In ancient Middle East, inheritance rights are usually passed
from father to son but they can also be passed from father to
grandson, or from father to son-in-law. As a side issue, one
example of the latter was when Jacob and Rachal fled from Laban,
Rachal stole the household gods, which caused Laban to chase after
them all through the night. Archaeological evidence suggests that
these household gods (trafim) are used as the token of inheritance
which can be passed from father to son usually but also to the
son-in-law in the event that there are no surviving sons.
The blessing on the death bed is like making a will in modern times
and it is binding. So Isaac mistakenly left his inheritance to
Jacob instead of Esau when he blessed Jacob thinking he was the
hairy Esau. Jacob on his deathbed decided to bless Ephraim and
Manasseh, giving them an equal share in his estate as their uncles.
So Joseph is over-represented, having two tribes to his name. The
tribes of Joseph therefore refer to the tribes of Ephraim and
Manasseh.
So, in one sense, if you count Ephraim and Manasseh as separate
tribes, you get 13 in all, not twelve. However, if you consider the
tribes that inherit land, there are only 12, because the tribe of
Levi do not own any land. Their inheritance is the Temple of
God.
