Personality is defined with following important factors: period you were born, genes and society.
For example, woman in 21st century is independent and equal with man. She is very ambitious and successful in her career. Today, woman is trying to live up to her potential in her profession. If she was born in 15th century, she would be inferior to man. Her only task would probably be taking care of children and her husband. She would be raised to become a housewife.
Genes are the basic units of heredity in a living organism so it's logical that they are affecting our personality. We are inheriting genes from our parents. To explore how much our genes are affecting our behavior, wishes and dreams, experts were making researches on twins.
Twins are two offsprings resulting from the same pregnancy, usually born in close succession and they have very similar genetic code. They started research on twins while babies weren't born yet. With modern technology like 3D cardiotocography they noticed that one twin was more dominant.
Twins have very similar genetic code so they are adequate for making researches on topics like how much culture and society affect our identity. Experts decided to make some tests on twins that were growing up together in the same family, and on twins that had been separated as babies and were growing up in different families and different cultures.
Twins that were growing up in the same family have more similar habits, opinions, ambitions than twins that were raised in different cultures. Twins raised in different families have almost nothing in common.
Experts made conclusion that personality also depends on family, culture and upbringing.