Naming Convention
Balancer chromosomes are named for the chromosome they serve to stabilize and for the phenotypic or genetic marker the balancer carries. 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 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 (i.e. TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (i.e. 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 2nd and 3rd chromosome in Drosophila are widely available.
Read more about this topic: Balancer Chromosome
Famous quotes containing the words naming and/or convention:
“Husband,
who am I to reject the naming of foods
in a time of famine?”
—Anne Sexton (1928–1974)
“Mankind owes to the child the best it has to give.”
—United Nations Convention on the Rights of the Child, 1989.