Balancer chromosome
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).
For that reason, balancers are also used in other model organisms, most notably the nematode worm Caenorhabditis elegans and the mouse.
Balancer chromosomes were first used in the fruit fly by Hermann Muller, who pioneered the use of radiation for organismal mutagenesis.
[2] In the modern usage of balancer chromosomes, random mutations are first induced by exposing living organisms with otherwise normal chromosomes to substances which cause DNA damage; in flies and nematodes, this usually occurs by feeding larvae ethyl methanesulfonate (EMS).
Equally as important, they also realized that they could limit crossing over in these populations as well as give them very consistent genetic compositions.
[6] In diploid organisms, mutations without recessive lethal (or sterile) phenotypes can simply be bred to homozygosity and maintained stably and indefinitely by crossing homozygotes.
However, recombination between the balancer and the homolog containing the mutated allele may also result in the de novo creation of a wild-type chromosome.
Any such cross that does not produce marker-negative animals is likely the result of a recessive lethal mutation in the non-balancer chromosome.
Generally, mutations with easily observable dominant phenotypic traits that are often homozygous lethal are used to ensure that all progeny are heterozygous.
All flies containing the TM3, Sb balancer will have shortened or stubbly hairs on the back of their abdomens, which are easily seen when viewed through a microscope.
Such stocks are often used to provide sources of easily traceable traits when breeding two different lines together, so that the correct progeny of each cross can be selected.
Using old balancer chromosome methods, genetic screening only allowed for the selection of heterozygous dominant mutations.
[12] Another published paper that employed the use of balancer chromosomes is "Inhibition of RNA Interference and Modulation of Transposable Element Expression by Cell Death in Drosophila".
A line was established that exhibited low levels of cell death and was named EGFPir hs-hid.
