Concordant vs Discordant – Twin Studies

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Concordant
Both twins have disease or trait.

Discordant
Twin pairs do not share the same trait.

Relevance: 
In twin studies, if a trait is genetic, monozygotic twins should be 100% concordant, dizygotic twins should be ~50% concordant/discordant. If the discordant rate is higher than the concordant rate in twin studies, this trait is most likely environmentally based.