Every living thing has genes that largely determine its appearance. Humans (magical or Muggle) look like humans because they have human genes; dogs look like dogs because they have dog genes; cats look like cats because they have cat genes. The set of genes that make up an individual person or dog or cat is called the “genotype.” Every individual has a unique genotype, with the exception of identical siblings such as the twins Fred and George Weasley.
But genes alone do not determine our appearance. Genes interact with the environment throughout our lives to produce our characteristics, otherwise known as our “phenotype.” Some features of our phenotype such as eye color or blood type are pretty much determined only by our genotype, with hardly any input from the environment. Other phenotypic characteristics, such as our weight, may be determined more by the environment, such as what we eat and how much we exercise. Even identical twins may develop different phenotypes based on how their genes interact with their environments, for example to produce mirror-image facial features or different personalities.
For most of us, our phenotype is relatively fixed. We may gain or lose weight, but our eye color doesn’t change. We can easily recognize our friends and relatives because each day they look pretty much the same as they did the previous day. Some magic folk, known as Metamorphmagi, have the ability to transform their appearance at will, but these are superficial magical changes, in some respects similar to Muggles dying their hair a different color or having plastic surgery. The underlying human remains basically unchanged; after all, we still retain our human genotype. And of course there are potions that can allow magical humans to transfigure, but these are temporary changes, akin to a Muggle donning a disguise.
In contrast there are a small number of witches and wizards known as Animagi who have the ability to transfigure at will into a different animal species and remain in their animal form while retaining their self-awareness for an indefinite period of time. This is exceedingly difficult to accomplish, and in most cases the Animagus learns to develop this ability only through years of practice.
When a specific Animagus transfigures, it is always to the same animal species and that animal always has the same appearance. Thus, the phenotype of the animal phase of a specific Animagus is as fixed and recognizable as that of the human phase. The question then arises, does this animal phenotype reflect the genotype of the Animagus the same way that eye color reflects the genotype of the human phase? And if it does, how can the genotype of a human create the phenotype of a cat or a dog?
The answer seems to lie in what could be called the “personality phenotype.” In all living things, parts of the genotype are expressed in other than physical characteristics. Many human personality traits and skills are a good example, as they are likely to have a genetic as well as an environmental component. For instance, artistic ability seems to have a genetic basis as it frequently runs in families, but education and training can develop the phenotype of artistic talent to a much greater degree. This trait does not have an overtly physical manifestation; you can not tell if a person is talented just by looking at them. It is their artwork that reveals their ability.
Similarly, other traits that make up our personality phenotype seem to be an interaction of genotype and environment as well, and for most of us there is no physical manifestation of these either. As for artistic talent, one can not tell if a person is courageous or cowardly just by looking at them. It is our actions, not our appearance, that reveal our personality phenotype.
However this does not appear to be true of an Animagus. The animal that the person transfigures into shows in an obvious physical way the most basic characteristic of his or her personality phenotype. Someone who is loyal to his friends, so much so that he would give his life for them rather than betray them, would transfigure into that most faithful of animals, the dog. Thus the animal phase for Sirius Black was a dog. Someone who is a natural born leader, someone who would defend his family against all odds, someone like James Potter, would become a stag. And of course someone whose basic personality phenotype is that of a treacherous sneak, someone who would “rat” on his closest friends, someone like Peter Pettigrew, would literally become a rat.
Because the animal phase is an expression of their most basic personality phenotype, Animagi have no ability to choose the animal they will become. If others understand the meaning of the animal phase, this can be very embarrassing indeed. Who would be friends with a rat if they knew the implications of that animal phase? Thus, many Animagi may have chosen to remain secret and not register officially, which would reveal their true personality to anyone who did understand the implications. Professor McGonagall is not afraid to be known as that independent and intelligent animal, the cat, but Rita Skeeter would rather risk blackmail than reveal that at heart she is nothing more than an insect.
It is interesting to speculate on what animal form others would take if they were Animagi, based on knowledge of their personality phenotype. Here are a few suggestions:
- Professor Albus Dumbledore. As he once was the Hogwarts Transfiguration Master, it is almost certain that Prof. Dumbledore could transfigure himself into another animate creature. Widely known as the greatest sorcerer of his time, it would seem almost inevitable that he would become a powerfully magical creature of great age, possibly a phoenix.
- Harry Potter. Harry would turn into something monumentally brave and heroic, perhaps an eagle or griffin. If it didn’t seem such a dangerous animal, his lightning bolt scar, Quidditch ability, and knowledge of Parseltongue would hint that he might transfigure into a fiery, flying reptile, the dragon.
- J. K. Rowling. Her personality phenotype is that of the consummate “auteur,” and she would undoubtedly transfigure into an “otter” if she became an Animagus (Sch2).
Here's a biographical note provided by the author:
“Professor Koniphorous Swamp is a vegemagus, an extremely rare type of magical person who can at will turn into a plant. As a result of this unusual ability, Prof. Swamp has devoted her career to the investigation of the biology of magic,Transfiguration in particular. She theorizes that there may be many undiscovered vegemagi, particularly among witches with botanical names such as Lavender, Lily, Myrtle, Narcissa, Olive, Pansy, and Poppy.”