They have all of their genes in common. Dizygotic twins, or so-called fraternal twins, on the other hand, are no more genetically similar than any other set of siblings. In general, either type of twin have shared environments as well as they grow up in the same household. As a first approximation, monozygotic twins can give an estimate of the effect of shared genes and dizygotic twins can give an estimate of the effect of shared environment. An additional twist on this research has also been very helpful because occasionally, where some of the effects of shared environment are eliminated by only studying twins who happen, through adoption, to have been raised apart.
Twin studies can produce information about the heritability ratio.
This is a number that suggests how much of the trait is environmental and how much is genetic. As the heritability ratio approaches zero, the trait is primarily environmental. As it approaches, the trait is primarily genetic. For instance, twin studies in diabetes have shown that there is a pronounced difference in the genetics of the juvenile onset and the maternity onset form. If one monozygotic twin has maternity onset diabetes, the other is very likely to also develop late onset diabetes.