$\ldots$ Pleiotropy is a phenomenon in which a single gene mayexpress more than one trait. Sometimes, one trait will be very evident and other will be less evident, e.g., a gene for white eye in Drosophila also affect the shape of organs is male responsible for sperm storage as well as other structures. Multiple allelism is a series of three or more alternative or allelic forms of a gene, that can occupy the same locus.
Mosaicism is the occurrence of cells that differ in their genetic component from other cells of the body.
Polygeny refers to a single characteristics that is controlled by more than two genes. (It is also known as multifactorial inheritance).