To be of a line or ancestry (of people with a certain characteristic); stock, breed (of animals); strain (of plants).
Such as generations of relatives; descendants of a social class or one particular person.
This water buffalo of mine is of a stock of big water buffalos.
That person is of a line of imams.
That person is of an ancestry of rich people.
This papaya is of a strain of big (lit. American) papayas.