Question

In a twin study, there is about 100% concordance between monozygotic twins and about 50% concordance between dizygotic twins for a certain trait. What can we say about this trait in regards to the impact of genes vs. the environment?

Answer 1

From this we can say that the trait has an complete impact of genes and that the environment does not happen to play a role.

Monozygotic twins have 100% concordance in their genes as they are formed from the same ova and the same sperm while the dizygotic/fraternal twins share an average of 50% of the genes as they are formed from different ova and different sperm.

Now there are 3 aspects which could cause variation - genetic make up, common environment and special environment. Any differences in trait between the monozygotic trait would indicate an effect of special environment experienced by them while a difference in traits among dizygotic twins could be a genetic effect or a special environment to which the 2 twins were exposed to.