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- For other uses, see Concordance.
Concordance as used in genetics means the presence of the same trait in both members of a pair of twins, or in sets of individuals. The strict definition is the probability that a pair will both have a certain characteristic given that one of the pair has the characteristic.
A twin study examines the concordance rates of identical twins having the same trait, especially a disease, which can help determine how much the disease is affected by genetics versus environment. Because identical twins have identical DNA, it follows that they would be affected by the same genetic diseases. This is usually true for monogenic diseases, but is not always true of polygenic diseases.
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 its DNA from each parent, parents and children, siblings, and fraternal (dizygotic) twins have a concordance around 50% using this measure.