Twin Studies And Concordance
Insight into such questions can be gleaned by several types of studies that compare twins. Comparison of MZ twins reared apart is one type of study but is hampered by the extreme rarity of such twin pairs. Another type of study, comparing MZ twins to DZ twins, is more commonly done, because there are many hundreds of thousands of such twin pairs worldwide. Data on twins have been collected by numerous research groups who have created large and growing databases (registries) that can be mined for information.
Determining a characteristic called concordance plays a crucial role in most such studies. A twin pair is said to be concordant for a trait if both members show it. If neither twin shows the trait, the pair is also concordant, but for the absence of the trait. For instance, twins are concordant for Alzheimer's disease if both develop it. They are discordant if one does have the disease but the other does not.
If a trait is strongly influenced by genes, more MZ twin pairs should be concordant than DZ twin pairs, because MZ twins share more genes. Comparing concordance rates between the two groups, and applying some mathematical analysis, allows researchers to estimate the genetic contribution to a trait, as shown in Table 1.
|PAIRWISE CONCORDANCE FOR PARKINSON'S DISEASE|
|Concordant Pairs||Discordant Pairs||Pairwise Concordance||Risk of Concordance if MZ|
|MZ||DZ||MZ||DZ||MZ||DZ||RR (95% CI)|
|First twindiagnosed <50||4||2||0||10||100.0%||16.7%||6.00 (1.69-21.3)|
|First twin diagnosed >50||7||8||58||68||10.8%||10.5%||1.02 (0.39-2.67)|
Medicine EncyclopediaGenetics in Medicine - Part 4Twins - Developmental Mechanisms, Monozygotic Versus Dizygotic Twins, Similarities And Differences Between Monozygotic Twins, Amazing Twin Similarities - Conclusion