Great question! Anthropologists like to say that in humans, anything which isn't biological is cultural. This means that whatever behaviours we may have, including those which make up our personality, much of it is shaped by social and cultural learning. Behavior does have its roots in biology and our instinctive drive to fulfill certain needs- food, drink, sleep, shelter, and reproduction. Socialization, too, can be considered a need. Humans are highly social creatures, and socialization is so engrained in humans as an adaptive strategy that we do very poorly without it.
There have been some studies done on children who were deprived of social contact during their development. The most famous case is that of Genie, who spent almost all of her early life by herself in her room. In cases like this, where socialization does not play a significant part in development, biology takes charge. Genie has fascinated anthropologists and psychologists alike because she had essentially not learned any culture. Even her language capacity was minimal.
Where personality is influenced by biology, some behaviours or mannerisms are easy to explain. When we feel threatened emotionally or physically, we tend to act defensive so as to protect ourselves. Similarly, how outgoing or introverted someone is can be based on the amount of stress or feel-good hormones in response to socialization. It is difficult to draw many direct lines between personality and genetic programming towards increasing fitness. At our very cores, that is really what humans are trying to do in every action- survive. Culture and socialization serve to shape the unique ways we tackle the challenge of survival.
Because all humans have the same basic, biological needs, one could say that personality is almost entirely shaped by culture. There is some biological variation which can influence personality, as in someone with an anxiety disorder caused by a chemical imbalance in the brain. Beyond such physiological variation, personality is almost entirely cultural or social.