Twin Studies

Identical (monozygotic) twins have identical DNA sequences, but fraternal (dizygotic, nonidentical) twins do not; they are no more closely related than other full siblings. If a response is studied in sets of identical and fraternal twins, the response will be much more concordant (similar) in the identical twins than in the fraternal twins if the response is hereditary as shown below for the urinary elimination of isoniazid.

Urinary elimination of isoniazid (INH)

Identical (monozygotic) twins Fraternal (dizygotic) twins

Urinary elimination of isoniazid (INH)

Identical (monozygotic) twins Fraternal (dizygotic) twins

Sex

INH eliminated (% dose ingested)

Sex

INH eliminated (% dose ingested)

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