What genetic criteria distinguish a case of extranuclear inheritance from: (b) a case of X-linked inheritance?

Answer the following fundamental questions:

How was extranuclear inheritance discovered?

What genetic criteria distinguish a case of extranuclear inheritance from: 

(a) a case of Mendelian autosomal inheritance

When performing a complementation test, how do you know if two mutations complement?

How can you tell if two genes are epistatic?

Two heterozygous organisms are crossed, and the F₂ phenotypic ratio is 12:3:1. What form of epistasis do these two genes exhibit?