Though we all know an organism’s genes have an impact on its outward appearance, this relationship is rarely one-to-one. Dealing with the division between ‘inside’ and ‘outside’ — between genes and their effects — is where the terms genotype and phenotype come in handy.
Genotype is a given organism’s genetic makeup: it describes the alleles (versions of genes) it has at any given locus (point in its genetic code). A bit of clarification: genomes and genotypes are distinct. A genome describes the general genetic makeup of an entire species (i.e. The Human Genome Project), whereas a genotype describes the genetic makeup of a single individual (i.e. The Richard Dawkins Genotype Project – which I just made up to illustrate this point).
Phenotype is a given organism’s ‘outward appearance’, and it’s a product of its genotype and the influences of its environment. Eye color, nose shape, blood type – these are all phenotypic traits of people. Hollywood provides an excellent example of the effects of environment on phenotype: no genes actually code for orange skin, and yet orange skin is a terrifyingly prevalent phenotype in Hollywood. In this case, artificial tanning salons count as part of the ‘environment’ in the following, simple equation:
phenotype = genotype + environment
This is not a naturally-occuring human phenotype.
More ‘natural’ environmental effects include the likes of malnourishment, injuries, disease, and the like. Lack of nutrients, for example, can stunt growth, even if the genes for tallness are present in an individual. In this case ‘environment’ is a catch-all term for any effects not strictly genetic in origin.
The dynamics between genotype and phenotype got really interesting, however, when Richard Dawkins put forth the idea of the extended phenotype in his aptly-titled book, The Extended Phenotype (1982). According to Dawkins, he considers the theory of the extended phenotype to be his principle contribution to evolutionary theory. I’ll try to give a brief overview of the theory here, and hopefully the implications will make you as delightfully dizzy as they made me when I first heard about this concept in an ecological parasitology class at UVM.
The primary fact is this: the only thing genes (and genotypes) influence directly is the synthesis of proteins. Genes code for specific proteins. That’s it – every other effect of genes we can see – fur length, wing shape, bone thickness, brain structure, behavior – all are indirect effects of the gene. We call this manifestation of genes (genotypes) the phenotype.
Yet we customarily only apply the term phenotype to a single organism. We consider the shape of a beaver’s tail part of its phenotype, as it results from interactions between its genes and the aforementioned catch-all ‘environment’.
What about beaver behavior? Well, we all accept that behavior is also influenced, to some extent, by genes. If nothing else, brains are the source of behavior, and genes are the source of brains, so. We’re still within the bounds of conventional phenotypes here.
What about a beaver dam? Or, better yet, the entire pond that results from the construction of a beaver dam? We’re happy to consider this an effect of beaver behavior – it’s one of the great marvels of ecology the way beavers create an entire habitat type. I think you see where I’m going here.
If behavior can be considered part of the phenotype, and the pond itself is a result – however indirect – of the beaver’s genes and the environment, then there is no logical reason not to consider beaver ponds part of a beaver’s phenotype. (Dawkins, in referring to such far-reaching phenotypic effects as the extended phenotype, is at least making a small concession by qualifying the term.)
A beaver dam
This method of thinking can be applied to all sorts of natural phenomena. Consider brood parasites – birds which lay their eggs in the nests of other species. Their offspring then take advantage of the surrogate parent, manipulating them via various cues to, essentially, force them to provide parental care. Since these effects on the host (victim) species’ are a result of the genes in the baby brood parasite, then the behavior of this entirely separate host species qualifies as part of the extended phenotype of the brood parasite.
I won’t run on and on with examples here (read The Selfish Gene and The Extended Phenotype). If, in reading this, you have become slightly disoriented, then I’ve successfully communicated the crux of this theory. Hopefully the once-clear lines between genes and the organisms that carry them are blurring a bit for you too.
The Selfish Gene by Richard Dawkins
The Extended Phenotype by Richard Dawkins
Wikipedia (genotype) Wikipedia (phenotype)
The genotype of an organism is its pure genetic make-up: what traits its DNA code for. The phenotype, on the other hand, is what physically shows up in the individual: the physical molecules and proteins that make up the body, dictated by the genotype. Environmental factors can affect the phenotype, though; for example, the genes of a flamingo may encode for white feathers, but their diet sometimes causes the feathers to turn pink.
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