Explain homologous chromosome pairs
Describe how homologous chromosomes carry the same genes at corresponding loci but may have different alleles.
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
What can differ between homologous chromosomes at the same locus?
What you will learn
- Explain homologous pairs as chromosomes with the same genes at corresponding loci
- Relate maternal and paternal homologues to possible allele differences
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Usual diploid precursor | 46 individual chromosomes; 23 pairs |
| Normal reduction | 46 ÷ 2 = 23 per gamete nucleus |
| Single gamete | 23 individual chromosomes; one set |
| Four-nucleus model | 92 across four boundaries; still 23 in each |
Build the genetic explanation
A diploid nucleus contains homologous pairs. The two chromosomes in a pair have the same genes at corresponding loci and similar structure, with one homologue inherited from each parent. Their alleles may differ, so a cell can carry two different alleles of a gene. Homologues pair in meiosis I and then separate into different nuclei.
Check the evidence and limits
Homologous chromosomes are not identical copies. Sister chromatids are replicated copies of one chromosome; homologues are the maternal and paternal versions of the same chromosome type.
Pause and explain
A diagram shows a maternal and paternal chromosome with the same gene positions but a different allele at one locus. What can be inferred?
Worked example
Two chromosomes carry gene G at the same locus but have alleles G and g. What is their relationship?
Show the worked solution
- The gene is at corresponding loci.
- The alleles differ between the parental homologues.
Answer They are homologous chromosomes carrying different alleles of the same gene at corresponding loci.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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 why two homologues can carry different alleles but still be homologous.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- They have the same genes at corresponding positions.
- Alleles are alternative versions of a gene.
Homologues have the same genes at corresponding loci and similar chromosome structure, but the maternal and paternal versions can carry different alleles.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A replicated chromosome has two sister chromatids. Are these the pair of homologous chromosomes?
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Sister chromatids come from replication of one chromosome.
- Homologues are the parental pair of the same chromosome type.
No. Sister chromatids are copies of one replicated chromosome. Homologues are the corresponding maternal and paternal chromosomes in a diploid cell.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A diagram shows a maternal and paternal chromosome with the same gene positions but a different allele at one locus. What can be inferred?
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- The chromosomes are a homologous pair.
- The individual is heterozygous at that locus if both alleles are present in the same diploid nucleus.
They are homologous chromosomes. If they occur in the same diploid nucleus, the individual is heterozygous at the locus where the alleles differ.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Calling homologues identical
- Confusing sister chromatids with homologous chromosomes
What can you explain now?
What can differ between homologous chromosomes at the same locus?
Compare with the explanation
The alleles
The gene position is corresponding, while the alleles may differ.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do homologous chromosomes differ from sister chromatids?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.