Each species of multicellular organisms has a set number of chromosomes that carry all genes required for development and living processes. However, some species in both the plant and animal kingdoms have extra chromosomes, called B chromosomes, which are dispensable. They only exist in some individuals of a species and often are variable in number. Variation in number of the normal chromosomes is highly detrimental, but B chromosomes seldom have any detrimental effects unless many copies are present. Thus, they are considered to be mostly inert, being neither required nor detrimental. They exist in populations because they have properties that foster their accumulation; normal chromosomes do not possess these properties. In some plant species, particularly corn, they have been used to make translocations with the normal chromosomes, which have proven to be useful for mutation mapping, chromosomal dosage studies, and recently production of engineered minichromosomes.