As noted in an earlier post, high heritability might mean that the similarity between twins or a set of close relatives is associated with the similarity of yet-to-be-identified genetic factors [but] the factors may not be the same from one set of relatives to the next, or from one environment to the next. This is one reason why translation from estimation of heritability to hypotheses about measurable factors is difficult. One response to this situation is to
[r]estrict attention to variation within a set of relatives. Even if the underlying factors are not yet known, high heritability still means that if one twin develops a trait (e.g., type 1 diabetes), the other twin is more likely to as well. This information might stimulate the second twin to take measures to reduce the health impact if and when the disease starts to appear.
[N]otice that this path assumes that the timing of getting the condition differs from the first twin to the second. Researchers might well then ask: What factors influence the timing? How changeable are these? How much reduction in risk comes from changing them? To address these issues researchers would have to identify the genetic and environmental factors that influence the development of the trait. To do so would require larger sample sizes than any single set of relatives allows. The question then arises whether the initial results would carry over from one set of relatives to others. This issue is an empirical one; there is a risk… that the proportion of fruitful investigations will be low compared to those confounded by factors not carrying over well from the initial set of relatives.
Additional complications follow from asking what to do about the possibility of underlying heterogeneity of the genetic and environmental factors.
Moreover, this advice to relatives does not depend on looking for, or finding, the genes. (Including this entry in the 50 whys runs the risk of bolstering the idea that heritability, which comes from analysis of traits, has something to do with whether a trait is heritable in the sense of caused by genes transmitted from parents to offspring.)
The quotes above are from page 31 in Taylor, Peter J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.
(Introduction to this series of posts)