Agreement in types of data that occur in natural pairs - in autism a pair of identical twins is "concordant" if BOTH are affected or unaffected. It is "discordant" if one of them only is affected.

As applied in behaviour genetics, the similarity in psychiatric diagnosis or in other traits within a pair of twins.

Presence of the same characteristic in both members of a pair of twins (or set of individuals). The opposite to discordant.

kon-KOR-dance A measure of the inherited component of a trait. The number of pairs of either monozygotic or dizygotic twins in which both members express a trait, divided by the number of pairs in which at least one twin expresses the trait. 279

discordance

For two or more loci or traits typed in offspring from a backcross or RI strain, the presence of alleles (or expression of a trait) derived from the same parental chromosome.

The probability that a person who stands in a particular familial relationship to a patient (e.g., an identical twin) has the same disorder as the patient.

When both members of a pair of twins exhibit the same trait they are said to be concordant. If only one twin has the trait they are said to be discordant.