[THE BIOLOGY OF PETGIRLS]

Petgirls exist due to several genetic mutations that have become common in humans. These mutations are passed from mother to child, but since there is no single "pet gene" it's not as simple as saying women give birth to regular children and pets give birth to pets. Both can give birth to both, though daughters of petgirls are more likely to be pets too.

The mutations do nothing unless all of them are present. If this is the case they trigger the production of a modified estrogen. This new hormone, estroxin, has three key effects in females:

1. Identity shift. Girls start to identify with and accessorize/dress as an animal species. How the species is determined is unknown, but a genetic or epigenetic component is suspected because it's common for second-generation petgirls to identify with a similar species as their mothers. Some girls almost instinctively know what they are, while others may be aware that they're pets but need more time to work out their affinity.
2. Personality change. Girls lose interest in careers and seek out owners instead. They tend to become more playful, approachable, and submissive, but this is not universal. There is a high degree of variability.
3. Sterility. Pets can't get pregnant unless they receive fertility treatments.

Women are not the only ones affected. The mutations are present in males as well, but due to lower amounts of estrogen (and therefore lower amounts of estroxin) they experience almost no symptoms. Some doubt there are any mental effects at all. Others say it makes them **** and more understanding, and that they have a greater affinity for animals and petgirls. The only undisputed facts are that it lowers the virility of their male sperm cells (meaning they have more daughters than sons), and that their daughters are more likely to be pets as well, though not to the same degree as the daughters of pets.

Ashley, for reasons the doctors are still trying to figure out, started producing estroxin much later than normal, which allowed her to go through regular estradiol-guided puberty. As a young woman her estroxin levels are still low. The result is that she has not lost her interest in a career, and may still be fertile.

Ashley's case is rare enough that it has the doctors stumped, but not totally unique. What happens when a male produces too much estroxin is hinted at in the alternate start "A Fish Called Tibby."