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.
Discordance, in genetics typically means that a similar trait is not shared between twin members. Studies of twins have shown that genetic traits of monozygotic twins are fully concordant whereas in dizygotic twins, half of genetic traits are concordant, while the other half are discordant. Discordant rates that are higher than concordant rates express the influence of the environment on twin traits.
Studies.
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. Other studies have involved looking at the genetic and environmental factors that can lead to increased LDL in women twins.
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:
Genotyping studies.
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.