CRISPR and reproductive screening
Fri, Nov 6, 2026 · Week 11 · Genetics of Disease (Medical Interventions)
Today's goal: Explain how CRISPR-Cas9 edits DNA, including off-target risk, and apply it to a reproductive screening case.
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.
Parallel case: A 16-year-old with severe sickle cell disease is offered a CRISPR-based therapy. Doctors would collect some of the patient's own blood stem cells and use a guide RNA to steer Cas9 to an enhancer, a switch region of the BCL11A gene that is active in red-cell precursors. BCL11A is what turns fetal hemoglobin off, so cutting the enhancer lowers BCL11A in those cells and fetal hemoglobin comes back. Before the edited cells are returned, the patient receives chemotherapy (busulfan) to clear space in the bone marrow; the edited cells then grow into red cells that make protective fetal hemoglobin. The patient and their guardians consent after counseling.
Claim: In this case CRISPR-based somatic therapy is an appropriate treatment to offer, because the edit is limited to the patient's own body cells, is not passed to future generations, and is chosen with informed consent to treat serious ongoing harm.
Evidence 1: The edit is somatic, not germline. It changes blood-forming cells in one person and is not inherited by that person's future children, so any mistake affects only the consenting patient and does not spread to descendants.
Evidence 2: Off-target risk is real but is being managed and consented to. The guide RNA is about 20 letters long against a genome of roughly 3 billion letters, so Cas9 can cut a wrong site that partially matches, and a bad off-target cut could disrupt a tumor suppressor or another important gene. However, the edited cells can be screened before they are returned, and the patient is weighing that known risk, plus the risks of the chemotherapy conditioning such as infection and infertility, against the daily harm of untreated sickle cell disease.
Reasoning: Because the change stays in one consenting person and is not heritable, the ethical stakes are lower than a change that every future generation would carry. The off-target danger does not disappear, but it can be reduced by checking the cells and is being accepted knowingly to relieve a serious, present illness. When a competent patient consents, the harm is severe, and the risk is bounded to that individual, offering the somatic edit is justified.
This model shows the level of evidence and organization needed to complete: Models the CER format on a parallel gene-editing case: a claim, two pieces of evidence, and reasoning about whether to use CRISPR-based somatic therapy in a consenting teenage patient with sickle cell disease. This mirrors the structure and depth students must produce today without answering the embryo screening prompt.
- 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: Submit your annotation and reproductive screening CER in Schoology.
Turn in: Annotated CRISPR article (guide RNA, Cas9 cut, repair step marked; off-target defined) plus a CER on using CRISPR in the reproductive screening case.
Go to Schoology to turn this in.
Submit one PDF. Schoology attaches your name to your upload; on the page itself, write your period and code. Name the file: FirstName LastName - Assignment Title - YYYY-MM-DD.pdf.
Open Schoology PDF upload helpIf you cannot get in, see Mr. Mendoza. Do not skip the work.
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: Explaining CRISPR-Cas9 targeting and off-target risk.
- 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.