Balancer chromosomes (or simply balancers) are a type of genetically engineered chromosome used in laboratory biology for the maintenance of recessive lethal (or sterile) mutations within living organisms without interference from natural selection. Since such mutations are viable only in heterozygotes, they cannot be stably maintained through successive generations and therefore continually lead to production of wild-type organisms, which can be prevented by replacing the homologous wild-type chromosome with a balancer. In this capacity, balancers are crucial for genetics research on model organisms such as Drosophila melanogaster, the common fruit fly, for which stocks cannot be archived (e.g. frozen). They can also be used in forward genetics screens to specifically identify recessive l