Explain how meiosis produces different gametes
Relate crossing over and independent assortment to genetic variation in gametes.
Section 16.1 · Passage of information from parents to offspring
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Between which chromatids does crossing over occur?
What you will learn
- Explain crossing over between non-sister chromatids of homologous chromosomes
- Explain how independent assortment of homologous pairs produces different combinations of chromosomes
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Two type choices | AA, AB, BA, BB: four possibilities |
| Three independent choices | 2 × 2 × 2 = eight possibilities |
| Complement remains | n = number of chromosome types per complete product |
| Ten-record sample | 4, 2, 1, 3 counts give 40%, 20%, 10%, 30% |
Build the genetic explanation
During prophase I, homologous chromosomes pair and non-sister chromatids can exchange corresponding segments by crossing over. This creates recombinant chromatids with new allele combinations. At metaphase I, each homologous pair can orient independently of the others; separation at anaphase I distributes maternal and paternal homologues into different combinations. The combination of crossing over and independent assortment produces genetically varied haploid gametes.
Check the evidence and limits
Crossing over occurs between non-sister chromatids of homologues, not between unrelated chromosomes. Independent assortment changes which whole homologues enter a nucleus; it is distinct from exchange of segments.
Pause and explain
A student says independent assortment changes the DNA sequence of each chromosome. Correct the statement.
Worked example
How can two gametes from one individual differ at linked loci?
Show the worked solution
- Crossing over can exchange segments between non-sister chromatids.
- Recombinant chromatids can carry new allele combinations.
Answer Crossing over during prophase I can create recombinant chromatids with new combinations of alleles at the loci.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain two ways meiosis produces genetically different gametes.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Crossing over recombines alleles.
- Independent orientation distributes homologues in different combinations.
Crossing over between non-sister chromatids creates recombinant allele combinations. Independent orientation of homologous pairs at metaphase I then distributes maternal and paternal chromosomes into different combinations.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A chromosome pair has linked alleles AB on one homologue and ab on the other. What can crossing over produce?
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- A crossover between non-sister chromatids can exchange a segment.
- The recombinant chromatids can carry Ab and aB.
A crossover between non-sister chromatids can produce recombinant chromatids Ab and aB, in addition to parental combinations AB and ab.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A student says independent assortment changes the DNA sequence of each chromosome. Correct the statement.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- The chromosome sequences need not change.
- Different intact homologues are distributed into gametes.
Independent assortment does not itself change a chromosome DNA sequence. It changes which maternal or paternal homologues are distributed together into each gamete.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Calling independent assortment mutation
- Saying crossing over occurs between sister chromatids
What can you explain now?
Between which chromatids does crossing over occur?
Compare with the explanation
Non-sister chromatids of homologous chromosomes
They exchange corresponding segments during prophase I.
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Make it stick
How do crossing over and independent assortment differ?
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