SNP and PTC case
Mon, Oct 19, 2026 · Week 9 · Genetics of Disease (Medical Interventions)
Today's goal: Connect a single-nucleotide polymorphism to a phenotype using the PTC-tasting genotype dataset.
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, not your readout: I used a different single-letter difference, the lactase SNP that sits just before the LCT gene. At the one tested position, T goes with keeping the ability to digest milk sugar as an adult, and C goes with losing it. The three samples below are practice samples for this worked example only.
Does the allele track with the trait? Yes, in these three samples. Samples with at least one T kept the ability to digest milk sugar, and the only sample that lost it was CC. Evidence: Sample A (TT) and Sample B (CT) both digest milk sugar as adults; Sample C (CC) does not, so T tracks with the trait and behaves as dominant here.
Carrier prediction: Sample B is heterozygous (CT). B shows the dominant trait but carries one C allele, so B could pass the C allele to a child.
Your own table uses your readout and your letters, so your genotypes and your carrier call come from your data, not from this example.
| Sample | Genotype | Zygosity | Digests milk sugar as an adult |
|---|---|---|---|
| A | TT | homozygous | yes |
| B | CT | heterozygous | yes |
| C | CC | homozygous | no |
This model shows the level of evidence and organization needed to complete: Completes the SNP case analysis: a genotype-to-phenotype table for three individuals with homozygous or heterozygous labels, an evidence sentence, and a carrier prediction.
- Name the variables and include units.
- Enter observations without changing the raw values.
- Check labels, calculations, and patterns before interpreting the data.
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 genotype-to-phenotype table and prediction as one PDF to Schoology assignment Wk9 Data table: SNP and PTC case.
Turn in: Genotype-to-phenotype table for three individuals (homozygous/heterozygous labeled) and a written carrier prediction.
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: Linking a SNP genotype to phenotype and identifying carriers.
- 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.