is the size of your boobs genetic?
Yup. It all depends on your genes, and sometimes depending on how big your relatives breasts are, including your mom, aunts, grandmother(s). It all depends. My mother was an A cup all her life until she had kids. I'm a 32B and soon I'll be a C ^-^. My mom is now a DD o_o. ANYWAY, It really matters about your genes. Your mother could be as flat as a board, while you have a nice rack on your chest xD. So it depends on your genes and your body type. And like the answer above, it also depends on your weight as well.
Yes, breast size is related to genes, although other things such as weight can influence it as well.