Naming convention in Drosophila 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 in D. melanogaster 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 an M , for "multiply inverted . "  The M is followed by a number to distinguish balancers of the same chromosome. Additionally, the genetic marker or markers of the balancer are listed after the name and separated by a comma. Generally  mutations with easily observable dominant phenotypic traits that are often homozygous lethal are used to ensure that all progeny are heterozygous. 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 " . A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (for example, TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (for example, D/TM3,Ser). Most balancer chromosomes also carry a recessive allele such as the "ebony" mutation that is only manifest in these stocks with two balancer chromosomes. These 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 might be selected. Stocks double-balanced at both the second and third chromosomes in Drosophila are widely available  .
Naming convention in Drosophila 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 in D. melanogaster 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 for second, and T for third. The small fourth chromosome does not undergo recombination and therefore does not require balancing. This letter is then followed by an M for "multiply inverted " . The M is followed by a number to distinguish balancers of the same chromosome. Additionally, the genetic marker or markers within the balancer are listed after the name and separated by a comma. Generally , mutations with easily observable dominant phenotypic traits that are often homozygous lethal are used to ensure that all progeny are heterozygous. For example, the commonly used TM3, Sb balancer stabilizes the third chromosome and carries a mutant Sb ("stubble") gene as a dominant marker. 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. The 3 distinguishes this balancer from other third-chromosome balancers such as TM1 and TM2 . A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (for example, TM6,Tb/TM3,Ser) on one chromosome and a homozygous-lethal, heterozygous-visible mutant on the other, wild-type chromosome (for example, D/TM3,Ser). Most balancer chromosomes also carry a recessive allele such as the "ebony" mutation that is only manifest in these stocks with two balancer chromosomes. 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. Stocks double-balanced at both the second and third chromosomes in Drosophila are widely available from fly stock repositories .
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