associative plurals

[Gontzal Aldai]

> The associative plural construction consists of a noun N - usually a
> proper name or a kinship term - and a marker (often the same as the
> regular plural marker of the language). The meaning is 'N and his
> family (or friends; or associates)'. An example is Japanese:
>    Tanaka-tachi 'Tanaka and his friends'

As Paolo Ramat says, perhaps we are going a bit too far here. BUt I was
just wondering: What is the origin of the Italian surnames in -I related
to one person's first name? Do they come from genitives?, from plurals?,
from plurals of genitives? Same question can be applied to Spanish family
names in -EZ, Portuguese in -ES, Catalan in -IS, and probably many others
(Basque in -IZ?).