Pedigree and risk CER
Wed, Oct 28, 2026 · Week 10 · Principles of Biomedical Technology (Principles of Biomedical Science)
Today's goal: Students construct a CER linking pedigree evidence to a calculated genetic risk for an offspring.
This is a model of the work you should turn in today. Use it to check your own: match the structure and the level of detail, do not copy it. Your data and wording should be your own.
Claim: There is a 50% chance that this couple's next son will be colorblind, and a 0% chance that a daughter will be colorblind.
Evidence: Red-green colorblindness is an X-linked recessive trait, so the allele sits on the X chromosome. The pedigree shows that the mother has a colorblind father, which means she inherited one affected X and is a carrier (X^A X^a), while the father has normal color vision (X^A Y). A Punnett square of X^A X^a mother by X^A Y father gives four equally likely children: X^A X^A, X^A X^a, X^A Y, and X^a Y. Among the two possible sons (X^A Y and X^a Y), one is affected, and neither of the two possible daughters is affected because each daughter still inherits at least one normal X^A from the father.
Reasoning: Because the trait is recessive and carried on the X chromosome, a son is affected whenever his single X carries the recessive allele, since he has no second X to mask it, giving 1 of 2 sons affected, or 50%. A daughter would need two recessive alleles to be affected, but the father can only pass a normal X^A, so no daughter in this cross can be colorblind, which is why the daughter risk is 0%. This reasoning assumes the mother is truly a carrier, which the pedigree supports because her father was colorblind and each daughter of a colorblind man must receive his X^a. A limitation is that the pedigree relies on reported vision status rather than a formal color-vision test, so mild cases could be missed or misreported, and confirming the mother's carrier status with genetic testing would make the 50% son estimate more reliable.
| Punnett (Aa x Aa) | A | a |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
This model shows the level of evidence and organization needed to complete: A claim-evidence-reasoning paragraph that states a quantitative genetic-risk for an offspring, supports it with pedigree and Punnett square evidence, and names at least one limitation.
- Write one defensible claim.
- Choose specific evidence that supports the claim.
- Explain the scientific rule that connects the evidence to the claim.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: turns in the CER on the class site under the Pedigree CER assignment before the end of the period.
Claim ceiling: Today's evidence supports a classroom claim about today's lesson. It cannot prove causation, diagnose a real patient, or justify action outside this room.
One exam-style question that uses exactly what you practiced today. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Using a Punnett square to calculate offspring risk from carrier parents.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.