Well, generally introversion is a genetically inherited personality trait (research shows that it is connected with certain brain structures). However, environmental factors also play a role in forming one's personality. Thus, a mother's availability could influence a baby personality toward extroversion. In social science, more often there are many factors that influence individuals than a single one. Usually, when there is a genetic predisposition to something, environmental factors could strengthen it or restrict it. So in this case, if someone has a predisposition to be an introvert, what happens later during their upbringing (environmental factors) could strengthen their introversion or restrict it.
The example I gave with my husband's mother is only to show the fun fact that we actually met at his home as her friend suggested a decade earlier :)
I see, so there is a predominant biological factor too, quite interesting. A very particular anecdote that you have told us, thanks for sharing it with us.