twin studies

Meaning: Monozygotic (MZ) twins come from one fertilized egg and share essentially all their genes. Dizygotic (DZ) twins share about 50% of their varying genes on average, like ordinary siblings. Both kinds of pair usually share a family environment. So if a trait is more concordant (both twins have it) in MZ pairs than in DZ pairs, the extra similarity is attributed to genes. A larger gap suggests greater heritability. Studies of MZ twins raised apart test the same idea: similarity despite different homes points to genes, while differences between MZ twins point to environment.

On the MCAT: A passage gives concordance rates or correlations for a trait such as schizophrenia, a personality measure or IQ. You compare MZ with DZ pairs and decide how much genes and environment contribute. MZ concordance well below 100% shows that environment matters even for a trait with a genetic component. Questions may also ask how twin studies differ from adoption studies, which compare adopted children with biological and adoptive relatives.

Common confusion: Heritability describes how much of the variation in a population is linked to genetic differences. It does not say how much of one person's trait is genetic. High MZ concordance also does not prove genes alone cause a trait, because MZ twins may be treated more alike. The DZ comparison and MZ twins reared apart help control for this.