Twin study
Twin studies can help determine the proportion of cases of a disease that are due to inherited versus environmental factors.[1]
Classification
Hawkes proposed the following types of twin studies:[1]
Classical twin method
The classical twin method compares outcome concordance rates in monozygotic and dizygotic twins. The classic twin study calculated the following measures:[2]
- 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
Prenatal environment
Twin studies assume that monozygous and dizygous twins have similar prenatal environments; however, the fetal environments of monozygous and dizygous twins may differ.[3][4]
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.[3] The importance of this can be shown in studies that find different rates of concordance between monozygotic twins that are monochorionic verus dichorionic.[5] Some experts propose that twin studies should exclude monozygotic monochorionic pairs and monozygous dichorionic pairs with different birthweights.[3]
Methods of recruiting twins for study
Twins should be recruited from regional and national registries such as the Swedish Twin Registry.[6]. Recruiting volunteer pairs may bias towards monozygotic concordance.[1]
References
- ↑ 1.0 1.1 1.2 Hawkes CH (1997). "Twin studies in medicine--what do they tell us?". QJM 90 (5): 311–21. PMID 9205666. [e]
- ↑ Spector, Tim D., Harold Snieder, and Alex J. MacGregor. 2000. Advances in Twin and Sib-pair Analysis. 1st ed. Greenwich Medical Media. ISBN 1841100048
- ↑ 3.0 3.1 3.2 Phillips DI (1993). "Twin studies in medical research: can they tell us whether diseases are genetically determined?". Lancet 341 (8851): 1008–9. PMID 8096890. [e]
- ↑ Sørensen TI (2002). "Contributions and potentials of Swedish twin research". J. Intern. Med. 252 (3): 181–3. PMID 12269999. [e]
- ↑ Loos RJ, Beunen G, Fagard R, Derom C, Vlietinck R, Phillips DI (2001). "Twin studies and estimates of heritability". Lancet 357 (9266): 1445. DOI:10.1016/S0140-6736(00)04594-3. PMID 11360946. Research Blogging.
- ↑ Lichtenstein P, De Faire U, Floderus B, Svartengren M, Svedberg P, Pedersen NL (2002). "The Swedish Twin Registry: a unique resource for clinical, epidemiological and genetic studies". J. Intern. Med. 252 (3): 184–205. PMID 12270000. [e]