The Extended Phenotype is an idea expounded by the biologist Richard Dawkins in his book of that title.
In the book Dawkins starts from the ideas of his earlier book The Selfish Gene which portrayed the organism as a survival machine constructed by its genes to maximise the chances of their replication. In a much more technical presentation than the earlier book, Dawkins devotes a significant portion of this work to an attempt to remedy incomprehension and rebut criticism of The Selfish Gene.
In the main portion of the book, arguing that the only thing which genes control directly is the synthesis of proteins, Dawkins points to the arbitrariness of restricting the idea of the phenotype to apply only to the phenotypic expression of an organism's genes in its own body. Dawkins develops this idea by pointing to the effect that a gene may have on an organism's environment through that organism's behaviour citing as examples caddis houses and beaver dams. He then goes further to point to first animal morphology, and ultimately animal behaviour, which appears advantageous not to the animal itself, but, rather, to a parasite which afflicts it.
In conducting this argument Dawkins aims to strengthen the case for a gene-centred view of life, to the point where it is recognised that the organism itself needs to be explained. This is the challenge which he takes up in the final chapter entitled "Rediscovering the Organism".