What concordance rate would be expected for monozygotic twins if a trait were completely genetic?

What concordance rate would be expected for monozygotic twins if a trait were completely genetic?

• A. 100%
• B. 50%
• C. 0%
• D. 25%

Answer: (A)

Concordance rate is the chance the second twin shows a trait given that the first twin does. Monozygotic twins are genetically identical, so if a trait is completely genetic with no environmental influence, the second twin would necessarily have the trait whenever the first does—giving a 100% concordance. In real life, even highly genetic traits often show less than perfect concordance because of environment, development, or measurement limits. The other numbers reflect different genetic sharing (fraternal twins share about half their genes), or no genetic influence, or other inheritance patterns, but for a trait that is entirely genetic in monozygotic twins, the expected concordance is 100%.

Concordance rate is the chance the second twin shows a trait given that the first twin does. Monozygotic twins are genetically identical, so if a trait is completely genetic with no environmental influence, the second twin would necessarily have the trait whenever the first does—giving a 100% concordance. In real life, even highly genetic traits often show less than perfect concordance because of environment, development, or measurement limits. The other numbers reflect different genetic sharing (fraternal twins share about half their genes), or no genetic influence, or other inheritance patterns, but for a trait that is entirely genetic in monozygotic twins, the expected concordance is 100%.