What does concordance measure in twin studies?

• A. The probability that both twins share a characteristic
• B. The percentage of shared chromosomes
• C. The difference in environments
• D. The age of twins

Answer: (A)

Concordance measures the probability that both twins share a characteristic. In twin studies, researchers look at how often both twins in a pair show the trait and express this as a concordance rate. If identical twins (who share almost all their genes) have a much higher concordance than fraternal twins (who share about half their segregating genes), this points to a genetic influence on the trait. If concordance is similar for both types of twins, environments may play a larger role. For example, if 60 out of 100 identical twin pairs both show the trait, the concordance is 0.60; if only 30 out of 100 fraternal pairs do, the larger gap suggests genetics contribute to the trait. This measure is not about the percentage of shared chromosomes, nor about differences in environments or the twins’ ages.