A phenotype covers every trait but the genome, from blood groups to beaver dams
Biologists split living things into two layers: the genotype, the inherited genetic material, and the phenotype, everything else that can be observed or measured. That second layer reaches further than it sounds, taking in a peacock's courtship display, a person's blood group and, for Richard Dawkins, a beaver's dam.
The Danish botanist Wilhelm Johannsen drew the line between the two in 1911, separating hereditary material from the forms, sizes, colours and structures that naturalists had always catalogued. It echoed August Weismann's earlier split between germ plasm and body cells, and Dawkins later recast the pair as replicators and vehicles in The Selfish Gene in 1976. The word itself comes from Greek roots meaning to show and type.
A phenotype is not only what meets the eye. Proteins, RNA and blood types rarely show up in appearance but can be revealed by laboratory tests, so they count. Behaviour counts too, from personality patterns to the traits that define some psychiatric syndromes. The collection of all such traits in a cell, organism or species is called its phenome, a term first proposed in 1949, and in 2009 researchers began linking health records to DNA banks to scan for gene-trait associations across the whole phenome.
Genes alone never settle the result. A simple formula adds environment to genotype, but a subtler one stresses that genes act only through a living organism interacting with its surroundings. Identical seeds of the hawkweed Hieracium umbellatum, landing on alternating habitats along the Swedish coast, grow bushy with broad leaves on sea cliffs yet flat and narrow-leaved among sand dunes. Chance plays a part as well: a fruit fly's left and right eyes can differ in their number of facets as much as flies of different genetic strains. This flexibility matters because natural selection acts on phenotypes, reshaping gene pools only indirectly.
Dawkins pushed the idea outward in The Extended Phenotype in 1982, arguing that bird nests, caddisfly larva cases and beaver dams express genes just as teeth do, and that a host bird feeding a cuckoo chick is extending the parasite's phenotype. Many biologists accept the notion but see it mainly as an explanation rather than a source of testable experiments.
Source: Phenotype
