certain breeds can be crossed and registered, mainly with tica. Example Bengal x Oriental short hair is a first generation serengheti cat, only registered with tica if both parents are registed active.

if a siamese was mated by a persian the kittens are cross bred so no, kittens would not be registered. i'm not sure why people do this, unless it's an accidental mating.

I know back when people were introdcing new breeds they crossed cats but it's then done proper and for a purpose and i guess off spring weren't registered in early generations? not sure.