Balancer chromosome

A balancer chromosome is a special chromosome that distinguishes genetic research with Drosophila melanogaster from that with other organisms and gives it a decisive advantage. The balancer chromosome prevents recombination ( crossing over ) in the flies, so that recessive lethal mutations can be kept as stable stocks. Balancer chromosomes thus use the mechanism of balanced polymorphism .

The idea of ​​the balancer chromosome goes back to Hermann Joseph Muller , who discovered in 1918 that chromosome C1B suppressed recombination on the X chromosome . He found out that this chromosome is no longer able to enter into homologous recombination with the sister chromosome due to various inversions . When these chromosomes are combined with a recessive marker , they represent a very useful tool in fly genetics. Only flies with a balancer chromosome and the recessive lethal mutation on the other chromosome can survive in stocks with such a balancer chromosome . If these chromosomes did not exist, each new generation would have to be selected for its genotype to ensure that the mutation is not lost.

Balancers are available for chromosomes X, 2 and 3. The most efficient are balancer chromosomes, which suppress recombination over the entire length of the chromosome. In cases in which a cross-over nevertheless takes place, faulty chromosomes are generated because large parts of a chromosome are duplicated or deleted in the germ cells that are formed.

There is an exception for X chromosomal balancers: since the hemizygous males would not survive a recessive lethal marker because they do not have a healthy X chromosome but instead the Y chromosome, markers that are not lethal are used for the X chromosome, for example but only have a sterile female phenotype. Then female flies can emerge that carry a double balancer, but they cannot reproduce.

literature

• Ralph J. Greenspan: Fly pushing. The Theory and Practice of Drosophila Genetics. ISBN 0-87969-711-3