One reason to look for the genetic basis for an observed trait is to understand how the living world operates at molecular level.
A complication is evident in the case of phenylketonuria (PKU) in humans. PKU has been shown to be associated with having two copies of a malfunctioning gene for the enzyme phenylalanine hydroxylase (PAH). The cognitive development of such individuals is extremely impaired by the level of phenylalanine present in normal diets. However, the detection of PKU in newborns is possible on the basis of a biochemical test, which was widely applied before the genetic basis was known. Biochemical, not genetic screening, continues to be the basis for initiating the diet needed to prevent the cognitive impairment that had been associated with PKU. The more general issue is, if we want to understand how the living world operates at molecular level, when do we need to go beyond the biochemistry to look for the genes?
(Introduction to this series of posts)