Balancer chromosome

A balancer chromosome is a genetic tool used to prevent crossing over (genetic recombination) between homologous chromosomes during meiosis. Balancers are most often used in "Drosophila melanogaster" (fruit fly) genetics to allow populations of flies carrying heterozygous mutations to be maintained without constantly screening for the mutations. Balancer chromosomes have three important properties: they suppress recombination with their homologs, carry dominant markers and negatively affect reproductive fitness when carried homozygously.

For example, flies that carry a mutation a researcher wants to study on a normal chromosome and a wild-type ("wt") balancer version of the homologous chromosome can be thrown in a tube with some fly food, then several months or generations later, flies with the same genotype will still be in the tube. Without balancer chromosomes then flies would carry genetic markers in completely different combinations after several generations.

How balancer chromosomes work

To suppress crossing over, balancer chromosomes are the products of multiple, nested chromosomal inversions so that synapsis between homologous chromosomes is disrupted. If crossing over between a balancer chromosome and the balancer's homolog does occur during meiosis each chromatid ends up lacking some genes and carrying two copies of other genes. Recombination in inverted regions leads to dicentric or acentric chromosomes (chromosomes with two centromeres or no centromere). Progeny carrying chromosomes that are the products of recombination between balancer and normal chromosomes are not viable (they die).

Dominant markers such as genes for green fluorescent protein or enzymes that make pigments allow researchers to easily recognize flies that carry the balancer chromosome. By suppressing reproductive fitness when carried homozygously a balancer chromosome ensures that the population it is carried in does not become fixed for the balancer chromosome.

Naming convention

Balancer chromosomes are named for the chromosome they serve to stabilize and for the phenotypic or genetic marker the balancer carries. [Fly Pushing: The Theory and Practice of Drosophila Genetics By Ralph J. Greenspan. Page 13] The naming of balancer chromosomes has been standardized as follows: the first letter of the chromosome's name represents the number of the chromosome it stabilizes. F stands for the first chromosome, S stands for second, and T stands for third. (The small fourth chromosome does not undergo recombination and therefore does not require balancing.) This letter is then followed by M, for "multiply inverted". The M is followed by a number to distinguish balancers of the same chromosome. Additionally, a list of genetic marker or markers (generally mutations with easily observable dominant phenotypic traits. Such mutations are often homozygous lethal to ensure that all progeny are heterozygous.) of the balancer are listed after the name and separated by a comma. For example, the commonly used "TM3, Sb" balancer is a balancer chromosome that stabilizes the third chromosome and carries a mutant "Sb" ("Stubble") gene as a marker. All flies containing the "TM3, Sb" balancer will have shortened (or stubbly) hairs on the back of the fly, which are easily seen when viewed through a microscope. The "3" is to distinguish this from other third chromosome balancers, such as "TM1" and "TM2".

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