Isogamy is a form of sexual reproduction that involves gametes of similar morphology (similar shape and size), differing in general only in allele expression in one or more mating-type regions. Because both gametes look alike, they cannot be classified as "male" or "female." Instead, organisms undergoing isogamy are said to have different mating types, most commonly noted as "+" and "-" strains, although in some species there are more than two mating types (designated by numbers or letters). Fertilization occurs when gametes of two different mating types fuse to form a zygote.