Rebecca, I feel your pain as I was in the same boat. My biological mom (I was adopted but raised to know her, she is like a best friend to me now) had size 34DD breasts by the time she was 16, yet I was flat chested until about 16, then only got to a 32B, had a baby at 17 got to a 34C while pregnant and breastfeeding, after breastfeeding right down to a saggy 32AA! If it's even possible for a AA to be saggy, mine was lol. So I always wondered why she was big chested and I never was. I then met my biological dad when I was 18, he was more of my body type, short, skinny, petite. He showed me pictures of family members from his side and ALL the women were no bigger than a B cup, so it was like ding ding, that's where my booby genes came from, thanks dad
So what I'm saying here is, look at your dad's side of the family, what are the women like on his side? Maybe you took after him more than your mom? I've also talked to women who both sides of the family were well endowed but they were not, so it can all vary. It really isn't up to your genes, but up to the individual how much they grow during puberty, you could have not gotten the booby genes at all from either side, it happens. Also diet and exercise while in puberty determine your size ALOT more than genes do, so does stress levels. If you aren't getting enough nutrition, exercising too much for them to gain anything, and life is a major stress, more than likely you won't grow much. Every big chested gal I talk to that grew naturally during puberty I've asked how their diet, exercise and stress was during their teens. They all say they ate alot but healthy if they look fit, didn't exercise much, and had very little stress. Me on the other hand, I barely ate most the time for many reasons, I exercised some, and stress levels were VERY high for me during puberty. So it makes sense that these are big factors.
But there is hope hun, read through the forum, do your research and find what you want to try to grow. Good luck!