossing over or independent assortment, which create new allele combinations without normally changing chromosome count. A genetic counsellor would discuss test results sensitively and accurately; one chromosome result does not describe a whole person. The real case helps students explain why correct chromosome separation matters alongside the specification’s focus on variation between gametes.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.10 understand the role of meiosis in ensuring genetic variation through the production of non-identical gametes as a consequence of independent assortment of chromosomes in metaphase I and crossing over of alleles between chromatids in prophase I
Use the science
How does a gamete with an extra chromosome differ from one made variable by crossing over?
Explore a possible response
An extra chromosome changes chromosome number. Crossing over exchanges sections between homologous chromatids and changes allele combinations while normally preserving the haploid chromosome count.
Explore the source
- About Down syndromeNational Human Genome Research Institute