Use a bounded X-linked recessive model for red-green colour blindness
Use the required example without claiming male-only expression or a universal colour-vision model.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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
Which is the required sex-linkage example here?
What you will learn
- Classify the supplied red-green model genotypes
- Distinguish the inheritance example from all colour-vision differences
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| XNXN | No model trait; no n contribution |
| XNXn | No model trait; heterozygous n carrier |
| XnXn | Model trait expressed in an XX genotype |
| XNY and XnY | No trait and trait respectively; no counterpart allele on Y |
Classify the supplied red-green model genotypes
Red-green colour blindness is the required example of sex linkage. Red-green colour-vision differences can make some colour distinctions difficult; this is not the same as seeing every scene only in black and white. The sequence uses a simplified X-linked recessive model to practise inheritance, not a test of a learner’s vision or a complete account of every biological variant.
The key uses XN for the normal-vision model allele and Xn for the recessive red-green-associated model allele. In the usual XX/XY model, XNXN and XNXn do not express the specified feature; XNXn carries n. XnXn expresses it. XNY does not express it and XnY does. These labels follow one supplied locus and are not complete genome records.
Distinguish the inheritance example from all colour-vision differences
The one-copy XnY case has no N counterpart at the locus on Y, so the recessive feature can be expressed with one n contribution. An XX individual requires the two n copies under the classroom rule. This supports the explanation that the X-linked recessive example is more common in males, but does not mean females cannot express it. XnXn is the supplied expressing female genotype.
Other kinds of colour-vision difference can have different causes and inheritance patterns. Even red-green biology involves more detail than this one-locus symbol model. Keep conclusions limited to the explicitly supplied genotypes and normal teaching assumptions. A phenotype, a colour image on screen or a model pedigree is not a personal diagnosis; no population frequency or individual clinical outcome is being measured here.
Sources for this lesson
Pause and explain
Which supplied female genotype expresses the recessive feature?
Worked example
Classify five genotypes under the stated usual XX/XY red-green colour-vision model.
Show the worked solution
- Read the recessive rule: XNXN and XNXn do not express the feature; XnXn does. The XNXn genotype is an unaffected carrier.
- For XY individuals, XNY has N and is unaffected; XnY lacks an N counterpart and expresses the feature. This bounded model does not describe every colour-vision difference.
Answer Model classification: XNXN unaffected; XNXn unaffected carrier; XnXn expresses the feature; XNY unaffected; XnY expresses the feature.
Try a topic test
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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.
Classify XNXN, XNXn and XnXn by expression and carrier status under the stated model.
Give me a hint
Apply the N-dominant expression rule to the two X copies.
Compare my reasoning
- Identify N in the first two records.
- Distinguish the n-carrying heterozygote.
- Identify two n copies in the third.
XNXN does not express the feature and does not carry n; XNXn does not express it and is an n carrier; XnXn expresses it.
Look for these in your work
- Correct expression categories
- Correct carrier distinction
Compare XNY and XnY, including why the Y does not make XnY an unaffected carrier in this model.
Give me a hint
The Y has no corresponding N allele at this locus.
Compare my reasoning
- Identify N in XNY and n in XnY.
- Apply the one-copy expression rule.
XNY does not express the specified feature. XnY expresses it because it has n and no N counterpart on Y; it is not the unaffected heterozygous-carrier case.
Look for these in your work
- Correct XY expression categories
- Correct absence of N counterpart explanation
Correct the claim that an X-linked recessive example must affect males only and every kind of colour-vision difference follows the same rule.
Give me a hint
Use the supplied female expressing genotype and the model’s scope.
Compare my reasoning
- Identify XnXn as an expressing female genotype.
- Separate the red-green example from other causes and types.
Females can express the feature as XnXn in this model. It can be more common in males without being male-only, and other colour-vision differences need not follow this simplified red-green inheritance rule.
Look for these in your work
- Rejects male-only claim using XnXn
- Keeps the model specific to the supplied example
Common mistakes
- Saying all colour-vision difference follows this one model
- Claiming females can never show the X-linked recessive feature
- Giving Y an N counterpart that the model does not contain
What can you explain now?
Which supplied female genotype expresses the recessive feature?
Compare with the explanation
XnXn
Both X-locus copies are n under the model rule.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, classify the supplied red-green model genotypes and distinguish the inheritance example from all colour-vision differences. 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.