Human Heredity 11th Edition by Michael Cummings

Exercise 11

Is Going to Medical School a Genetic Trait?
Many behavioral traits follow a familial, if not Mendelian, pattern of inheritance. This observation, along with twin and adoption studies, reveals a strong genetic component in complex behavioral disorders. In most cases, these phenotypes are not inherited as simple Mendelian traits. Researchers thus are faced with the task of selecting a model to describe how a behavioral trait is inherited. Using this model, further choices are made to select the methods used in genetic analysis of the trait. A common strategy is to find a family in which the behavior appears to be inherited as a recessive or an incompletely penetrant dominant trait controlled by a single gene. Molecular markers are then used in linkage analysis to identify the chromosome that carries the gene controlling the trait.
If researchers are looking for a single gene when the trait is controlled by more than one gene or genes that strongly interact with the environment, the work may produce negative results, even though preliminary findings can be encouraging. To illustrate some of the pitfalls associated with model selection in behavior genetics, researchers deliberately selected attendance at medical school as a behavioral trait and then determined if the inheritance of this trait in families is consistent with a genetic model. They surveyed 249 first- and second-year medical students. Thirteen percent had first-degree relatives (parents, siblings, or children) who also had attended medical school, compared with 0.22% of individuals in the general population with such relatives. Thus, the overall risk factor among first-degree relatives for medicalschool attendance was 61 times higher for medical students than for the general population, indicating a strong familial pattern. To see if selecting the right model would show this behavior as an inherited trait, researchers used statistical analysis that supported inheritance and rejected the model of no inheritance. Pedigree analysis supported a single-gene recessive pattern of inheritance, although other models, including polygenic inheritance, were not excluded. Using a further set of statistical tests, the researchers concluded that the recessive inheritance of this trait was just at the border of statistical acceptance.
Similar results of borderline statistical significance are often found in studies of other behavioral traits, and it is usually argued that another, larger study would confirm the results. Although it is true that genetic factors may partly determine whether one will attend medical school, it is highly unlikely that a single recessive gene controls this decision, regardless of the outcome from this family study and segregation analysis of the results.
The authors of this study were not serious in their claims that a decision to attend medical school is a genetic trait, nor did they intend to cast doubts on the methods used in the genetic analysis of behavior. Rather, their work was intended to point out the folly of accepting simple, single-gene explanations for complex behavioral traits.
What role would twin and adoption studies play in selecting a model for determining whether attendance at medical school is a heritable trait? What heritability values would make you take the study to the next level?