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?
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.
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