Broader Term: non-Mendelian inheritance

Epistasis

Phenomenon in genetics that one gene (a pair of alleles) suppresses the operation of another gene (pair of alleles). Epistasis is a type of a non-Mendelian inheritance pattern. Epistasis comes in different types.

Note:

Epistasis may also occur between three or more genes, in quantitative genetics.
NT: dominant epistasis ↔ recessive epistasis ↔ cumulative isomery ↔ non-cumulative isomery ↔ reciprocal dominant epistasis ↔ reciprocal recessive epistasis
RT: polygenes
BT: non-Mendelian inheritance

Multiple alleles

Phenomenon in genetics that three or more alleles (example: A, a1) occur for a singular gene. A diploid plant contains just two alleles from this allelic series. As a result more than two phenotypes occur. Multiple alleles is a type of a non-Mendelian inheritance pattern.

Note:

For both multiple alleles and epistasis a 3:1 segregation ratio is very common in the offspring of a directed cross between two different phenotypes. An allelic test can give conclusive evidence which inheritance system applies.
RT: allelic test
BT: non-Mendelian inheritance