Bad biology in fanfiction is exactly what you would think it is: improper portrayal of organisms. Bad biology may affect anatomy, physiology, genetics, diseases, and more. It is a common source of squick.
Fics with poor biology are encountered by most departments, but fics where biological charges are the main issue usually end up in the Department of Bad Slash or Division of Bad Het, since they handle stories that revolve around the fundamentally biological processes of sex and (sometimes) reproduction. It is arguable that the Department of Mary Sues sees the most illogical genetics, which is a standard feature of many Mary Sues.
While biology is a topic that is taught in most schools, for some reason it escapes many writers... and even if one did not study biology in school, ignorance is not an excuse when resources are just a click away on the Internet. In this respect, it's similar to bad psychology.
Signs of Bad Biology Edit
Nonsensical Afflictions Edit
Inaccurate portrayal of diseases, chronic conditions, and other health problems is a common manifestation of bad biology. A nonsense affliction could be a misrepresented World One health issue such as cancer or a concussion, or it might take the form of a made-up, uncanon illness that doesn't make sense. The cliché of foreshadowing the presence of a deadly affliction with a vague "soap opera cough" is also a form of bad biology.
Nonsensical Cures Edit
Contrary to the belief of some, athelas does not actually cure colds, flu, cancer, or syphilis. Despite what Hollywood might lead you to think, getting shot in the shoulder is bad, especially in a world without modern means of sterilization. Broken bones hurt a lot, and slapping a makeshift splint on it doesn't necessarily mean you'll be able to get up and keep fighting. From misapplied medicine to healing sex, bad biology can take the form of cures that are simply not medically sound.
Illogical Genetics Edit
Mary Sues really love things that can make them special without any effort on their part, and what's easier than to simply be born? Illogical genetics are very common in badfic, often lending speshul qualities to the product of said genetics. Whether the bad character is a crossbreed of two incompatible species (or even three or more incompatible species), or an impossible child of something that can't procreate, illogical genetics are there. Tamer versions might just involve improbable inheritance of real traits (e.g. "Both of my parents are blond with blue eyes, but I have natural jet-black hair and emerald-green eyes"), while more exotic versions might claim non-heritable traits or powers from their parents: "I'm a child of Sauron, and I inherited my red eyes and black hair from him because that's all sinister!" or "My elf heritage lets me cast magic that not even elves can cast," etc.
Bad Anatomy Edit
Anatomy is complex and requires research and study to learn. But this doesn't explain badfics where even the most obvious anatomical facts are absent. While this can apply to any organism and concern any bodily system, bad anatomy of humans and humanoids (and the sex organs thereof) are the most common... which is distressing, considering most if not all authors are human themselves... but then again, this sometimes causes nonhuman characters to mysteriously manifest mammal/human sex organs.
Mpreg can also be related.
Unpleasant Actions Edit
Without comprehension of the human (or otherwise) body, authors may command their characters to do unpleasant things without realizing that they are impossible, or at least excruciatingly painful. A failure to grasp biology produces such bad sexual situations as Not There, Dubious Lube, and violations of the Mechanics. But perhaps even more common are the impossible, agonizing actions that happen in badfic all the time, such as forcing horses to run for hours and hours at a time without water, or somehow contorting the body impossibly in combat to show off "fighting skills."
It should be noted that what constitutes bad biology can vary depending on canon, as more fantastical or comedic worlds are likely to feature biology which doesn't work in the Real World. For example, in Gorillaz canon, head injuries make one's hair turn blue (and one's children will inherit that hair), and in The Land Before Time, dinosaurs have eyelashes. In cases like the aforementioned, it's generally best to assume that things work the way they do in the Real World unless canon specifies otherwise; one canonical biological inaccuracy does not mean ficcers can throw all the laws of nature out the window.