Concordance

Concordance refers to the probability of two people in a pair having a certain phenotype (like a disorder) given that one of the two in the pair has it. It's commonly used to showcase how strong a genetic component is in a certain disorder. The "pair" mentioned is usually a pair of monozygotic twins.

Monozygotic twins and dizygotic twins are often compared, as monozygotic twins share 100% of their genes, while dizygotic twins share 50% of their genes. If there's higher concordance in monozygotic twins, this functions as evidence for a genetic component.

Example

As an example, there is high concordance of schizophrenia in monozygotic twin pairs. If one monozygotic twin has schizophrenia, the other has a 40-50% risk of developing it too. Between dizygotic twins, the concordance is 10-15%. This means that there is a strong genetic component to schizophrenia.