Phenotype - Difficulties in Definition

Difficulties in Definition

Despite its seemingly straightforward definition, the concept of the phenotype has hidden subtleties. It may seem that anything dependent on the genotype is a phenotype, including molecules such as RNA and proteins. Most molecules and structures coded by the genetic material are not visible in the appearance of an organism, yet they are observable (for example by Western blotting) and are thus part of the phenotype. Human blood groups are an example. It may also seem that this goes beyond the original intentions of the concept with its focus on the (living) organism in itself, meaning that the lowest level of biological organization compatible with the phenotype concept is at the cellular level. Either way, the term phenotype includes traits or characteristics that can be made visible by some technical procedure. Another extension adds behavior to the phenotype, since behaviors are also observable characteristics. Indeed there is research into the clinical relevance of behavioral phenotypes as they pertain to a range of syndromes. Often, the term "phenotype" is incorrectly used as a shorthand to indicate phenotypical changes observed in mutated organisms (most often in connection with knockout mice).