A lot of people really believe that genetics play a much bigger role in vocal ability and talent than it actually does. So we’ll start there. The first place genetics has an influence is on the size of your vocal folds. Thicker vocal folds can vibrate at lower frequencies. Just like a piano, the lower the note, the thicker the string needed. Men’s voices undergo a structural change during puberty where their vocal folds grow in length (length also influences ability for low notes) and thickness, which enables their voice to reach lower notes.
The sound of the vocal folds all by themselves, is not too exciting. If you could chop off someone’s head and hear just the sound of the vocal folds, you would only hear a buzzing sound, essentially. There are a good amount of things that your vocal folds can do that influence the sound you hear, but really they only account for a small percentage of the vast amount of sounds a person is able to produce.
So what makes the huge variety of sounds that we are able to hear from someone’s “voice”? It’s actually their mouth, throat and nasal cavity! (And lots of little things that can move within that area to influence your sound.)
You could compare this with a trumpet player. If you could see a trumpet player do the lip motions required to play the trumpet, without a trumpet in his hands (essentially a trumpet version of the “air guitar”) you would see that there isn’t much of a “sound”. When he applies that lip motion to an actual trumpet though, you get LOTS of sound!
Same thing with your voice.
So your mouth, throat, and nasal cavity act as the trumpet. Here is where genetics have a factor. Basically, the larger your mouth and throat are, the darker your voice is able to be. Everyone can make darker or brighter sounds in their voice (see my other articles for an explanation of that) but how dark specifically will vary with the size of your mouth/throat.
To relate this to a real world example, imagine the way someone with extreme dwarfism would sound…likely they would have a bright, often high voice, right? Why is this? Partially it’s their vocal folds but it’s also that they have a small mouth cavity. Now imagine someone with giantism, you would imagine that they would have a deeper, darker-sounding voice. Same thing happening here.
So part of what makes your voice unique, is genetics, but there are other factors at work, namely:
Vocal Skill Level
Musical Skill Level
Your Speaking Voice/Accent
The Vocal Music You’ve Heard and Imitated
Personal Style/The Sounds You Imagine
The Demands of the Genre You’re Singing