If a trait that's present in one twin is just as often present in the other regardless of whether the twins are fraternal or identical then the trait is likely caused by environmental conditions. On the other hand, if a trait is generally found in both identical twins but only in one of a set of fraternal twins, it's likely that the trait is inherited, since identical twins have the same genetic material but fraternal twins do not.
Similar studies also can be done for linked traits, such as height and IQ. But while scientists could determine that a pair of traits is passed down genetically, they could not further resolve whether inherited traits were linked due to the same genes influencing both traits, called "pleiotropy," or because people who have those traits are more likely to mate with each other, known as "assortative mating."
The new CU-Boulder study solves this problem by including the parents of twins in its analysis. While this has occasionally been done in the past for single traits, information on parents has not previously been used to shed light on why two traits are genetically correlated. In part, that's because existing twin registries, where information for heritability studies is drawn, don't often contain information on the parents.
Additionally, creating the computer programs that are necessary to crunch the data for multiple traits from twins and their parents in order to understand environmental effects and both types of genetic effects is difficult.
"These designs have never taken off because they're very difficult to code," Keller said. "It's a challenge. They're very complicated models."
For this study, the research team used data collected from 7,905 individuals including twins and their paren
|Contact: Matthew Keller|
University of Colorado at Boulder