Why have scientists studied identical and fraternal twins to try to tease out how much variation within a population can be attributed to heredity and how much to environment?

Question

Why have scientists studied identical and fraternal twins to try to tease out how much variation within a population can be attributed to heredity and how much to environment?

BlackTom AI · Accepted Answer

Monozygotic (identical) twins arise from a single fertilized egg and therefore share essentially 100% of their genetic sequence, whereas dizygotic (fraternal) twins come from two separate eggs and on average share about 50% of their segregating genes.

1) Relevant knowledge points and concepts
- Heredity vs. environment: traits can be influenced by genes, by environmental factors, or both.
- Twin-study logic: comparing similarity (concordance/correlation) for monozygotic (MZ) versus dizygotic (DZ) twins lets researchers estimate genetic contribution because MZ twins share all genes while DZ twins share half on average.
- Assumption: twins typically experience similar family and prenatal environments, so differences in similarity between MZ and DZ pairs are attributed mainly to genetic differences.

2) Step-by-step reasoning
- Measure how similar twin pairs are on a trait (e.g., IQ, height, disease status) and compute an average similarity metric for MZ twins (r_MZ) and for DZ twins (r_DZ).
- If r_MZ is much higher than r_DZ, genetic factors likely explain much of the variation; if r_MZ ≈ r_DZ, shared environment likely explains more.
- A common shorthand formula for broad-sense heritability from twin correlations: h^2 ≈ 2 × (r_MZ − r_DZ).

3) Example calculation (intermediate steps)
- Suppose r_MZ = 0.80 and r_DZ = 0.50; then h^2 ≈ 2 × (0.80 − 0.50) = 2 × 0.30 = 0.60 (60% genetic contribution).

4) Why the correct answer is correct
- The key reason twin studies work is the known difference in genetic relatedness: MZ = ~100% shared genes, DZ = ~50% shared genes. That contrast allows separation of genetic vs. environmental influences.

5) Why other options are incorrect
- "Same prenatal process": incorrect, MZ and DZ twins can have different prenatal conditions (e.g., placentation) and they are not both products of the same zygotic origin.
- "Epigenetics is the same in identical twins": false; epigenetic patterns can diverge with age and environment even in MZ twins.
- "Identical twins have more similarity in their experience": not generally true and not the primary reason twin studies isolate genetic effects; the method relies on genetic relatedness differences.

6) Conclusion
This reasoning confirms the correct answer: Because fraternal twins share, on average, 50 percent of their genes, and identical twins share 100 percent of their genes.

Why have scientists studied identical and fraternal twins to try to tease out how much variation within a population can be attributed to heredity and how much to environment?Single choice

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