Mendelian inheritance

Generally applies to qualitative characters

Some examples:

Is the plant resistant to fungus x or not?

Is the flower red or white?

Does the plant need a certain daylength to flower or not?

This situation arises when a trait is controlled by one (or a small number of ) "major" genes.

There are many examples of this in breeding:

Disease resistance

Dwarfness

Flower colour

These characters are often quite easy for the breeder to handle by simply looking at the phenotype