What do genetics have to do with writing fiction? Everything.
The debate of Nature versus Nurture will likely rage on until the end of time, but the fact remains that many of our traits are down to simple genetics. Hair and eye color are just some of these traits that you have to keep in mind when creating a character. Here's how to get your genetics right.
A Fictional History
Let's start with some background. Draw up a basic family tree for your character. When I say basic, it can be as basic (or not) as your story requires from you. Go back as far as your characters great-grandparents - note their names, eye and hair colors, relations and anything in their back-story that aids your story now: For example, is there a family history of pattern baldness or cancer that affects your current character's story arc or life?
First, here's the most basic fact about genetics: Certain traits, when put together, will create other traits. This means that the product of two blonde parents will very, very likely be a light hair-colored child – not always, but likely. This is something a fiction writer has to get right: There's no point in selling a character that couldn't possibly, based on basic genetics, be the son of who the story claims.
Not sure which combinations will result in what? Don't worry about it. That's what the internet is for.
What else can be genetic? Characters can also have traits like pattern baldness; the way their mouth turns might look like someone in their family; they might have a nose reminiscent of their grandfather. It can also be more serious like inherited diseases or health problems that simply 'run in the family'. Cancer, high blood pressure or heart disease are some; rarer illnesses like Marfan Syndrome can be hereditary.
Even things like susceptibility to addiction or tolerance for drugs or alcohol can be part of one's DNA coding. (Don't believe me? Rock legend Ozzy Osbourne had his genes sequenced and yes, according to his autobiography I AM OZZY his drug tolerance is part of his wiring.)
Mental illnesses like depression, bipolar disorder or schizophrenia can be hereditary too. Consider this if you want your fiction to portray the facts accurately.
Also think about genetic throwbacks. These are genetic traits that skip a generation, or several. This can, for example, lead to a seemingly white couple unexpectedly producing a black baby or mental illness skipping one generation and hitting the next instead.
If your character is adopted, you'll still have to delve into the background. Ask yourself some extra questions here, like why they were given up for adoption, who they grew up with and where. Which brings us to...
Nature versus Nurture
....the fact that not all traits are genetic. When considering the fact that your character's genetic history has made them who they are, let's spare a thought for the non-genetic things that turn your character into the person the story needs them to be. For example, a lifelong fear of spiders is likely to come from a terrible childhood experience whereby the person had the wits scared out of them by a spider.
The way people behave, act and react is down to their collective experiences. Someone who has experience with high-pressure situations might, for example, cope better in a natural disaster situation. Someone who spends too much money on luxuries could be compensating for something else. Someone who was caught up in a previous hostage stand-off might react differently to a car backfiring in the neighborhood, and a character whose dad used to take them to the shooting range as a kid would likely be a better shot faced with an attacker.
Now think about yourself and your own experiences. Why you react a certain way to things and why. Do you have standard reactions for certain things, or pet hates because they remind you something? Who you are is a mixture of your genetic make-up and your experiences; the same should always be true for your characters. As I've said before, characters are made of flesh, blood and ink. The DNA, of course, flows from your keyboard.
Don’t know where to start? The internet is a great resource, and you don’t have to do any of the math yourself!
Quick Trait Calculator
This quick trait calculator includes traits like Eye Color, Hair Color, Widow’s Peak (Yes, really!), Cleft Chin, Dimples and Earlobes – attached or unattached, apparently.
Quick Trait Calculator II
This second quick trait calculator helps you figure out other traits like Alcohol Flush Reaction, Blood Type and Lactose Intolerance. Who knew?
Here’s one specifically for eye color from BabyMed.
Feeding your head? Take this free course from MIT OpenCourseware on the science of genetics.