Twin studies can help determine the proportion of cases of a disease that are due to inherited versus environmental factors.
Hawkes proposed the following types of twin studies:
Classical twin method
The classical twin method compares outcome concordance rates in monozygotic and dizygotic twins. The classic twin study calculated the following measures:
- A: additive genetic variance
- D: dominance genetic variance
- C: common environmental variance
- E: unique environmental variance
Inheritability is the difference in concordance rates between the monozygotic and dizygotic twins. For example if 100% of the pairs of monozygotic twins are concordant whereas 50% of the dizygotic twins are concordant, then the inheritability is 100%.
Co-twin control method
In the co-twin control method, discordant monozygotic are closely compared which creates matched pair control. In this design, a single pair of twins may sometimes suffice.
Biometrical genetic methods
Problems with twin studies
As a separate problem, about one third of monozygotic twins are dichorionic. Dichorionic type twins have separate sets of fetal membranes due to their common ovum having divided at an early stage. The importance of this can be shown in studies that find different rates of concordance between monozygotic twins that are monochorionic verus dichorionic. Some experts propose that twin studies should exclude monozygotic monochorionic pairs and monozygous dichorionic pairs with different birthweights.
Methods of recruiting twins for study
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