Tattoos arenīt acquired genetically.
It is a symbol of SeeD.

Adel is easily explained.

I theorize that SeeD (and "white SeeD") has these powers because they are alien beings known as Propagators.
They ruled Centra but camed from another planet.
Propagators are the monsters Squall and Rinoa fought on the airship on outer space.
These monsters have the ability of acting as a single entity and can physically become one through some artificial processes.This technology exists somewhere on the Galbadia prison.
This single entity is Adel.
if you look at Adel you will see that he resembles Squall (on space),Zell(FMV beggining fight),Seifer (same) and Irvine(Through the long pushair).The girls resemblance are difficult to see but are easily spotted through the clothes.