Concordance (genetics)

Concordance, as used in genetics, usually means the presence of the same trait in both members of a pair of twins. However, the strict definition is the probability that a pair of individuals will both have a certain characteristic, given that one of the pair has the characteristic. For example, twins are concordant when both have or both lack a given trait. The ideal example of concordance is that of identical twins.

A twin study compares the concordance rate of identical twins to that of fraternal twins. This can help suggest whether a disease or a certain trait has a genetic cause. Controversial uses of twin data have looked at concordance rates for homosexuality and intelligence.

Because identical twins are genetically virtually identical, it follows that a genetic pattern carried by one would very likely also be carried by the other. If a characteristic identified in one twin is caused by a certain gene, then it would also very likely be present in the other twin. Thus, the concordance rate of a given characteristic helps suggest whether or to what extent a characteristic is related to genetics.

There are several problems with this assumption:

In genotyping studies where DNA is directly assayed for positions of variance (see SNP), concordance is a measure of the percentage of SNPs that are measured as identical. Samples from the same individual or identical twins theoretically have a concordance of 100%, but due to assaying errors and somatic mutations, they are usually found in the range of 99% to 99.95%. Concordance can therefore be used as a method of assessing the accuracy of a genotyping assay platform.

Because a child inherits half of his or her DNA from each parent, parents and children, siblings, and fraternal (dizygotic) twins have a concordance that averages 50% using this measure.

...
Wikipedia