Gene expression and the concept of the phenotype.

While the definition of the 'genotype' has undergone dramatic changes in the transition from classical to molecular genetics, the definition of the 'phenotype' has remained for a long time within the classical framework. In addition, while the notion of the genotype has received significant attention from philosophers of biology, the notion of the phenotype has not. Recent developments in the technology of measuring gene-expression levels have made it possible to conceive of phenotypic traits in terms of levels of gene expression. We demonstrate that not only has this become possible but it has also become an actual practice. This suggests a significant change in our conception of the phenotype: as in the case of the 'genotype', phenotypes can now be conceived in quantitative and measurable terms on a comprehensive molecular level. We discuss in what sense gene expression profiles can be regarded as phenotypic traits and whether these traits are better described as a novel concept of phenotype or as an extension of the classical concept. We argue for an extension of the classical concept and call for an examination of the type of extension involved.