My game came with dominus <| sep |> It is genetic , but hard to trace for the same reason that all secondary sexual traits are hard to trace . Half of your genes come from each parent , and very few are carried exclusively by the Y chromosome that comes from dad . The genes that influence penis size , shape , and development are complex and come from both parents . Of course your mother is just a carrier of these genes , as she doesn ' t have one herself , so it ' s very difficult to follow the inheritance through the maternal side . Additionally , like all secondary sexual characteristics , including voice pitch , body hair , breast development , etc . they don ' t exist in childhood and are later activated by the presence of testosterone and estrogen . Trouble is , a person ' s natural testosterone and estrogen levels are also genetic and inherited from both parents . It ' s also believed that the mother ' s blood testosterone levels while a male baby is developing in the whom can also tweak things in one direction or the other . In other words penis size is absolutely genetic . But the genetic influence comes in multiple developmental stages , some direct and some indirect through hormone levels , and is difficult if not impossible to trace all of these back through both sides of a family to really see the connection .