Syntax

Syntactic actants

Actant 1: whose buttocks they are
(a) Genitive not Common
(b) Possessive
Толстый зад нашего учителя не помещался на стуле. Our teacher's fat ass wouldn't fit in the chair.
Тётка заслонила проход своим широким задом. The woman blocked the way with her big wide ass.
NB When the word зад functions as a subject and has no descriptive modifier, the construction зад у + Genitive is often preferred to (a) and (b), above:
Зад у Петьки был худой и костлявый. Pet'ka had a skinny, bony butt.