Measuring Inheritance Over Time and in Twins
How can a study measure risk forward in time, and how can comparing twins separate inherited risk from environmental risk?
💡 A cohort follows exposure forward to outcome, and a uses identical versus fraternal concordance to estimate heritability without naming a single gene.
Prerequisite check
- A starts with the outcome (cases versus controls) and looks backward at the suspected exposure; it is the workhorse for a rare outcome like a (about 1 in 700 births).
- The is the odds of exposure in cases divided by the odds in controls; above 1 suggests a , equal to 1 means no association, below 1 means less common in cases.
What you will learn
Goal: Students will explain how a follows exposure forward to outcome, how a compares identical and fraternal concordance to estimate heritability, and what high heritability does and does not tell us about a single gene.
- A starts with the exposure and follows people forward to the outcome; a prospective cohort records exposure before the outcome, so it is largely immune to .
- A compares identical (about 100 percent shared DNA) and fraternal (about 50 percent shared) concordance to estimate heritability, after confirming zygosity.
- For lip with or without cleft , identical concordance was 50 percent versus 8 percent in fraternal twins, giving heritability above 90 percent.
- High heritability says genes explain most of the variation in risk across a population; it does NOT name one gene and is compatible with many small-effect genes plus environment.
Model: A cohort in motion, and a real twin study with its numbers
A reverses the case-control arrow. Instead of starting with the outcome, it starts with the exposure and follows people forward. Imagine enrolling two groups of pregnant women at the start of pregnancy, one with a chosen exposure and one without; no one has had their baby yet. You follow every pregnancy forward and record which babies are born with a . Because you measured the exposure before any outcome existed, no one can misremember it. The catch: clefts are rare, about 1 in 700 births [DOI:10.1002/bdr2.2216], so a forward cohort must be very large and patient.
Researchers linked a national database to a national twin registry, verified which pairs were identical versus fraternal, excluded syndromes, and asked: when one twin has a cleft, how often does the other also have one (the concordance) [PMID:21423016]? Identical (monozygotic) twins share nearly 100 percent of their DNA; fraternal (dizygotic) twins share about 50 percent, like any siblings, and both kinds share a womb at the same time. Their results for cleft lip with or without cleft : identical twins 50 percent concordance, fraternal twins 8 percent. From models like this, the study estimated heritability above 90 percent for clefting [PMID:21423016].
Explore (work the model before reading on)
- In the cohort model, does the study start with the exposure or the outcome, and which way in time does it look?
- In the , what was the concordance for identical twins? For fraternal twins?
- Both identical and fraternal twins share a womb at the same time. Why does the gap between 50 percent and 8 percent point to genes rather than to shared environment?
- A cohort measures exposure before the baby is born. Explain how that timing protects it from the that threatened the .
- The study found heritability above 90 percent. A student concludes there must be one gene that gets passed down. Predict at least one reason that conclusion could be wrong even though genes clearly matter a lot.
- In one sentence, what does comparing identical and fraternal concordance let us estimate that a single cohort cannot?
Guided notes
The forward design
- A starts with the ____ (exposure) and follows people forward in time to the outcome; a prospective cohort records exposure before the outcome exists, so it is largely immune to .
- Its native measure is the (RR): risk in the exposed group divided by risk in the unexposed group; the price is size and patience for a rare outcome.
Twins as nature's experiment
- A uses identical and fraternal twins to estimate heritability after confirming each pair's zygosity; the engine is concordance, how often the second twin is affected given the first one is.
- If identical concordance is much ____ (higher) than fraternal concordance, genes are doing the work; if the two are about equal, shared environment is. For clefting, 50 percent towered over 8 percent, giving heritability above 90 percent.
What heritability does NOT say
- High heritability says genes account for most of the variation in risk across the population; it does not say one gene is responsible or that a cleft is inherited in a simple pass-it-down pattern.
- A trait can be highly heritable and still depend on many genes each adding a little, plus environment; the 50 percent identical discordance shows non-genetic factors still operate.
Reading the Research
- Skim the title and abstract first to get the gist.
- Circle the one sentence that states the main claim.
- Box the evidence the authors give for that claim.
- Mark one sentence that confuses you, and move on.
Vocabulary (the same words your classes use)
Track your progress today
Check these off as you work through the lesson, then submit. This tells Mr. Mendoza how you're doing so he can help the class. It does not replace turning in your producible.
Use the code Mr. Mendoza gave you, not your name. Saved on this device.
- Read the Model and answered the Explore questions.
- Filled in the guided notes in my own words.
- Defined the new vocabulary with an example.
- Built the producible: Present the twin result at the team meeting: state the identical and fraternal concordances and which is higher, explain in one sentence why this points to genes rather than the shared womb, and answer in two sentences whether heritability above 90 percent means Mateo's future siblings will almost certainly have a cleft.
- Wrote my Claim, Evidence, and Reasoning exit ticket.
Exit ticket (Claim, Evidence, Reasoning)
- Claim: Genes contribute strongly to risk.
- Evidence: The found identical concordance of ____ percent and fraternal concordance of ____ percent.
- Reasoning: The gap between those numbers is evidence for genes and not for shared environment, because both twin types share the womb equally, so the main thing that differs is how much ____ they share.
| Criterion | Proficient | Developing | Beginning |
|---|---|---|---|
| Complete | Every required part of the artifact is present and filled in. | Most parts are present, but one is missing or left blank. | Several parts are missing. |
| Accurate | The science and data are correct and match the evidence. | Mostly correct, with a small factual slip. | Key science or data is wrong. |
| Scientific reasoning (CER) | States a claim, backs it with specific evidence, and explains the reasoning. | Has a claim and evidence, but the reasoning is thin or missing. | Gives an answer with no evidence or reasoning. |
| Professional communication | Clear, organized, and labeled the way a clinician or scientist would write it. | Readable but disorganized or missing labels. | Hard to follow. |
| Submitted | Turned in the right way (Schoology for routine work) and confirmed. | Turned in, but in the wrong place or unconfirmed. | Not turned in. |
- CompleteProficient: Nothing is left blank: the model fills every part of "Present the twin result at the team meeting: state the identical and fraternal concordances and which is higher, explain in one sentence why this points to genes rather than the shared womb, and answer in two sentences whether heritability above 90 percent means Mateo's future siblings will almost certainly have a cleft.".
- AccurateProficient: Every number and claim matches the case evidence.
- Scientific reasoning (CER)Proficient: It names a claim, cites the specific evidence, and explains the reasoning, not just the answer.
- Professional communicationProficient: It is organized and labeled like a real chart note.
- SubmittedProficient: It would be turned in on Schoology and confirmed.
Where this leads: careers
What's next: The tells us genes carry most of the risk, but heritability is just a number; it does not name a single gene. Out of roughly twenty thousand genes, how do scientists track down the specific risk gene hiding among millions of bases?