Explain sex linkage through the gene location and supplied chromosome model
The supplied X-linked locus has two copies in XX and one in XY, with no counterpart allele on Y.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes a characteristic sex-linked?
What you will learn
- Locate a sex-linked gene on a sex chromosome
- Explain unequal expression opportunity in a stated X-linked recessive model
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Gene location | On a sex chromosome rather than an autosome |
| XNXn at this locus | Two different copies; heterozygous carrier in the model |
| XnY at this locus | One allele copy; Y has no counterpart |
| Frequency interpretation | Expression opportunity differs; no universal population percentage follows |
Locate a sex-linked gene on a sex chromosome
A sex-linked characteristic is a feature whose responsible gene is located on a sex chromosome. The gene’s location affects its inheritance pattern and can make the characteristic more common in one sex than the other. A feature being culturally associated with one sex is not evidence of sex linkage. The question needs genetic location and an inheritance rule, not a stereotype about behaviour.
In the usual XX/XY teaching model, an XX individual has two X chromosomes and an XY individual has one X and one Y. For the specific X-linked locus followed here, the Y has no corresponding allele. An XY individual therefore has one copy at that locus rather than a pair of alternative alleles. Do not label XNY or XnY homozygous or heterozygous for this one-copy X locus.
Explain unequal expression opportunity in a stated X-linked recessive model
The plain-text labels XN and Xn mean an X chromosome carrying the normal-vision model allele N or the recessive red-green model allele n. They correspond to X with allele superscripts in conventional genetic diagrams. In the supplied rule, N prevents expression of the n-associated feature in an XNXn heterozygote, while XnY lacks an N counterpart and expresses it.
This explains why an X-linked recessive feature can be expressed more often in XY males than XX females at comparable allele frequencies: one n copy is sufficient in the one-X model, whereas the stipulated XX expression requires two. It is not a numerical frequency prediction for every population or a rule for every sex-linked gene. Read whether the gene is X-linked or Y-linked and the specific expression pattern before drawing conclusions.
Pause and explain
Why is XnY not heterozygous at this supplied locus?
Worked example
Compare XnY and XNXn at the supplied X-linked recessive locus.
Show the worked solution
- Count the X-locus alleles: XnY has one n copy and the supplied Y has no corresponding allele. XNXn has N and n copies.
- Apply the recessive model: the N copy prevents expression in XNXn, whereas XnY lacks that counterpart. Do not classify the one-copy XY locus as homozygous or heterozygous.
Answer For the supplied X-linked recessive locus, XnY has one n copy and no N counterpart on Y; XNXn has an N copy under the stated expression rule.
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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.
State the evidence needed to classify a characteristic as sex-linked and distinguish it from an autosomal trait.
Give me a hint
Use gene location rather than appearance or a social association.
Compare my reasoning
- Locate the responsible gene.
- Determine whether the chromosome is a sex chromosome or an autosome.
Sex linkage requires the responsible gene to be on a sex chromosome; an autosomal trait’s gene is on an autosome. A cultural association alone does not establish that location.
Look for these in your work
- Correct sex-chromosome location
- Correct distinction from autosomal location
Count the allele copies at the supplied locus in XNXn and XnY. Explain which can be called heterozygous at that locus.
Give me a hint
The Y has no corresponding allele here.
Compare my reasoning
- Count N and n on two X chromosomes in XNXn.
- Count only n in XnY.
- Apply the two-different-copy definition.
XNXn has two different X-locus alleles and is heterozygous. XnY has one allele copy at this locus and is not heterozygous or homozygous for the model gene.
Look for these in your work
- Correct two-copy and one-copy counts
- Correct locus-specific terminology
Explain why n can be expressed in XnY but not in XNXn under the supplied recessive rule, without predicting a population percentage.
Give me a hint
Compare the presence of an N counterpart.
Compare my reasoning
- Identify N in the heterozygous XX record.
- Identify its absence at the locus in XnY.
- Use the expression rule rather than a frequency assumption.
XNXn has N and does not express the n-associated feature under the rule. XnY has no N counterpart and expresses it. This explains a possible sex-frequency difference without establishing any population percentage.
Look for these in your work
- Correct N-counterpart explanation
- No unsupported numerical frequency claim
Common mistakes
- Calling a stereotyped feature sex-linked without gene-location evidence
- Putting an invented n or N allele on Y at this model locus
- Calling an XY one-copy X locus heterozygous
What can you explain now?
Why is XnY not heterozygous at this supplied locus?
Compare with the explanation
It has one X-locus allele and no corresponding allele on Y
Heterozygous refers to two different allele copies at the particular gene.
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Make it stick
Without your notes, locate a sex-linked gene on a sex chromosome and explain unequal expression opportunity in a stated x-linked recessive model. Use the supplied allele key and state what the evidence can establish.
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.