Genetics -- why you look like your father, or if you don't, why you should.
I vaguely recall some DNA study that found roughly 1 in 5 Americans wasn't sired by the person they believed was their father. This may exaggerate, but it's always something to consider as possible. You really don't know what your ancestors were up to, and your ancestors may not either!
Funny story told by someone I used to know, who traced American Indian genealogies for a living:
Mary gave a presentation at some outfit like "Daughters of the Pilgrims" (I forget exactly) and afterward a woman came up, genealogical scroll in hand, and proudly proclaimed that she was descended from Famous Pilgrim Woman (whose name I also forget).
Mary said, "No, you're not."
The woman pointed at her scroll and insisted, "Yes I am; here she is in my pedigree!"
Mary said, "No, you're not. Famous Pilgrim Woman was sterile; she adopted 13 Indian children."
Needless to say, this did not go over well.
I can show you dog pedigrees where a recessive trait, not rare in dogs overall but in a given breed only sourced from one foundation animal, was in that breed (ie. ethnic population) vanishingly rare or not seen at all for 10 or more generations.